Evolutions in Gaseous and Liquid Fuel Cook-Stove Technologies

Abstract

The rapidly growing global demand for pollutant-free cooking energy has proliferated the research and development of energy efficient and clean cook-stoves. This paper presents a comprehensive review on the gradual improvements in cook-stove designs, focusing on gaseous and liquid fuel-operated cook-stoves around the world. Various literatures concerning the technical aspects such as design and testing, are brought together to provide an insight into the present status of developments in cook-stoves. This review of cook-stove performance covers topics such as stable operating conditions, flame propagation aspects, heat transfer and temperature distribution within the burner, fuel consumption, thermal efficiency, and emissions. Covering both laboratory-scale and field studies, the various cook-stove technologies reported so far are summarized with relevant comments regarding their commercial viabilities. The numerical modeling of combustion in cook-stoves; human health and the environmental impacts of unclean cooking technologies; and various schemes, strategies, and governmental initiatives for the promotion of cleaner cooking practices are also presented, with suggestions for future work.

1. Introduction

Clean and efficient cook-stoves and fuels are essential to address energy demand and ensure safe cooking for billions of people across the globe. Though cooking has a history of thousands of years, the methods, technologies, and fuels used have changed from time to time. Currently, nearly 2651 million people in developing countries (Africa—910 million, Asia—1674 million, Central and South America—57 million, and Middle East—10 million) are without clean cooking facilities [1]. In the previous decades, there was not much difference in the cooking technologies and the fuels used between the rural and urban households. However, due to improved living standards, sophisticated lifestyles, access to technology, and the availability of more resources to the urban people, a greater change in cooking technologies and availability of fuels has taken place between the urban and rural communities. For instance, while urban households started using cooking fuels such as natural gas, Liquefied Petroleum Gas (LPG), kerosene, biogas, and electricity, the people living in rural areas are still using traditional cooking fuels such as firewood, dried cow-dung cakes, dried leaves, and plant residues. Significant variations in access to clean cooking between urban and rural areas has led to disparities in infrastructure and the availability of clean fuels and technologies [2]. There is an increasing trend of the adoption of gaseous (LPG, natural gas, and biogas) and liquid fuels (kerosene, methanol, ethanol, etc.) in low and middle-income countries [3]. This is growing rapidly due to the global initiatives to promote clean energy cooking fuels [4,5,6]. Though electric stoves are pollution free at a household level, they may not be feasible for the developing nations such as India to adopt for cooking applications in the next few decades due to the rapid increase in electrical demand for the industrial and agricultural sectors. Further, some parts of the rural population in India still lack a consistent power supply, and a few villages even lack access to electricity. Major parts of the electricity generated in India is through coal, and the country is still on its way to shift towards renewable sources for electricity production. There is also a lack of an adequate supply of coal, which makes the generation of electricity inconsistent. Thus, shifting from gas stoves to electric stoves would only add a burden to the government.

Almost four million deaths per year have been reported due to pollutants released from the burning of kerosene, charcoal, wood, etc., for cooking [7]. Exposure to pollutants from cooking with such fuels can lead to chronic obstructive pulmonary disorder, heart disease, childhood pneumonia, lung cancer, etc. [8]. Among the 17 Sustainable Development Goals (SDG) of the United Nations (UN), providing access to clean cooking solutions has been given top priority in SDG 7 [2]. Electricity from renewable energy sources with electrical cooktops can be conceptualized as a possible solution; however, recent multi-model analyses [9,10,11,12,13,14] in various countries have shown the complexity of cooking fuel transition and questioned its feasibility in the diverse populations of developing countries. The potential economic and human health benefits of the transition to cleaner cooking technologies and fuels have motivated many researchers to focus on improved cook-stoves [15]. Previously, people used to construct chimneys to vent out the emissions from the kitchen. However, since the emissions were directly released into the atmosphere, this practice created outdoor pollution, and hence they have slowly moved towards the usage of clean cooking fuels [16].

In recent years, improvements in cook-stove designs of gaseous and liquid fuels have been studied both experimentally and numerically, as evident from the vast variety of literature. The reviews by Malico and Mujeebu [17], Ismail et al. [18], Khan et al. [19], and Datta et al. [20] cover most of the literature. Malico and Mujeebu [17] focused on cook-stoves with heat recirculating burners and the influence of burner size and materials on their performance. Ismail et al. [18] highlighted the scope of further research in the development of a numerical model on cook-stoves with heat recirculating burners. Khan et al. [19] described the performances of some of the LPG-operated cook-stoves and their variations with design modifications. Datta et al. [20] provided a detailed review of natural gas and LPG-based domestic cook-stoves operating on flame impingement heat transfer. These review articles provide information on the performance of some cook-stoves only from the perspective of combustion scientists and failed to gather information on the user’s point of view. This lack of understanding from the user’s point of view is one of the biggest hurdles towards the design of cleaner cook-stoves. For further improvement in the efficiency of cook-stoves and advancement in the technology for a better future, there is a need for an updated compilation of data available on the design and performance parameters of cook-stoves developed across the globe. Hence, in this review paper, the abovementioned shortcomings are addressed by providing a comprehensive review on the evolution, development, applications, and prospects of various gaseous and liquid fuel-based cook-stove technologies and their impact on society. Moreover, the performance of cook-stoves evaluated both in the laboratory and the field is documented for a better understanding of the readers. In addition, innovative technologies adopted to develop improved cook-stoves are tracked and their current technology readiness is presented, which helps in deciding the path for future research.

2. Operating Principles of Gaseous Fuel-Based Cook-Stoves

Based on the stage of fuel and air mixing, burners of gaseous fuel cook-stoves are mainly classified as aerated and non-aerated; the fuel and air will be premixed in the former, while they interact only at the reaction zone in the latter. In most aerated burners used in domestic cook-stoves, additional air is supplied to ensure cleaner combustion. These burners have been developed to overcome the shortcomings of non-aerated burners, such as unstable combustion and high manufacturing costs [21]. The cook-stove burners can also be classified according to the mode of air supply as forced-draft (air is supplied externally) and natural draft or self-aspirated (air is induced naturally). The phenomenon of the natural induction of air in a typical self-aspirated burner is shown in Figure 1.

The fuel is supplied through an orifice and is ejected at a high velocity. The surrounding air is entrained due to the pressure difference created by the high velocity of the jet. The entrained air and fuel then form a mixture and are passed through the mixing tube. The mixture is ignited downstream. Mainly two types of combustion methods have been reported in the literature: the bunsen burner principle and the heat recirculation principle.

2.1. Cook-Stoves with Bunsen Burner Principle

In Bunsen burner-based cook-stoves, the fuel–air mixture is delivered through radial ports on the burner head. They operate on the mechanism of Free Flame Combustion (FFC), in which the occurrence of combustion of fuel–air mixture is above the top surface of the burner head, and the burnt products depart through the downstream section of the flame (Figure 2).

The location of flame is controlled by restricting the reaction zone within a desirable limit by controlling the flame speed. The flame speed is in turn affected by several factors such as the composition, amount, and speed of the fuel–air mixture. The energy transfer is by direct impingement of the turbulent jets, i.e., the flame on pan surface. During combustion, some part of the total energy content of the fuel is lost by incomplete combustion, and the rest is released as heat energy in the product gas. This heat energy is transported to the cooking load mainly by convection; however, a considerable quantity of heat energy is lost to the surroundings by radiation and convection. As the FFC-based burners are characterized by the release of combustion products with low emissivity and thermal conductivity, the contributions of radiation and conduction from combustion products to the load are negligible [22]. Consequently, the FFC-based burners (including the commercial cook-stoves) are less fuel-efficient, and also possess unenviable characteristics such as inferior stable flammability limits, a high emission of toxic pollutants, and low power density [23].

2.2. Cook-Stoves with Heat Recirculation Principle

As discussed above, only a part of the heat energy from the hot gas is transported to the load in FFC-based cook-stoves. This can be solved only by the efficient combustion of fuel–air mixture and an enhanced transfer of heat energy to the load, which could be effectively accomplished by providing means to recirculate the heat energy from the hot combustion products to the incoming fuel–air mixture without their direct mixing. The process of transferring enthalpy from the combustion products to the incoming fuel–air mixture is often termed ‘excess enthalpy burning’. Furthermore, the combustion process in burners operating on the heat recirculation principle can sustain in much leaner conditions than the flame in conventional burners.

Higher burning velocities and flame temperatures are the characteristics of such flames, hence the name ‘superadiabatic’ combustion. Many researchers reviewed the heat-recirculation principle based burners and described their operation [24,25,26,27]. Cook-stoves with the heat recirculation principle can be classified into two groups: external heat recirculating burners and internal heat recirculating burners. The example of external heat recirculating burners is the Swiss roll burner [28], where the heat is transferred from hot product to cold reactant in a double spiral heat exchanger. Another example is the one which was developed by Jugjai and Sanitjai [29], where a Porous Medium (PM) was used to convert the enthalpy of the exhaust gases to thermal radiation which is then fed back to the burner inlet. On the other hand, internal heat recuperation is realized by facilitating the combustion in a PM. In the presence of a PM, the heat energy is transferred from the combustion zone (CZ) to the fresh reactants predominantly by conduction and radiation, leading to excess enthalpy or superadiabatic combustion [30]. Therefore, Porous Radiant Burners (PRBs) offer wider power modulation and flame stability range, lower emissions, and higher thermal efficiency than their FFC-based counterparts [31]. Figure 3 shows the heat transfer mechanism occurring in a double-layered PRB. The incoming fuel–air mixture is pre-heated by the heat recirculated from the CZ. The preheated mixture then undergoes combustion in the CZ. The preheating zone (PZ) with lower porosity also serves as a flame-arrestor [32]. The PM is also subjected to heat loss via radiation, convection, and conduction.

Based on the flame location, PRBs can be further classified as submerged-flame type or surface-flame type; in the former, the flame is located fully inside the PM, while the flame is stabilized at the PM surface in the latter. The performance of PRBs could be improved by the careful selection of the material and dimensions of the PM.

3. Liquid Fuel-Based Cook-Stoves Operating Principles

In liquid fuel burners, the fuel is converted into droplets either by vaporization or atomization before being injected into a combustion device. Based on the mechanism of fuel vaporization and mixing with air/oxidizer, liquid fuel-based cook-stoves are classified as wick type, and pre-vaporizing and vaporizing type. The working of a wick type cook-stove is the same as that of a candle (Figure 4). In a wick type cook-stove, a thin film of fuel is formed on the surface of the wick that is soaked with the fuel and is heated by the flame initiated at the surface of the wick. As the fuel film evaporates, a diffusion flame sustains at the surface of the wick. Liquid fuel flows up from the tank to the flame portion through the wick by capillary action. Kerosene, plant oils, and alcohol are the most commonly used liquid fuels in wick type cook-stoves.

In the case of a pre-vaporizing type cook-stove, fuel is pre-vaporized in a separate chamber or pipeline (tube or coil) before combustion (Figure 5). The vaporizer vaporizes the liquid fuel at a steady rate by providing the required heat energy for vaporization. Pre-vaporizing burners are generally employed in pressurized liquid fuel cook-stoves. This type of cook-stove is also available with porous burners as illustrated in Figure 6. In the case of vaporizing type cook-stoves, liquid fuel directly enters the combustion chamber, where it gets vaporized and combusted instantly. Alcohols are the standard fuel used in these types of cook-stoves. In these cook-stoves, the liquid fuel vaporizes on the top of the hot surface. This type of burner is used in canister-based cook-stoves as depicted in Figure 7.

4. Cook-Stove Designs

Since the advent of cook-stoves, numerous improvements have been reported on their design. The research works on design improvements mostly include parametric studies and investigations of the influence of operating parameters. To provide an insight into the chronology of developments, a review of the related literature (based on fuels) is presented in this section.

4.1. Gaseous Fuel Cook-Stove Designs

4.1.1. Liquefied Petroleum Gas (LPG)

The traditional LPG cook-stoves are designed based on partially-aerated type burner design, as described by Berry et al. [34]. The traditional free-flame cook-stoves are generally based on the Bunsen burner design. In these designs, there are several holes located on the burner head where the flame jets are anchored. Most of the burner heads are made of metals such as cast iron or brass. The details of the various improved cook-stove designs (Figure 8) are presented in Table 1. A venture center in Pune, India, developed a swirl flow-based LPG cook-stove for commercial cooking [35]. Zhen et al. [36] redesigned the traditional domestic cook-stove by modifying the burner cap to induce swirl flows (Figure 8b). The first design, Swirl Burner 1 consisted of 10 guided vanes that produced a continuous swirl. The radius and the inclination angles of the curvy channels were 3 mm and 10°, respectively. The second design, Swirl Burner 2 included both inward and swirling motion. The concave surfaces had an inclination angle of 30° towards the horizontal plane while the slots were at 10° off the normal direction of the surface. With the insertion of an annular metal insert and an extended spill-tray, Das et al. [37] improved the performance of traditional domestic cook-stoves. These improvements increased the heat transfer to the load by guiding the combustion products and air more towards the load.

As already mentioned, utilization of PM for improving the performance of traditional cook-stoves was demonstrated by Jugjai and Sanitjai [30] in their Porous Radiant Recirculated Burner (PRRB). PRRB is different from PRB, as it has an FF, and PM only promotes heat recirculation by recirculating the exhaust gas within the PM. Further, Jugjai and Rungsimuntuchart [38] developed a PRRB, i.e., PRRB (CB), using a ring burner. However, the swirling central flame ring burner (SB) and the PRRB were combined, i.e., the PRRB (SB), to further increase the burner’s thermal efficiency. Compared with their previous design [30], they scaled up the capacity of the burner from 5 kW to 30 kW. Mujeebu et al. [39] added another feather in the field of PRB cook-stoves by exploring the operation of double-layered PRBs on the surface and submerged combustion modes. These modes of PMC operation at the same power input were obtained by using different PMs. For a power input of 0.62 kW, a PRB with PZ and CZ of Alumina foams of different porosities was used to obtain surface stabilized flame, while PRBs formed by porcelain foam and alumina spheres were used to obtain a matrix stabilized flame. Another improved domestic PRB cook-stove based on the flat-flame operation was reported by Wu et al. [40]. The PRB was of diameter 50.8 mm and thickness 3 mm, and the PM was formed with bronze pellets of diameter 0.5 mm with a porosity of 0.237. Mishra [41] presented an improved cook-stove with a double-layer PRB for commercial cooking (5–10 kW). The PRB was of 120 mm diameter, and SiC and Alumina formed the CZ and PZ of PRB, respectively. Similarly, for domestic applications, Herrera et al. [42] developed 160 mm diameter PRB, where the preheater material was uniquely chosen from Alumina grinding wastes and the CZ comprised of SiSiC ceramic foam. Another milestone of PRB for domestic cooking was achieved by Mishra and Muthukumar, who developed a self-aspirated double layered PRB cook-stove operating at a power input range of 1–3 kW, marking the first of its kind [43]. The CZ and the PZ comprised of SiC reticulated foam and Alumina filter. These were enclosed in a housing made of refractory cement casing. This design was based on submerged PMC.

Figure 8. Improved LPG cook-stoves. (a) Improved cook-stove developed by Venture centre [35]; (b) Swirl burners developed by Zhen et al. [36]; (c) Porous metal burner developed by Wu et al. [40]; (d) Medium-scale PRB developed Mishra [41]; (e) Porous Radiant Burner developed by Herrera et al. [42]; (f) Domestic Porous Radiant Burner developed by Mishra and Muthukumar [43].

Figure 8. Improved LPG cook-stoves. (a) Improved cook-stove developed by Venture centre [35]; (b) Swirl burners developed by Zhen et al. [36]; (c) Porous metal burner developed by Wu et al. [40]; (d) Medium-scale PRB developed Mishra [41]; (e) Porous Radiant Burner developed by Herrera et al. [42]; (f) Domestic Porous Radiant Burner developed by Mishra and Muthukumar [43].

Table 1. Design features of improved LPG cook-stoves.

S. No.	Design Details	Fig.	Ref.
1.	Swirl Burner I: Ten guided vanes; Ten distinct swirling flame jets are created from the air/fuel mixture as it is steered through the wavy channels. The curved channel has a radius of 3 mm and a 60° inclination angle. Non-dimensional burner port spacing: 0.8. Swirl Burner II: The slots are constructed at an angle of 10° off the normal direction of the surface, and the concave surface of the burner cap has an inclined angle of 30° towards the horizontal plane. This causes a skewed channel, similar to that of the guided vane. Non-dimensional burner port spacing: 0.8.	Figure 8b	[36]
2.	Combustion rate: 1 kW, Porous medium: Plug (diameter 50.8 mm, thickness 3 mm) average diameter of the bronze pellet was 0.5 mm and had a porosity of 0.237	Figure 8c	[40]
3.	LPG composition: 60% butane and 40% propane LPG supply pressure: 1.5 bar Combustion rate: 5–15 kW Porous medium: PZ (Al2O3 matrix)—120 mm diameter thickness 10 mm with through holes (1.5 mm diameter 463 holes); CZ (SiC reticulated foam)—diameter 120 mm and thickness 25 mm and porosity 90%). Port: 4 ports with 21 mm diameter Slot’s length and width: 30 mm and 10 mm Orifice diameter: 0.25 mm Body Materials: Casing: castable cement	Figure 8d	[41]
4.	LPG composition: 60% propane and 40% butane LPG supply pressure: 0.023 bar Air pressure: Dry air at 0.7 bar Combustion rate: 98.5–244 kW/m2 Porous medium: PZ: Average equivalent diameter of 11 mm. 85 mm in height amorphous Al2O3 particles resulting from grinding wastes. CZ: SiSiC ceramic foam disks, 20 ppi Height 15 mm and diameter 16 cm Body Materials: Steel housing	Figure 8e	[42]
5.	LPG composition: 60% butane and 40% propane LPG supply pressure: 1.2 bar Combustion rate: 1–3 kW Porous medium: PZ: (Al2O3 matrix)–80 mm diameter and thickness 10 mm with through holes (1.5 mm diameter 204 holes); CZ: (SiC reticulated foam)—80 mm diameter, thickness 20 mm and porosity 90%. Port diameter: 21 mm Slot’s length and width: 30 mm and 10 mm Orifice diameter: 0.35 mm Body Materials: Casing: castable cement	Figure 8f	[43]

Table 1. Design features of improved LPG cook-stoves.

S. No.	Design Details	Fig.	Ref.
1.	Swirl Burner I: Ten guided vanes; Ten distinct swirling flame jets are created from the air/fuel mixture as it is steered through the wavy channels. The curved channel has a radius of 3 mm and a 60° inclination angle. Non-dimensional burner port spacing: 0.8. Swirl Burner II: The slots are constructed at an angle of 10° off the normal direction of the surface, and the concave surface of the burner cap has an inclined angle of 30° towards the horizontal plane. This causes a skewed channel, similar to that of the guided vane. Non-dimensional burner port spacing: 0.8.	Figure 8b	[36]
2.	Combustion rate: 1 kW, Porous medium: Plug (diameter 50.8 mm, thickness 3 mm) average diameter of the bronze pellet was 0.5 mm and had a porosity of 0.237	Figure 8c	[40]
3.	LPG composition: 60% butane and 40% propane LPG supply pressure: 1.5 bar Combustion rate: 5–15 kW Porous medium: PZ (Al2O3 matrix)—120 mm diameter thickness 10 mm with through holes (1.5 mm diameter 463 holes); CZ (SiC reticulated foam)—diameter 120 mm and thickness 25 mm and porosity 90%). Port: 4 ports with 21 mm diameter Slot’s length and width: 30 mm and 10 mm Orifice diameter: 0.25 mm Body Materials: Casing: castable cement	Figure 8d	[41]
4.	LPG composition: 60% propane and 40% butane LPG supply pressure: 0.023 bar Air pressure: Dry air at 0.7 bar Combustion rate: 98.5–244 kW/m2 Porous medium: PZ: Average equivalent diameter of 11 mm. 85 mm in height amorphous Al2O3 particles resulting from grinding wastes. CZ: SiSiC ceramic foam disks, 20 ppi Height 15 mm and diameter 16 cm Body Materials: Steel housing	Figure 8e	[42]
5.	LPG composition: 60% butane and 40% propane LPG supply pressure: 1.2 bar Combustion rate: 1–3 kW Porous medium: PZ: (Al2O3 matrix)–80 mm diameter and thickness 10 mm with through holes (1.5 mm diameter 204 holes); CZ: (SiC reticulated foam)—80 mm diameter, thickness 20 mm and porosity 90%. Port diameter: 21 mm Slot’s length and width: 30 mm and 10 mm Orifice diameter: 0.35 mm Body Materials: Casing: castable cement	Figure 8f	[43]

4.1.2. Biogas

Biogas-based cook-stove technology has been widely implemented in several countries such as India [44], China [45], Bangladesh [46], and Pakistan [47] with the subsidization of biogas plants. These cook-stoves are similar to the traditional LPG-based cook-stoves working on FFC [48] or PMC [49]. A biogas cook-stove usually has a single or double burner with different biogas consumption rates (Domestic burners: 1.2 to 5.5 kW; Commercial burners: 5.5 to 17 kW) [50]. The burner itself has several parts viz., Jet (Injector orifice), air intake holes, mixing tube (diffuser), flame port, baffle, burner manifold, frame, and the brackets welded on the top of it. There are two types of FFC-based biogas cook-stove designs: one is a modified design of the existing LPG cook-stoves and the other is the original design based on the properties of biogas [51]. In the early 1990s, Chandra et al. [52] presented analytical expressions for designing a burner in a biogas cook-stove. With the help of these expressions, for a given biogas pressure and pressure drop across the orifice, the design parameters of a biogas burner such as the orifice exit diameter, the spacing between the mixing tube and the orifice, the area of the burner head, and the number of burner port could easily be selected. In 1996, Fulford [50] presented a detailed design equation for a biogas cook-stove consistent with the scientific and technical criteria for low-pressure burners. They also presented a typical design calculation for a DCS cook-stove to supply about 1.5 kW for cooking. Using similar design principles, Kurchania et al. [53,54] developed a biogas cook-stove for cooking and baking corresponding to biogas consumption of 1 m³/h and 0.375 m³/h, respectively. Analogous designs of domestic cook-stoves can be found in the literature [48,55,56,57,58,59,60]. Recently, by using PMC, Kaushik et al. [49] developed a domestic double layer PRB cook-stove for biogas following the design of Mishra et al. [41] meant for LPG. The various biogas cook-stove designs are illustrated in Figure 9, and their basic design specifications are listed in

Table 2. Design features of biogas cook-stoves.

S. No.	Design Details	Fig.	Ref.
1.	KVIC model	Figure 9a	[55]
2.	Biogas supply pressure: 0.01 bar Biogas flow rate: 0.471 m3/h Injector size: 2.1 mm Port hole: 20 holes of 5 mm diameter (total burner port area = 390 mm2) Materials: cast iron, aluminium, mild steel, ceramics	Figure 9b	[50]
3.	Orifice diameter: (do) 16 mm Diameter of mixing pipe: (d) 97 mm Length of air intake hole (Max., Min.): (Lmax, Lmin) 679, 131 mm Diameter of mixing chamber: (D) 126 mm Length of mixing chamber: (L) 146 mm Number of ports: (n) 35 Diameter of flame port: (dH) 2.5 mm	Figure 9c	[56]
4.	Length of gas and air mixing tube: 960 mm Inner diameter of tube: 19 mm Number of air holes for primary aeration: 16 Diameter of air holes for primary aeration: 18 mm Number of perforated tubes for burner: 2 Number of ports: 56 Diameter of each port: 2.5 mm Distance between pan and burner: 35 mm Orifice diameter: 3 mm	Figure 9d	[53]
5.	Length of the gas and air mixing tube: 850 mm Inner diameter of tube: 14 mm Number of air holes for primary aeration: 4 Diameter of air holes for primary aeration: 4 mm Number of ports: 49 Diameter of each port: 2 mm Distance between pan and burner: 35 mm Orifice diameter: 2 mm Crown diameter: 80 mm	Figure 9e	[54]
6.	Material: Sheet metal (2 mm thickness) Jet diameter: 2.3 mm Primary air inlet: No of holes—2, Area of holes—96 mm2 Gas air mixing tube: Length 158 mm, Throat dia. 15.63 mm Port: No. of port—180, Dia. of port—2 mm Manifold: Cylindrical shape, 260 mm external dia.	Figure 9f	[58]
7.	Estimated gas flow rate: 5.69 m3/h Jet diameter: 11 mm Diameter of mixing pipe: 66 mm Maximum length of air intake hole: 462 mm Minimum length of air intake hole: 89.1 mm Diameter of mixing chamber: 86 mm Length of mixing chamber: 99 mm Number of holes: 24 Diameter of flame port hole: 2.5 mm	Figure 9g	[60]
8.	Injector diameter: 2.5 mm Internal throat diameter: 18 mm Number of burner ports: 71 Mixing tube length: 170 mm Internal manifold diameter: 100 mm Flame port diameter: 2.5 mm Air inlet diameter: 5.0 mm	Figure 9h	[48]
9.	Porous medium: PZ (Al2O3 ceramic)—90 mm diameter and thickness 20 mm and porosity 31%; CZ (SiC foam)—porosity 90%, diameter 90 mm and thickness 20 mm. Body Materials: Casing: high-temperature–resistant castable cement Biogas supply pressure: 1.2 bar	Figure 9i	[49]

.

4.2. Liquid Fuel Cook-Stove Designs

Liquid fuels used for cooking applications can be categorized based on their sources as fossil fuels (e.g., kerosene), alcohols (e.g., ethanol, methanol), and plant oil (Figure 10). Liquid fuels can be easily stored, and most of them, being derived from plant-based sources, are renewable. Similar to gaseous fuel cook-stoves, the design features of liquid fuel cook-stoves are influenced by the burner parts that enable the smooth flow and combustion of the fuel. The most important design improvements involve the selection of the wick materials in the case of wick type cook-stoves and the design of the vaporizer in the case of pressurized cook-stoves.

4.2.1. Kerosene

There are currently two kinds of kerosene-fueled cook-stoves: wick stove [61], which relies on the capillary action for the transfer of fuel, and a pressure cook-stove [61,64] with the vapor-jet arrangement, which aerosolizes the fuel using manual pumping. Different designs of both types of cook-stoves and the burner head assembly of pressure cook-stoves are shown in Figure 11. In terms of safety, the pressure stove is ranked higher than the wick stove.

4.2.2. Alcohol

The production of alcohol shows an increasing trend around the world, and attempts have been made from the mid-2000s to commercialize alcohol-based cook-stoves (Figure 11). The use of ethanol and methanol as cooking fuels was first seen in Ethiopia through the ‘Project Gaia’ initiative [67,68]. The cook-stove under this project was a canister-based methanol-/ethanol-operated Origo stove [69,70] manufactured by a Swedish company. These stoves carry a 1.2 L canister and weigh around 1.8 kg. One fill of the canister in this cook-stove can operate for about 6–8 h. In another study, Nimbkar Agricultural Research Institute (NARI) developed an ethanol-pressurized cook-stove working on a 50% ethanol-water mixture [65]. The stove produces outputs comparable to that of the traditional LPG and kerosene stoves and provides for simple flame regulation. Another ethanol stove operating without a pressure system is the “VOAHAJA” stove. The burner is designed to convert the water in the ethanol into steam, and the resulting flame is blue and odorless [71]. In a different study, two prototype stoves operating on methanol and ethanol gel were reported by Masekameni et al. [66], but their details are not available. The details of the alcohol-based cook-stoves discussed above are summarized in

Table 3. Summary of the alcohol-based cook-stoves.

S. No.	Design Details	Fig.	Ref.
1.	Design stove capacity: 0.9 to 2.45 kW Fuel tank capacity: 2.6 L Fuel tank operating pressure: 0.5–1.5 bar	Figure 10a	[65]
2.	Fuel: Ethanol gel	Figure 10b	[66]
3.	Fuel: Methanol	Figure 10c	[66]

.

4.2.3. Plant Oils

At Hohenheim University, Elamar [72] created the first cook-stove based on plant oil. The power output range and efficiency of the developed plant oil stove were comparable to the existing kerosene pressure stoves. A plant oil stove called “Protos” was designed by Bosch and Siemens Home Appliances Group (BSH) with a power rating of 1.6 to 3.8 kW [73,74]. Natarajan et al. [75] modified the spray nozzle exit angle of a kerosene pressure cook-stove to operate with vegetable oil. The modified stove could operate at the pressure of 1.6–1.8 bar with a fuel tank capacity of 3 L. By winding a copper coil around the vaporizer, Murthy et al. [76] modified a horizontal type kerosene pressure cook-stove to burn cottonseed oil blended with kerosene. The modification of the stove could improve the thermal efficiency, and the blend ratio of cottonseed oil could be increased to 70% with higher thermal efficiency. Similarly, Pande et al. [77] modified the existing kerosene pressure cook-stove capable of operating with cottonseed oil blended with kerosene. Suhartono et al. [78,79] developed a vegetable oil-operated pressure cook-stove employed with a spiral coil pipeline. The main objective of their study was to highlight the combustion performance and economic viability of the developed stove. Kakati et al. [80] used PM (made of clay, Al₂O₃ powder, and sodium silicate) as an insert in the CZ of a kerosene pressure cook-stove to burn plant oil. Lapirattanakun and Charoensuk [81] developed a cook-stove using PM made of spherical ceramic balls of 2 cm diameter, which acted as a flame stabilizer; its performance was improved by varying the burner height to diameter ratio. Kaushik and Muthukumar [82] developed a novel PRB-assisted kerosene pressure cook-stove operating between 1.5–3 kW of power input to burn waste cooking oil. The PRB consisted of four tubes, two ascending and two descending, attached to a spherical tube (vaporizer) on top, with SiC as CZ placed inside a casing. The details of the aforementioned plant oil-based cook-stoves are consolidated in

Table 4. Summary of the plant oil-based cook-stoves.

S. No.	Design Details	Fig.	Ref.
1.	Power range: 1.6–3.8 kW Fuel: Diverse plant oils. Plant oil esters Fuel usage: 2 L oil per week for a family of 4–5 member Efficiency: 40–50%	Figure 12a	[73]
2.	Tank pressure: 1.6–1.8 bar Burner: Commercially available burner (Copper plate brazed to the fuel injection and vaporization pipe) Nozzle: Spray nozzle Fuel tank capacity: 3 L Fuels used: Kerosene and vegetable oils	Figure 12b	[75]
3.	Fuel: Cottonseed oil blend with kerosene Operating pressure: 2 bar Fuel tank capacity: 2 L Stove modification: Fuel tank placed 100 mm above from the base and a capillary phenomenon is introduced before the fuel entering in the nozzle	Figure 12c	[81]
4.	Fuel: Waste vegetable oil Pressure: 2 bar	Figure 12d	[82]

. The undesirable physical properties of oil viz., the density, high auto-ignition temperature, viscosity, and flash point have been the main focus of research on plant oil-based cook-stoves. Numerous studies on modified kerosene pressure cook-stoves that burn plant oil focused on raising the temperature of the entering oil to decrease viscosity and ignition time, which ultimately speed up the combustion of the plant oil. Images of various plant oil-based cook-stoves are shown in Figure 12.

5. Performances of Gaseous and Liquid Fuel Cook-Stoves

Performance evaluation of a cook-stove is not only an indispensable part of validating the proposed design improvements but also an effective process to recognize the superiority of a cook-stove under a certain operating condition. There are three testing methods commonly employed to test cook-stove performance: Water Boiling Test (WBT), Controlled Cooking Test (CCT), and Kitchen Performance Test (KPT). WBT is a laboratory-level test and is used to assess the technical performance in a controlled setting. Since WBT simulates cooking by boiling water, it does not represent the actual cooking conditions. The Controlled Cooking Test can be applied both as a laboratory test and as a field test. It evaluates the performance of a cook-stove using a standardized cooking menu predefined by consulting selected local users. The time and fuel savings for cooking the menu are recorded during the test. The shortcomings of WBT and CCT can be overcome by KPT, which is conducted in real cooking settings. The ensuing subsections review the performances of various gaseous and liquid-fuel cook-stoves under laboratory and field test conditions.

5.1. Performances of Gaseous and Liquid Fuel Cook-Stoves in Laboratory

5.1.1. Performances of Gaseous Fuel Cook-Stoves

LPG-Based Conventional Cook-Stoves

The traditional LPG cook-stoves available in the commercial market are of the radial type. As mentioned previously, the performance in terms of thermal efficiency and emissions mostly depends on two factors (i) the heat transfer characteristics, and (ii) the residence time of the species. These factors are in turn influenced by parameters such as Reynold’s number, the shape and size of the burner port, the burner port-to-plate distance, the spacing between two adjacent burner ports (S/d), and the surface properties of the vessel (impingement plate) [83]. These design features are common to most of the traditional radial cook-stoves; however, to meet the growing demand for energy conservation and pollutant reduction, improved design features are required. One such feature is the introduction of swirl, which is created by inclining the burner port at a certain angle with the axes of the burner head. Swirl motion enhances the heat transfer and increases the residence time by inducing a rotational motion to the flame. Another common design improvement is the introduction of a flame shield to suppress the dispersion of high-temperature flames and flue gases.

Some of the earliest investigations can be found in the works of Jugjai et al. [84], who developed an LPG-operated stove with a modified pan support and swirl flame. Within the fuel flow range of 0.8 to 1.0 lpm, the introduction of swirl flame increased the thermal efficiency by 15% with respect to conventional radial flow burners and the replacement of the pan support with a lighter one further increased the thermal efficiency by 3%. A study [85] on the performance of the available cook-stoves in Nepal reported an average thermal efficiency of 53.15%. Li et al. [86] studied the influence of the design parameters on the thermal efficiency and emissions from domestic LPG cook-stoves. The thermal efficiency and CO emissions were measured following the Chinese National Standard on domestic gas appliances (GB 16410). The increase in Reynold’s number was reported to increase the flame length, which led to incomplete combustion at the stagnation zone (plate surface) with an eventual rise in emissions and reduction in the thermal efficiency. Similar observations were made in the case of increasing equivalence ratio. However, the increase in Reynold’s number beyond a certain limit also increased the turbulence, which decreased the CO emission. On the other hand, the decrease in the burner port-to-plate distance led to similar results. However, beyond a certain limit, the location of the highest temperature point shifted farther away with an increase in the burner port-to-plate distance, thus reducing thermal efficiency. The CO emission decreased with the burner port-to-plate distance due to the increase in air entrainment. The S/d ratio influenced the thermal efficiency and emissions in two ways. Shorter S/d created an interference between the flame jets, thus inhibiting complete combustion and reducing thermal efficiency. However, with a further increase in S/d, the CO emission reduced due to increased air entrainment, but decreases temperature.

Hou et al. [87] found that the implementation of a shield along with a swirl flow increased the thermal efficiency of a domestic (2.82–4.41 kW) LPG cook-stove by about 12%. Basu et al. [88] investigated different burner head designs and fuel orifices for improving the efficiency of traditional domestic LPG cook-stoves. The burner cap was modified by changing the port diameter and inclination angle. The first three designs had straight ports of diameters 1.1, 1.8, and 2.2 mm and the fourth design had a port diameter of 1.8 mm with an inclination angle of 38°. The thermal efficiency was observed to vary with port diameter and nozzle size: the combination of port diameter 2.2 mm and smaller nozzle with gas flow rate of 1.534 m³/s yielded the maximum thermal efficiency of 61.2%, with reduced emissions of CO, unburnt hydrocarbon, NO_x (NO and NO₂), and soot. In another advancement, Hou and Chou [89] investigated the effects of swirl, loading height, and semi-confinement on the cook-stoves’ thermal efficiency and CO emission at a power input of 4.7 kW. Thermal efficiency and CO emissions were evaluated as per the protocol prescribed by Chinese National Standard (CNS) General no. 13605. The increase in the swirl angle from 0 to 56° increased the thermal efficiency. The addition of a shield was shown to increase thermal efficiency and reduce emissions. The effect of loading height with the inclination angle of the ports was also investigated. For a loading height of 20–30 mm, the highest thermal efficiency was reported at a loading angle of 15°, and beyond a loading height of 40 mm, the thermal efficiency was found to increase with the inclination angle. Too small loading height leads to sudden quenching of the post flame gases and eventual increase in the CO emissions while too high loading height leads to loss of the available heat. In another experimental investigation [90], the influences of the loading height and port-to-port spacing on the thermal efficiency and emissions were discussed. The increase in loading height was reported to decrease the thermal efficiency and CO emissions. The increase in the loading height increased the secondary air entrainment and the residence time, resulting in the decrease in CO emissions. However, the decrease in the flame jet temperature with the increase in loading height led to a decrease in thermal efficiency. On the other hand, the port spacing had an insignificant effect on the thermal efficiency and emissions and a particular trend of the dependencies of thermal efficiency and CO emissions on the port spacing could not be established. Zhen et al. [36] modified the burner cap of the traditional domestic LPG cook-stove to induce swirl motion. Two design modifications were prescribed; one that produced continuous swirl motion (Swirl Burner I) and the other that produced swirling and an inward motion (Swirl Burner II). The thermal efficiency was measured according to the National Standard of the People’s Republic of China, GB 16410–2007. At stoichiometric conditions, Swirl Burner I was reported to produce the highest thermal efficiency when compared with Swirl Burner II and the conventional benchmark burner. However, at an equivalence ratio of 1.5, both Swirl Burner II and the benchmark burner produced the highest thermal efficiency. At an equivalence ratio of 2.5, the Swirl Burner II was observed to produce the least CO emissions followed by Swirl Burner I and the benchmark burner. Hence, it can be inferred that incorporating inward flow along with swirl motion can widen the operating range while increasing the thermal efficiency and reducing CO emissions. A similar research was carried out by Samantray et al. [91] on the effects of flat, semi, and full swirl on thermal efficiency and emissions. At a maximum fuel flow rate of 0.55 m³/s, the highest thermal efficiency of 64.1% was obtained with a full swirl burner compared with semi swirl and flat flame burners. The CO and NO_x emissions of all the burners were observed to be below 400 and 100 ppm, respectively. Simultaneously, Agarwal et al. [92] studied the effects of loading weight, pan diameter, and loading height on the thermal efficiency and emissions of conventional LPG cook-stoves, where they followed IS 4246:2002 standard for the estimation of thermal efficiency. They observed that with an increase in the pot diameter, the thermal efficiency was found to increase due to the increase in the surface area for heat transfer, and the increase in the loading weight increased the thermal efficiency due to the increase in the absorbed heat. The thermal efficiency first increased and then decreased with loading height. A maximum thermal efficiency of 50% was obtained with a loading height of 24 mm. The emissions of CO₂, NO_x, and unburnt hydrocarbon were reported to increase with power input. In another study, Joon et al. [93] reported the emissions of harmful pollutants such as Particulate Matter (PM) 2.5 and CO from various burners commonly used in Jhajhar, India. One-way ANOVA analysis was used to calculate the emission level. The CO emission was the least from LPG cook-stoves (3.36 ppm), which was very low compared with the crop residue stoves (157 ppm). Similarly, the LPG stoves reported least PM2.5 emissions of 4.69 μg/m³ while dung cake stoves emitted a maximum PM2.5 emissions of 11,000 μg/m³.

LPG cook-stoves for commercial and large-scale applications require the operation of the burner at a higher power input rate and pressure compared with the domestic scale burners. One such commercial LPG cook-stove was developed under the Technology Refinement and Marketing Programme (TREMAP) by Venture Center (Entrepreneurship Development Center), NCL Innovation Park, Pune [35]. A cook-stove with a swirl flow offered 55% thermal efficiency, which is 20% higher than its traditional counterpart. Parametric studies on commercial LPG cook-stoves (KB-5 burner) available in Thailand were conducted by Aroonjarattham [94] by following the Thai Industrial Standard (TIS) 2312–2549. The influence of the inclination and number of outer and inner ports on thermal efficiency was investigated. The increase in the number of inner and outer ports increased the thermal efficiency by 5 and 7%, respectively. The maximum thermal efficiency obtained at an outer port angle was 54.8%. While the outer port angle increased the thermal efficiency by 2%, the effect of the inner port angle was insignificant.

Shen et al. [95] evaluated the performance of commonly available traditional domestic LPG cook-stoves in China, Uganda, Peru, and Cameroon by following the ISO IWA-11 (International Organization for Standardization, International Workshop Agreement) test protocol. The nominal thermal efficiency of the cook-stoves was reported to be 51 ± 6%. Emission factors were determined on useful energy delivered (MJd). Emission factors of Total Hydrocarbons (THC), NO_x, and CO were reported to be 130 ± 196, 46 ± 9, and 0.77 ± 0.55 mg/MJd, respectively. Most of the PM2.5 emission results were found to be below the detection limit of 0.11 mg/min. Fakinle et al. [96] tested the thermal efficiency and emissions from domestic LPG cook-stoves operating on FFC available in Nigeria. As per the Nigerian standards, the average thermal efficiency obtained was 66.2 ± 7.21% for power input of 1.5 ± 0.3 kW, and the toxicity potential of NO_x and SO₂ was found to be below unity while the same was more than unity for CO and Hydrocarbon (HC). Saad et al. [97] investigated the performance of double ring-type burners with different patterns of swirling flow. The highest thermal efficiency of 57% was obtained from the counter-swirl type burner, while the least thermal efficiency of 56.3% was obtained from the radial burner. International Standard EN-30 was followed to evaluate thermal efficiency and performance. However, the burners with swirl also produced higher CO emissions than the radial burners with no swirl. Another study was conducted by Sutar et al. [98] to find the suitability of different pot sizes for efficient cooking at different flame settings. The use of optimum pot size, low flame setting, and a smaller burner were found to reduce the annual consumption of LPG by one cylinder. Studies conducted by Lather [99] revealed that the existing domestic LPG cook-stove operates at a power input of 1.12 kW and the corresponding thermal efficiency is 60.7%. Moreover, investigations for thermal efficiency improvement were conducted by changing the loading height. A loading height of 5 mm was found to be the optimum that facilitated the yield of the maximum efficiency and lowest fuel consumption.

LPG-Based PRB Cook-Stoves

Meanwhile, developments in PRB for cook-stove applications had started taking place. The initial developments on the use of PRBs for cook-stoves are found in the research work of Jugjai and Sanitjai [29]. Using a PRRB design, they found that for a burner capacity of 4 kW, the temperature of primary air attained a maximum value of ~210 °C. They reported that PRRB showed a significant increase in burner efficiency compared with PRB because of the increased primary air temperature. They also reported CO emission increases with an increase in combustion rates in PRRB for a certain range, whereas these values were nearly constant in the case of PRB. In addition to that, the higher flame temperatures in PRRB increased NO_x emission with an increase in the combustion rate. Later, Jugjai and Rungsimuntuchart [38] proposed a PRRB (CB) cook-stove design capable of operating at a burner capacity range of 5–30 kW and reported that the proposed PRRB (CB) was 12% more efficient than a conventional burner (CB). They also found that the maximum thermal efficiency of PRRB (CB) was ~44%. Further, they modified the PRRB (CB) design by replacing the ring burner (CB) with the swirling central flame ring burner (SB), i.e., PRRB (SB) for further improvement in thermal efficiency. They reported that PRRB (SB) yielded a maximum thermal efficiency of ~60%. This significant improvement in the thermal efficiency of PRRB (SB) is because of the combined advantage of efficient heat recirculation and the swirling central flame. However, for the studied burner capacity range, CO and NO_x emissions were found to be higher for PRRB (SB), because of the smaller port area, which resulted in less primary air entrainment and led to incomplete combustion.

Later, major developments took place on PRB for domestic cooking; these developments comprised the exploration of various types of porous materials based on combustion and heat transfer performance, parametric studies, and studies on surface and submerged combustion in PM. Pantangi et al. [100] investigated the use of different types of PM for their improved thermal performance over a CB. The PRB of metal chips was found to yield the highest thermal efficiency of 73%, which was an improvement of 5% over the CB. In another study, Pantangi et al. [101] developed two-layered PRB cook-stove, and the thermal efficiency and emissions were studied for burners of different sizes. The power ranges studied were 0.89–1.82 kW, 0.85–1.3 kW, and 1.28–1.72 kW for PRB diameters of 60, 70, and 80 mm, respectively, and 1.11–1.8 kW for PRB diameters of 90 and 100 mm. The highest thermal efficiency reported was 68% for the 80 mm burner, against 65% for a comparable CB. The thermal efficiency was found to decrease with power input and equivalence ratio. The CO and NO_x emissions were found to be 25–350 mg/m³ and 12–25 mg/m³, respectively, lower than that of CB. Mujeebu et al. [39] studied LPG combustion in domestic scale PRB in surface and submerged combustion modes at 0.62 kW power input. Operating the PRB on the surface combustion mode, yielded a thermal efficiency of 71%, which was more than that obtained from the submerged combustion mode and CB. Both surface and submerged combustion modes of operation reduced NO_x emissions up to 76% compared with CB. The effect of PM porosity on the thermal performance of a domestic PRB was investigated by Muthukumar and Shyamkumar [102]. Maximum thermal efficiency was obtained with a CZ of 90% porosity. Furthermore, they reported that the maximum thermal efficiency yielded by PRB was 75%. Wu et al. [40] assessed the feasibility of a flat flame burner operated with LPG for domestic heating applications, in which a flame was stabilized on top of the metal PM composed of small bronze pellets forming a plug. They reported that in the flat flame burner, the flame temperature was high which resulted in higher heat transfer. Compared with a CB, the flat flame burner was found to have a higher ratio of maximum to minimum output power with a stable flame (turndown ratio). The turndown ratio, in the case of the flat flame burner, was 34 to 6.5, whereas it was only 6.2 to 3.17 for CB. The turndown ratio reduced with decreasing equivalence ratio for both the flat flame burner and CB. They claimed that, for the flat flame burner (equivalence ratio varying from 1 to 0.7), the measured thermal efficiency at an input power of 1 kW ranged from 41% to 56%, whereas for CB, it was only 38 to 49% (equivalence ratio varying from 1 to 0.9). Similarly, the NO_x and CO emissions were generally lesser than that of CB. However, the distance between the burner-exit and the pot/pan had only a nominal impact on efficiency and emissions of the flat flame burner. In another advancement, commercial cook-stoves developed by Mishra et al. [41] generated a maximum thermal efficiency of 56% at an equivalence ratio of 0.56 at a power input of 5 kW, which was higher by 13% compared with a CB. The thermal efficiency was observed to decrease with the equivalence ratio while the emissions of CO and NO_x were observed to increase with the equivalence ratio. Meanwhile, Herrera et al. [42] developed a PRB-based LPG burner for heat inputs of 98.5–244 kW/m² and tested the burner on two modes of heat transfer: radiation-convection mode and conduction mode. They observed that the radiation-convection mode did not increase the thermal efficiency, whereas the conduction mode test yielded thermal efficiency up to 14% higher than CB. CO emissions lower than 25 ppm were observed for critical heat rates of 154 kW/m², beyond which the CO emissions suddenly increased, due to quenching on the surface of the burner and moderate lift-off.

The self-aspirated cook-stove developed by Mishra and Muthukumar [43] showed a maximum thermal efficiency improvement of 15.5% and reductions in CO and NO_x emissions with respect to CB. As an extension to this study, Kaushik et al. [103] performed a techno-economic and cost-saving assessment, which showed an annual saving of US$ 27.06 with a payback period of 6 months. Control cooking tests performed on the domestic PRB cook-stove [41] revealed a total savings of 50 min of cooking time and 29% of fuel consumption. Another study on the lifecycle and techno-economic assessment was conducted by Kaushik and Muthukumar [104] on the self-aspirated medium-scale PRB cook-stove developed by Mishra et al. [41]. The life cycle energy efficiency was reported to be 38% at 5 kW, which amounted to an annual saving of US$ 563.39 compared with CB. The cumulative present worth was estimated to range between US$ 4684.28 and US$ 8102.1 for power inputs of 5–10 kW. Deb and Muthukumar [105] developed a Cluster Porous Radiant Burner (CPRB) as an improvement over the medium-scale PRB cook-stove developed by Mishra et al. [41]. The clustering of three single-PRBs (70 mm diameter) resulted in a thermal efficiency improvement of 13.8% and a reduction of CO emissions. Indian Standard (IS) 14612:1999 was followed for the testing of the cook-stoves. The design was further improved by increasing the diameter and an optimum individual PRB diameter of 80 mm yielded a 27% improvement in thermal efficiency over CB (namely, T-35, T-35, and M-22). The maximum reduction of CO and NO_x emissions up to 85% and 83%, respectively, were obtained with CPRB of 90 mm diameter.

In general, the studies reported so far portray that the thermal efficiency of LPG cook-stoves could be improved by appropriate design modifications to the FFC-based cook-stoves. The maximum thermal efficiency of domestic and commercial cook-stoves has been reported as 66.2% and 55%, respectively. These improvements were obtained by changing the loading height, port spacing, swirl angle, etc. However, with the introduction of PM, further improvements in thermal efficiency and reductions in emissions were obtained. The highest thermal efficiencies reported were 78% and 58% for PRB-assisted domestic and commercial cook-stoves, respectively. In addition, the newly developed PRBs were further improved by optimizing the PM materials, burner geometry, operating conditions, etc.

Biogas-Operated Cook-Stoves

Chandra et al. [106] tested the biogas cook-stoves of 14 different brands available in the Indian market. The efficiencies of these cook-stoves varied between 32 to 49% and 37 to 54% for the testing carried out under unsteady state and steady-state conditions, respectively. Further, they studied the effects of input pressure, pan size, and pan position over the burner. The results showed that for optimal utilization of the heat available from the cook-stove, optimization of all the three above mentioned parameters is very essential. Smith et al. [107] studied the domestic biogas cook-stoves (operating at ~1.4 kW) available in India and estimated the nominal combustion efficiency, heat transfer efficiency, and overall energy efficiency as 99.5%, 57.7%, and 57.4%, respectively. The Center for Energy Studies, Nepal, reported an efficiency of ~49.5% for biogas cook-stoves in Nepal, whereas it was ~45% for India [85]. Itodo et al. [56] tested the performance of cook-stoves by boiling water (1 L) and cooking 146.6 g of rice and 123.3 g of beans. They found that the cooking rate was 0.14 L/min, 5.13 g/min, and 2.55 g/min, respectively, for all three mentioned quantities and the biogas consumption rate was 0.69 m³/min, 2.87 m³/min, and 4.87 m³/min, respectively. The efficiency of the cook-stove for boiling water, cooking rice and cooking beans was 20%, 56%, and 53%, respectively. A report by the Netherlands Development Organization (SNV) pointed out the problems associated with the poor design of Asian and African biogas cook-stoves [108]. Using three different test standards viz. Chengdu Energy Environment International Cooperation (CEEIC), China, Department of Renewable Energy Sources (DRES), India, and Kiwa Gastec Certification (GASTEC), Apeldoorn, performance was assessed. Kurchania et al. [53] developed a community-scale biogas cook-stove that could yield a cooking efficiency of 44% for a fuel consumption of 1 m³/h. The cook-stove developed by Kurchania et al. [54] for a fuel consumption of 0.375 m³/h showed a thermal efficiency of ~60% and CO₂ emissions of ~150–180 ppm. A burner for an injera baking application with an operating power of 5.7 kW and a biogas consumption of 0.93 m³/h was developed by Kebede and Kiflu [109] and the measured thermal efficiency of the burner was only 25%. Obada et al. [110] designed a biogas-based burner for residential cooking. They reported the thermal efficiency recorded by WBT was 21% at a fuel consumption rate of 0.47 m³/h, while the thermal efficiency recorded by the cooking of rice was 60% at 2.87 m³/h. Tumwesige et al. [111] studied various biogas stoves available in the market of Sub-Saharan Africa and the average thermal efficiency of the tested stoves (KEJS, Reo, Tusk, Bremen, Ideal, Psem, Double, Psem L) ranged between 19.8 and 25.7% with input powers in the range of 5.1–13 kW. Syamsuri et al. [57] studied the impact of burner shape (regular and cyclone) on the thermal efficiency. Two cyclone-shaped burners were used for the experiments and were compared with the baseline regular burner. The cyclone-2-shaped burner produced a maximum thermal efficiency of 58.4% at 1.53 kW. Decker et al. [59] found a higher thermal efficiency of 56.8% (at 1.1 kW) with the optimized design (4 mm diameter circular ports) of the biogas stove; the CO and total HC emissions were 1.103 g/MJ and 0.071 g, respectively. Kaushik et al. [49] estimated the thermal efficiency of PRB based biogas stoves (using IS 8749:2002) to be in the range of 51–62% for a biogas flow rate of 177–530 L/h, and its maximum was observed at a 0.75 equivalence ratio and 0.177 m³/h flow rate of biogas. The CO and NO_x emissions were 29–80 ppm and less than 4 ppm, respectively. Petro et al. [48] achieved a thermal efficiency of 67% (obtained in simmering phase at a consumption rate of 8.18 g/min) which was higher than the efficiency of the locally available burners. The specific fuel consumption (for boiling 1 L of water) of the developed burner was 736 g/L compared with 920 g/L for CARMARTEC and 833 g/L for Simgas. Awulu et al. [60] measured the mean time for boiling water and cooking rice, yam, and beans and obtained 5 min, 34.33 min, 34.66 min, and 49.33 min with biogas consumptions of 0.474 m³, 3.254 m³, 3.285 m³, and 4.778 m³, respectively, and the corresponding efficiency values were 42%, 63%, 61%, and 30%, respectively.

From the above-discussed results, one can easily observe a considerable discrepancy between measured performances (see

Table 5. Summary of literature on the performance tests of biogas cook-stove carried out in the laboratory.

S. No.	Thermal and Emission Performances					Ref.
1.	Thermal efficiency: 55%					[51]
2.	Thermal efficiency: 32 to 49% (unsteady state), 37 to 54% (steady state)					[106]
3.	Thermal efficiency: 67% (Biogas consumption rate: 8.18 g/min)					[48]
4.	Activities performed	Efficiency (%)	Biogas Consumption rate (m3/min)			[56]
	Boiling water (1 L)	20%	0.69
	Cooking rice (146.6 g)	56%	4.87
	Cooking beans (123.3 g)	53%	4.87
5.	Country	Gas consumption (L/h), Thermal efficiency (%) and CO emission (ppm)	CEEIC	DRES	GASTEC	[108]
	Bangladesh	L/h	474.5	211	500
		%	57	64.5	52.1
		Ppm	>1180	5300	2800
	Cambodia	L/h	762	489	808
		%	55	48.1	45.6
		Ppm	>1180	2200	1700
	Ethiopia	L/h	252.5	537	633
		%	53	40.5	41.2
		Ppm	>1180	4350	4463
	India	L/h	597	400	261
		%	57	54.5	89.9
		Ppm	>1180	2840	85
	Lesotho	L/h	270.5	217	354
		%	41	45.1	45
		Ppm	28	4350	8
	Nepal	L/h	565.5	453	536
		%	55	42.1	42.2
		Ppm	>1180	4350	2140
	Rwanda	L/h	340	285	336
		%	60	53.8	54.6
		Ppm	>1180	2250	2200
	Vietnam	L/h	758	620	1039
		%	39	21.2	31.5
		Ppm	>1180	4350	1100
6.	Cooking efficiency: 43.96% (Biogas consumption rate: 1 m3/h)					[53]
7.	Thermal efficiency: ~60% (Biogas consumption rate: 375 L/h) Emissions: CO—~150–180 ppm					[54]
8.	Thermal efficiency: 56.8% Emissions: CO—1.103 g/MJ Total HC—0.071 g					[59]
9.	Thermal efficiency: 51–62% (Biogas flow rate: 177–530 L/h) Emissions: CO—29–80 ppm NOx—<4 ppm					[49]

). This implies that these cook-stoves are not properly tested, designed, and standardized.

5.1.2. Performances of Liquid Fuel Cook-Stoves

One of the earliest studies on the performance assessment of wick cook-stoves was reported by Zhang et al. [112]. CO emissions from the kerosene stoves was found to be higher than that of the LPG stoves. Thermal efficiencies of the alcohol fuel-based cook-stoves such as methanol-based CleanCook stove and the ethanol-based “NARI” and “VOAHAJA” stoves were reported as 56.3% at 0.7 kW [67], 44–46% at 0.9–2.45 kW [65], and 54.4–59% at 0.7–1.6 kW [71], respectively. MacCarty et al. [63] investigated the performance of a wick cook-stove using the 2003 University of California-Berkeley (UCB) revised Water Boiling Test (WBT) Version 3.0. Fuel use, carbon monoxide (CO), and particulate matter were found to be 0.223 g, 8 g, and 0.01 g, respectively. It took 2550 s to boil 5 L of water. Makonese et al. [61], using the Heterogeneous cook-stove Testing Protocol (HTP), measured the performance of wick and pressurized kerosene cook-stoves. The pressurized stove had lower CO emission compared with the wick stove. Conversely, the wick stove consumed lower specific time to boil water with a relatively higher thermal efficiency. Similarly, Masekameni et al. [66], using HTP, measured average CO and PM2.5 emissions, as well as the CO/CO₂ ratio and thermal efficiency for the kerosene wick stove and two alcohol stoves. Of all the compared factors, methanol stoves stood out as a better choice. CO_EF (g/MJ), CO/CO₂ (%) and PM2.5 (g/MJ) were found as 1.1 ± 0.2, 1.7 ± 0.3, and 0.0003 ± 0.00001, respectively, for methanol stoves, whereas the corresponding values were 2.4 ± 0.3, 4.7 ± 0.4, and 0.0051 ± 0.00004 for kerosene wick stoves. The thermal efficiencies of the studied stoves were 67 ± 1.2% at 0.76 ± 0.04 kW for methanol, 73 ± 0.8% at 1.20 ± 0.09 kW for ethanol, and 73 ± 1.2% at 1.17 ± 0.02 kW for kerosene.

With the arrival of PMC technology, the research and development of liquid fuel-based cook-stoves started brewing. Liquid fuel combustion in PRB is complicated because of the need for the complete vaporization of the fuel before combustion. The use of a suitable vaporization technique and an optimum vaporizer arrangement in the burner are the two essential criteria for proper and stable combustion of the liquid fuel in the burner. Studies on conventional pressurized kerosene burners based on PMC technology are also scarce. The first attempt was reported by Kakati et al. [80], from the Indian Institute of Technology Guwahati, who examined the performance of a domestic burner with porous inserts of pottery clay, sodium silicate, and sawdust in terms of thermal efficiency, kerosene consumption rate, and emissions. Later this work was extended by Sharma et al. [113] by using different PMs, namely, wire mesh rolls filled with metal balls, alumina, zirconia (ZrO₂), and silicon carbide (SiC). They reported that at an optimum fuel flow rate of 130–140 g/h and a vessel size of 260 mm diameter, the thermal efficiency increased for all PM. The burner with SiC was found to have a maximum of 7% increase in thermal efficiency. The efficiency was further increased by insulating the heat shield ring. In their subsequent study [114], the stove was further modified by incorporating a ceramic (alumina) heat shield, which could improve thermal efficiency by 15%. Sharma et al. [115] experimented with three different burner casings, viz. straight cylindrical, tapered, and conical in a cook-stove consisting of a two-layer PM of alumina balls and SiC honeycomb structure. They found that at a fuel flow rate of 220 g/h, the burner with a conical casing showed the highest improvement of 10% in thermal efficiency. They also found that 20 mm was the optimum thickness of the CZ to ensure lower emissions and higher thermal efficiency compared with the conventional cook-stoves. They recommended that the thermal efficiency could be improved further by modifying the vaporization technology and burner assembly. They also investigated how the burner diameter affected thermal efficiency, emissions, and temperature distribution at various air and fuel flow rates in the same burner [116]. They focused on the optimization of the PM configurations, and a combination of SiC (10 ppi, 20 mm thickness) and Al₂O₃ (2 layers with 7 mm diameter ball) was found to suffice the requirement in terms of the complete retention of the flame within the media. Out of the three diameters tested (60 mm, 70 mm, and 80 mm), the burner with 70 mm diameter performed the best, reaching a maximum thermal efficiency of 50% at 2 kW and low levels of CO and NO_x emissions (44 and 1.2 ppm, respectively). The above developed burners were operated with an external air supply, which makes these burners unsuited for domestic application. To overcome the aforementioned problem, with a new vaporizer design, Sinha and Muthukumar [117] developed a kerosene pressure type stove with self-aspirated PRB for the burner capacity of 1.5–3 kW, claiming around 15% improvement in thermal efficiency. However, a decline in efficiency (from 64% to 55.5%) was observed when burner capacity increased from 1.5 to 3 kW. They also reported that the optimum vessel diameter was 270 mm for the developed self-aspirated PRB. Their published patent contains information on the design and functionality of vaporizers [64,118]. Lapirattanakun and Charoensuk [81] developed a PRB using ceramic balls for the combustion of Waste Cooking Oil (WCO). They reported that within the firing rate range of 325–548 kW/m² with a water flow rate of 0.16 kg/min, the maximum thermal and combustion efficiencies were approximately 28% and 99.5%, respectively. Similarly, CO and NO_x emissions were ~171 and ~40 ppm, respectively (at 6% O₂). On the other hand, Kaushik and Muthukumar [82] found the maximum operational blending ratio (WCO/Kerosene) as 50% in PRB. With a 50:50 blend ratio, thermal efficiency of the porous and conventional burner was in the ranges of 45.3–37.8% and 36.2–28.6%, respectively. Such variation in operational limits was because of differences in design and the working principle of both the cook-stoves [81,82].

Natarajan et al. [75] conducted a Standard water-boiling test (SWBT; recommended by VITA, 1985) to determine the fuel consumption and thermal efficiency for the developed pressurized cook-stove. The maximum thermal efficiency reported was 47.67% and 34.78% at 2.7 kW for vegetable oil and kerosene, respectively. Murthy et al. [76] obtained a thermal efficiency of 47.66% and 45.8% with 20% cottonseed blend with kerosene in a normal kerosene stove and a modified kerosene stove, respectively. Kakati and Mahanta [119] reported the operation of a conventional pressure stove with a 20–30% blend of Jatropha oil with kerosene. The thermal efficiency was found to be in the range of 53–52%. Jambhulkar et al. [120] investigated the usage of blends of kerosene and spent soya bean cooking oil of various proportions in conventional kerosene pressurized stoves. They reported an efficiency of about 38% with an input energy of 3,91,710 kJ for 50% kerosene with 50% spent soya bean cooking oil blends. Suhartono et al. [78] investigated the performance of the vegetable oil pressure stove employed with a spiral coil pipeline. They reported that atomization of all the vegetable cooking oil produced fuel droplets with diameters of 0.155–0.650 mm with lower viscosity and lower ignition values and the efficiency of the cook-stove ranged between 20–32%. Wagutu et al. [121] investigated the performance of a wick stove with Kenyan FAME (Fatty Acid Methyl Ester) fuels and compared it with that of kerosene. They found maximum thermal efficiency of 47% at 1.19 kW with coconut FAME. Khan et al. [122] reported that in a wick stove, 5% of Used Frying Oil (UFO) blend with kerosene showed comparable results to that of pure kerosene. Further, they studied the Karanja oil blend with kerosene and found that a maximum of 10% Karanja oil blend with kerosene to be the best blending proportion [123]. Nagaraju and Gopal [124] reported difficulties in igniting a wick type stove operating with kerosene and Pongamia oil blends. Shetty et al. [125] thoroughly studied a wick stove fueled with different blends of kerosene and Pongamia oil. They found a peak efficiency of only 5.6% at 35 kg/h with 10–90% Pongamia oil and kerosene blends.

Thus, the liquid fuel cook-stoves are generally polluting in nature, and further research is needed to validate their combustion, emission, and thermal performances. The following remarks could be drawn from the review of laboratory tests on liquid fuel cook-stoves:

• Pressurized kerosene cook-stoves, either traditional or modified, show better performance than a wick stove when operated with either pure kerosene or its blends with plant oil;
• Proper combustion is challenging due to the high viscosity of plant oil;
• Only a few successful plant oil cook-stoves are available, but results related to their adoption and large-scale application are yet to be reported;
• PRB assisted cook-stoves for kerosene show improved performance compared with their conventional counterpart. Moreover, no attempt has been made to develop PRB assisted cook-stoves for alcohols;
• The potential risks associated with the flammability and toxicity of the alcohol fuel and low power rating of alcohol-based cook-stoves pose a major hindrance to accommodate various cooking practices in a household.

5.2. Performances of Gaseous and Liquid Fuel Cook-Stoves in Field

5.2.1. Performances of Gaseous Fuel Cook-Stove in Field

Kandpal et al. [55] conducted field tests KVIC model biogas cook-stoves in a kitchen of the size 2600 × 2800 × 2300 mm. They observed that using biogas for cooking application instead of wood and cow dung cake reduced the CO and suspended particulate matter and benzo[a]pyrene in the kitchen area. Saha et al. [126] tested the air quality in selected kitchens in the campus of a large institute in India to check whether CO₂, CO, and the temperature of the space fell within the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ (ASHRAE) standards (CO₂—1000 ppm, CO—9 ppm, and temperature—20–26.1 °C). They also developed a computational method for figuring out the temperature and hazardous gases (such CO₂ and CO) distribution in three dimensions (3D) within a working kitchen. Nandasena et al. [127] reported the particulate matter emissions from kitchens in Sri Lanka for various combinations of wood and LPG fuels. PM2.5 and PM10 emissions during cooking using 100% LPG cook-stoves were reported to be the least, the values of which were 53.6 μg/m³ and 69.5 μg/m³, respectively. These values were significantly lower than those reported from the 100% use of wood as a cooking fuel. A field study on the pollutant emissions of CO, CO₂, and PM2.5 from kitchens in southwest Nigeria was investigated by Giwa et al. [128]. Emissions were measured 20 min before the commencement of the cooking and 30 min after cooking and during the entire duration of various cooking procedures including boiling, stewing, and frying. The LPG kitchens were reported to emit 29 ppm, 895 ppm, and 0.000328 g/m³ of CO, CO₂, and PM2.5, respectively. Rupakheti et al. [129] presented the per capita fuel and energy use in rural Nepal with the help of field studies. As compared to wood, charcoal, agro-residues, and biogas, the fuel use of LPG was reported to be the least (60 g/day), which was translated into a per capita fuel and energy use of 12 g/day/person and 0.58 MJ/day/person, respectively. Another study was conducted by Johnson et al. [130] to determine the pollutant emissions from cooking in Asia and Africa. LPG-operated cook-stoves were found to emit PM2.5 within the prescribed interim limits of 0.105 g/h by World Health Organization (WHO). Weyant et al. [131] conducted similar studies on biogas and LPG-operated cook-stoves in Nepal. The emission factors from LPG cook-stoves for PM2.5 and elemental carbon were reported to be 0.0095 ± 0.0068 g/MJ and 0.0003 ± 0.0003 g/MJ, respectively. While these emissions were comparable to those emitted from biogas cook-stoves, significant reductions were obtained when compared with wood stoves. Simkovich et al. [132] presented a review on cross-sectional studies on the field performance of various cook-stoves and reported that the use of LPG cook-stoves saved a maximum of 1 h of cooking time when compared with biomass. The Kitchen Performance Test (KPT) is costly, time intensive, and error prone. To overcome the shortcomings of KPT, Ventrella et al. [133] used a sensor-based system to monitor the fuel consumption in Uganda and Burkina Faso. The KPT included weighing the fuel and recording moisture content while the Fuel Use Electronic Logger (FUEL) procedure required the systems to be hung from the roof and thermocouples to be attached to the cook-stoves. Comparisons were drawn between the performance obtained by the FUEL and KPT methods. While differences were reported on the performances of the FUEL on a daily level, both the methods were found to be comparable on an aggregate level. From the cost analysis, the breakeven point for the FUEL procedure was found to be 40 days. A comparative study of PM2.5 emissions from wood and LPG cooking was carried out by Johnston et al. [134] in Nepal. PM2.5 filter-based and real-time nephelometer data were collected for approximately 24 h in homes and outdoors. Results showed that the PM2.5 was significantly associated with fuel type and location and for both wood and LPG, PM2.5 levels exceeded the 24 h limit (0.000025 g/m³) proposed by the World Health Organization. Similarly, results reported by Sambandam et al. [135] corroborated these findings for kitchens of rural Tamil Nadu, India during 24 or 48 h observations. In biomass and LPG using homes, PM2.5 was found to be 0.000134 g/m³ and 0.000027 g/m³, respectively. Kephart et al. [136] studied the NO₂ exposures from LPG stoves in a cleaner-cooking intervention trial in the Peruvian Andes. From the results, they found that the kitchen area’s NO₂ concentrations exceeded the WHO indoor hourly guideline. However, the kitchen area’s NO₂ concentrations were substantially lower within the LPG households compared with that of biomass. Islam et al. [137] conducted emission measurements in two rural locations in northern and southern India for three months. They measured pollutants including fine PM (PM2.5), organic and elemental carbon (OC, EC), black carbon (BC), and carbon monoxide (CO). Pollutants emitted from LPG cook-stoves were reported to be 90% lower than biomass cook-stoves. However, emissions from LPG cook-stove in field tests were higher than those of laboratory ones.

5.2.2. Performances of Liquid Fuel Cook-Stove in Field

Pandit et al. [138] monitored indoor volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons from kerosene cook-stoves at Trombay, Mumbai. All the identified VOCs were found to be higher than the permissible limits. In another investigation, Andresen et al. [139] measured and compared 24-h gravimetric personal and indoor PM2.5 exposures due to kerosene and LPG cook-stoves in households of Mysore, India. For 24 h, the women carried an insulated bag with a personal pump; at the same time, precautions were made indoors in the room where the family spent most of their time. Kerosene users had nearly twice the amount of PM2.5 exposures than LPG users. A field study on the pollutant emissions of CO, CO₂, and PM2.5 from kitchens in southwest Nigeria was conducted by Giwa et al. [128]. Emissions were measured 20 min before the commencement of the cooking, 30 min after cooking and during the entire duration of various cooking procedures including boiling, stewing, and frying. The kerosene-stove kitchens were reported to contain 31 ppm, 897 ppm, and 0.000345 g/m³ of CO, CO₂, and PM2.5, respectively. Rajvanshi et al. [65] compared the field performance of NARI ethanol stoves with that of LPG and kerosene for a typical rural household. The tests were performed in a room of a size 4500 mm × 3100 mm and 3000 mm ceiling, which resembled a typical room size in a rural settlement. According to the results, more specific energy was needed for the ethanol stove than for the LPG stove and less for the kerosene stove.

Field tests of cook-stoves are costly and time consuming as they involve the participation of a large number of households over a long duration of time. Due to these factors, the authors believe that only limited field tests of cook-stoves have been reported. One method called FUEL [133] was suggested to counteract this issue. However, more studies are required to make it a regular method. According to the available literature, when compared to liquid or biomass fuels, LPG operated cook-stoves save time and emit lesser pollutants to the indoor kitchen environment. The few works of literature reported on gaseous and liquid fuel-based cook-stoves are on FFC-based cook-stoves. Field studies on PRB based gaseous and liquid fuel-based cook-stoves are yet to be conducted. As there are many factors that influence the savings in fuel and cooking time, an effective standard protocol on KPT is required to guide the researchers in performing these tests.

5.3. Numerical Modelling of Gaseous Cook-Stoves

5.3.1. The Modeling Approach

The fundamental phenomena taking place in the cook-stoves are the supply of pressurized or the natural induction of air and fuel, their mixing and subsequent combustion, and heat transfer. The governing equations used to describe these phenomena are the continuity equation (Equation (1)), momentum equation (Equation (2)), species equation (Equation (3)), and energy equation (Equation (4)). These equations are solved with the aid of computational fluid dynamics (CFD) tools.

$$\nabla ·\left({\rho }_g\overrightarrow{u}\right)=0$$

(1)

$$\nabla ·\left({\rho }_g\overrightarrow{u}\overrightarrow{u}\right)=-\nabla p+\nabla ·\left(\mu \nabla \overrightarrow{u}\right)$$

(2)

$$\nabla ·\left({\rho }_g\overrightarrow{u}{Y}_i\right)=-\nabla ·\left({\rho }_g{D}_m\right)\nabla Y_i+\dot{w_i}{W}_i$$

(3)

$$\nabla \left(c_g{\rho }_{g}T\overrightarrow{u}\right)=\nabla \left(\varphi c_g{\rho }_g{D}_t\right)\nabla T-{\displaystyle \sum }_i\dot{w_i}{h}_i{W}_i$$

(4)

where, $\overrightarrow{u}$, μ, and ${\rho }_g$ are the velocity vector, viscosity, and density of the gas, respectively.

In the case of cook-stoves based on PMC, since the patterns of fluid flow and heat transfer differ due to the presence of PM, the momentum and the energy equation are modified to account for the changes. In the momentum equation, the sink term S_i is added in the right-hand side of Equation (2) as shown below:

$$S_i=-\left(\frac{\mu }{K_1}\overrightarrow{u}+C_2\frac{1}{2}{\rho }_g\vee \overrightarrow{u}\vee \overrightarrow{u}\right)$$

(5)

The permeability of the porous media is represented by $K_1$ and the inertial resistance factor is represented by $C_2$ [140].

In the case of PMC, there exists thermal non-equilibrium between the solid and the gaseous phases in the porous domains. Therefore, separate energy equations are solved each for the gaseous phase (Equation (6)) and the solid phase (Equation (7)).

$$\varphi \nabla \left(c_g{\rho }_g{T}_g\overrightarrow{u}\right)=\varphi \nabla \left(k_g+c_g{\rho }_g{D}_t\right)\nabla T_g-\varphi {\displaystyle \sum }_i\dot{w_i}{h}_i{W}_i-h_v\left(T_g-T_s\right)$$

(6)

$$k_s\nabla \left(\nabla T_s\right)-h_v\left(T_s-T_g\right)=0$$

(7)

In Equations (3)–(7), T, W, $\dot{w}$, Y, h, and k represent the temperature, molecular mass, molar rate of production, mass fraction, molar enthalpy, and thermal conductivity, respectively. The subscripts ‘s’, ‘g’, ‘v’, and ‘i’, stand for solid, gas, volumetric, and species number, respectively. The terms $D_m$ and $D_t$ represent the mass diffusivity and thermal diffusivity, respectively. The volumetric heat transfer coefficient h_v is incorporated in Equations (6) and (7) to couple the gaseous and the solid phases. The boundary conditions applied vary as per the operating conditions and assumptions considered. The most common boundary conditions are adiabatic wall, pressure outlet and mass flow inlet boundary conditions for the fuel.

5.3.2. Numerical Studies on Cook-Stoves

Many researchers focused on the analysis of cook-stoves through mathematical modeling of the phenomena to understand their physics and to enable the optimization of cook-stove performance. To exemplify a few, Boggavarapu et al. [141] conducted 3D numerical simulations for design improvements on a traditional LPG cook-stove. The design modifications consisted of the use of an insert and a radiant sheet. The simulations were carried out on a simplified geometry comprising a 15° sector of the physical domain, and the results were validated with experiments. The modified design could improve the thermal efficiency by 2.5 to 5%. Dwivedi et al. [142] investigated the effect of the addition of a flame shield on the temperature and flame velocity of an LPG cook-stove with the help of a 3D model. They found that the modified cook-stove with a flame shield produced a higher temperature and flame velocity than a CB. By using a similar model, Dey et al. [143] studied the influence of mixing tube design parameters on the air entrainment in an LPG cook-stove. Circular shaped side ports were reported to entrain more air compared with the square or elliptical side ports. Turbulence closure was obtained by employing the k-ε turbulence model. The increase in the air pressure was found to increase both the fuel-flow rate and air-entrainment; however, the air-fuel ratio was unaffected. While the increase in the nozzle throat diameter increased the fuel flow rate, the air-fuel ratio was found to decrease. An extended study on the flow and combustion in the cook-stove was carried out by Das et al. [144]. The combustion of LPG was simplified to a three-step mechanism that included the formation of CO from the incomplete combustion of propane and butane and the dissociation of CO₂ to CO and O₂. The effects of fuel flow rate, equivalence ratio, pan diameter, and the distance of the pan from the burner on the thermal efficiency were investigated. Another numerical study on LPG combustion in a forced-draft CPRB was conducted by Deb et al. [145]. The flame movement in the burner was determined for stable range of equivalence ratios and power inputs. A two-step reaction mechanism was considered to simplify the combustion in the burner, and a thermal non-equilibrium model was chosen as the energy model after its superiority over the thermal equilibrium model was established.

It could be deduced that, even though the models presented so far could yield realistic predictions to a remarkable level, there is enough scope to establish an exact correlation between the thermal efficiency and emissions of common pollutants of interest such as unburnt hydrocarbon, CO, CO₂, NO_x, etc. The complexity in the combustion and heat transfer process poses a challenge in obtaining reliable results in terms of thermal efficiency, emissions, and burner stability for specific burner geometry and operating parameters. Therefore, future works shall focus on developing numerical models that help in optimizing the cook-stove design with minimal requirements of experimental prototyping.

6. Health Issues Related to Use of Gaseous and Liquid Fuel Cook-Stoves

When cook-stoves become an integral part of our daily life, their detrimental health implications should be a matter of serious concern. The World Health Organization (WHO) has set guidelines, listed in

Table 6. Guidelines on Household Air Quality, WHO [146].

Pollutants	Mean Concentration over Averaging Time					Comments
	15 min	1 h	8 h	24 h	1 year
CO (mg/m3)	100	35	10	7	-	-
NO2 (μg/m3)	-	200		-	40	-
PM2.5	-			25	10	24-h guideline max 3 days/year
PM10	-			50	20

, for the air quality in households [146].

A report published in 2011 [147,148] highlighted the impact of ethanol cook-stoves used in Madagascar on mortality and illness from conditions such as childhood pneumonia and chronic obstructive pulmonary disease (COPD). The ethanol stoves were found to be risk-free of all of these outcomes for the mothers and children. They also reported the impacts on Acute Lower Respiratory Infections (ALRI) in children and Chronic Obstructive Pulmonary Disease (COPD) and Ischemic Heart Disease (IHD) in adults. They found that ethanol as the primary household fuel could avoid the loss of 0.03 Disability-Adjusted Life Years (DALYs) per household per year compared with charcoal. Ohimain [149] investigated the impact of household air pollution (HAP) on placental growth markers. They found that HAP exposure from the use of kerosene during pregnancy may be deleterious for fetal development and growth. Adetona et al. [150] compared the impact of wood kerosene and LPG cook-stoves on creatinine-adjusted hydroxy-PAH (OH-PAH) concentrations in pregnant women in Trujillo, Peru. Women cooking with wood or kerosene cook-stoves had the highest creatinine adjusted OH-PAH concentrations compared with those using LPG cook-stoves. Epstein et al. [151] compared the impact of various Indian biomass, coal, and kerosene cook-stoves with LPG and biogas cook-stoves on low birth weight (LBW < 2500 g), and neonatal mortality (death within 28 days of birth). The results indicated that the risk of LBW and neonatal deaths were high in the case of highly polluting fuels. Dutta et al. [152] investigated the effects of HAP on chronic hypoxia in placenta. Two sets of pregnant women using firewood, kerosene, and bioethanol cook-stoves in Nigeria and natural gas cook-stove users in Chicago were monitored. Firewood and kerosene-based cook-stoves users were found to be more susceptible to chronic hypoxia. Alexander et al. [153] compared the impact of ethanol with kerosene and firewood cook-stove on pregnancy in Ibadan, Nigeria. They considered variables such as birthweight, preterm delivery, intrauterine growth restriction (IUGR), and occurrence of miscarriage or stillbirth. Ethanol cook-stoves reduced adverse pregnancy outcomes compared with biomass and kerosene cook-stoves. Steenland et al. [154] compared the health impacts of LPG cook-stoves with biomass cook-stoves. Around a 74% reduction of PM2.5 with LPG cook-stoves reduced the blood pressure in women above 50 years and incidences of childhood pneumonia and improved the lifespan of women. Bates et al. [155] investigated the risk factors of using different cooking fuels with the help of case studies in western Nepal. Findings revealed that women using LPG were more prone to pulmonary tuberculosis relative to the use of wood or biogas. Stapleton et al. [156] assessed pre- and post-bronchodilator lung function on 25 primary female cooks using LPG and biomass, and quantified exposures from 34 kitchens (PM2.5, PM < 40 μm, black carbon, endotoxin, and PM metal and bacterial content). The study reveals that fewer cooks of biomass cook-stoves had normal spirometry as compared with those using LPG cook-stoves. On the other hand, LPG kitchens had a higher concentration of bacteria, sulfur, and particulate matter compared with biomass kitchens. Imran and Ozcatalbas [157] estimated the impacts of cooking fuels on women’s health in rural Pakistan. Health expenditures of households using biomass were almost 25% higher than those using LPG. Arku et al. [158] performed an analysis on the adverse health impacts of cooking viz., cardiorespiratory effects and mortality due to kerosene cook-stoves used in China, India, South Africa, and Tanzania. Kerosene cook-stoves resulted in an increased risk of mortality and higher rates of respiratory failure.

There is plenty of literature on the various health issues due to indoor pollution caused by the use of FFC-based cook-stoves, and a detailed review is beyond the scope of this article. However, the cited literature shows that most of the cook-stoves were operated on fuels such as LPG, kerosene, alcohol-based, and plant oil. The cook-stoves with FFC-based burners are inherently harmful due to their incomplete combustion, and they show detrimental health implications on their users. Therefore, replacing kerosene with cleaner fuels is recommended for cooking, and LPG and alcohol can be better substitutes for biomass and kerosene. Another promising alternative is use of cook-stoves with heat recirculating burners owing to their better combustion and emission performances.

7. The Enablers and Barriers to the Adoption of Clean Cook-Stoves

Despite field tests reporting that an improved cook-stove is better in terms of health and financial savings, their large-scale adoption may still be hindered due to various socio-behavioral factors. Various researchers have studied the factors influencing cleaner cooking fuel and technology adoption. In this section, these studies are analyzed to identify the various enablers and barriers of improved cook-stove adoption. A global overview of the transition to cleaner cook-stove technologies is provided in

Table 7. Summary of literature on adoption of improved cook-stoves of gaseous and liquid fuels.

S. No.	Sample Size	Transition		Sample Location	Ref.
		To	From
1.	43 household	Traditional chulha cook-stove	LPG and improved biomass stoves	11 villages in Lag and Gadsa Valley, Himachal Pradesh, India	[159]
2.	200 households that had benefited from receipt of a free LPG kit project during the prior LPG promotion campaign	Electricity	LPG	Atteridgeville Township, South Africa	[160]
3.	458 households	Biofuels	LPG stove	Peru	[161]
4.	22 participants	Biomass stove	LPG stove	Peru	[162]
5.	10,168 households	Charcoal and firewood	Kerosene	Tanzania	[163]
6.	113 women	Biomass	BioLite and LPG	Ghana	[164]
7.	72 households	Firewood	LPG	Chiapas, Mexico	[165]
8.	23 married couples	Traditional cook-stove	Cleaner cook-stoves	West Pokot County in Northern Kenya	[166]
9.	4994 households	Traditional biomass	LPG	India	[167]

.

Wang and Bailis [159] studied the adoption of cleaner cook-stoves in Himachal Pradesh, India and found socio-economic factors (such as product price and household income) as the most important driver of cook-stove transition. A study by Kimemia and Annegarn [160] on the prospects and challenges of LPG intervention in South Africa suggested that higher success rates on making LPG an alternate cooking energy option could be obtained through better community engagement, long-term plans on distribution and maintenance, and recurrent LPG subsidies. Calzada and Sanz [161] evaluated the Peruvian Fondo de Inclusión Social Energético (FISE) program, which subsidizes the replacement of traditional stoves with LPG cook-stoves. A proper subsidy mechanism was found to be important to promote the diffusion of modern cooking technologies, and it should be according to the households’ income level. In another study in Peru, Williams et al. [162] analyzed the factors that influence the adoption of LPG when monetary costs associated with it were removed. They found that the social and cultural barriers to LPG adoption were significantly reduced and near exclusive adoption was possible. In a study [163] on the transition from biomass to kerosene in Tanzania, the availability of alternate fuels was found to pose a hindrance despite the changes in fuel prices. Agbokey et al. [164] performed a qualitative study and determined the enablers and barriers for the adoption of clean cook-stoves in Ghana. Some of the barriers identified were high fuel costs, unavailability of spare parts, and unfitting the stoves and kitchens, while the enablers were the low maintenance of stoves, a reduction in fuel usage, and the elimination of procuring firewood. A study was conducted by Troncoso et al. [165] on the transition to LPG cook-stoves from firewood in two rural communities in Mexico. The socio-economic status and the health awareness in women were shown to favor the easy transition from firewood to LPG. Ochieng et al. [166] studied the roles of women and men in decision-making on the adoption of cleaner cook-stoves. They found that the men are aware of the challenges associated with traditional cooking but are not interested in the transition to cleaner cooking fuels. Khanwilkar et al. [167] investigated the impact of Pradhan Mantri Ujjwala Yojana (PMUY) policy, which has provided poor households with LPG connections since 2016 in the Central Indian Highlands Landscape (CIHL). The results suggested that social status, education level, income, and proximity to the forest are the major factors that impact the transition towards LPG. However, the adoption and sustained use of heat recirculating burner-based cook-stoves have not been reported so far.

There is a coherence in the findings from various studies carried out on the adoption of improved cook-stoves in low and medium-income countries around the world. The reasons underpinning the lack of the widespread adoption from the reported literature can be summarized in the following points:

• Socio-economic status of the user
• Education level of the user
• Selection of target group in government policies
• The convenience of cook-stove and/or fuel, including purchase/gathering, transporting, and storing
• Safety of stove and/or fuel
• Availability of fuel
• The economy of cook-stove and/or fuel
• Quality of stove/fuel performance.

8. Suggestions for Future Research

From the various works on cleaner fuel and cook-stove technologies, it is clear that it requires a multidimensional approach. The following are some of the potential future works:

• Any modifications in cook-stove designs will have a large effect on the product’s performance and the user’s experience. However, to ensure commercial acceptability, new cook-stove designs must offer improved performance without compromising manufacturability, usability, strength, and durability. Factors such as the output power requirement for different cooking applications, affordability, fuel availability, and operational safety also need to be considered;
• Considering the proven record of improved performance, PRBs need extended field study to verify their durability, operational stability, and commercial viability, with different types of fuels;
• The reports on the performance of liquid fuels are limited. There is a need for a more thorough investigation on used cooking oil and other plant oils for rural reach as kerosene is being phased out of the market;
• Extensive field tests are needed to validate the performance of cook stoves with different types of fuels by following a standard protocol;
• Extended research is required on numerical modeling to ensure optimal burner design based on real operating parameters;
• The consumer must be given priority when making fuel/technology decisions. Understanding the tradeoffs between every technology or fuel, as well as the various manufacturing alternatives, could be made easier with the support of a consumer-centric approach. The market studies regarding consumer behavior and related studies would fetch a good framework for the policymaking related to future fuel/cooking technologies.

9. Concluding Remarks

An exhaustive and updated review of various cook-stove technologies in the world has been made based on the data available in the open literature. The following remarks are worth noting:

• Cook-stoves are basically designed to achieve an optimized combustion and heat transfer performance, which ensures maximum heat transfer to the load with minimal pollutant emissions;
• The combustion and heat transfer performance could be enhanced with the careful design of burner geometry and external mixing chamber, and the judicious selection of the burner materials;
• The commercially available cook-stoves for gaseous and liquid fuels are designed on the free flame combustion principle. These cook-stoves offer higher thermal efficiency and reduced pollutant emissions compared with biomass-based cook-stoves. However, due to stricter norms and an increasing awareness about energy security and health concerns, the demand for better and advanced cook-stove technology has been growing;
• Pressurized cook-stoves show better performance compared with wick stoves when operated with either pure kerosene or its blends with plant oil;
• The porous media combustion technology is a viable manifestation of excess enthalpy/superadiabatic combustion by internal heat recuperation. It gives the time-tested combustion technique a fresh dimension with numerous benefits. Over the past few decades, Porous Radiant Burners (PRBs) have gained popularity for both residential and commercial cooking applications. The effectiveness of PRBs and their potential to replace CBs are being determined. However, long-term stability is still a concern and the report in this direction is limited;
• LPG has gained popularity among the many cooking fuels, whereas research on kerosene and natural gas is still lacking. Self-aspirated PRBs for biogas application are very limited. The recent development of self-aspirated PRBs for cook-stoves is promising as they can be a cleaner and energy-efficient alternative to CBs;
• In recent times, Methanol, ethanol, and their blends are emerging as alternatives to wood, charcoal, kerosene, etc., for cooking applications in underdeveloped nations. To some extent, they are also used in developing countries, which in turn reduces the import of crude oil;
• Reports show detrimental health issues such as respiratory diseases due to indoor air pollution by cook-stoves, and LPG and alcohol-based fuels are recommended to combat such problems;
• Household composition, education, socio-economic status, and ease of access to the fuel are significant factors affecting the choice of fuels for cooking. Furthermore, effective government policies enable a shift towards sustainable fuel transition.