A Comprehensive Assessment of Clean Coal Fuels for Residential Use to Replace Bituminous Raw Coal

Abstract

Residential coal combustion is a major source of air pollution in developing countries, including China. Indeed, precisely measuring the real-time emission of major air pollutants is often challenging and can hardly be repeated at a lab-scale. In this study, for the first time, two clean coals initiated from raw bituminous coal were burned for real-time estimation of air pollution characteristics and their thermal efficiencies in different stoves. Moreover, thermodynamic equilibrium simulations were investigated for slagging parameters using Factsage 7.1 at reaction temperature 800~1600 °C. Results revealed that the firepower of clean coals (Briquetted coal and Semi-coke) was much higher (2.2 kW and 2.1 kW) than raw coal (1.8 kW) in a traditional stove. However, the thermal efficiencies were remarkably increased (13.3% and 13.5%) in an improved stove for briquetted coal and semi-coke, respectively. The emission of major air pollutants including carbon monoxide (CO), sulfur dioxide (SO₂), particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), non-methane hydrocarbons (NMHCs) from both coal and semi-coke was significantly reduced. Thermodynamic equilibrium calculations indicate that briquetted coal is not susceptible to slagging under the reaction conditions in the household stove. The current study provides guidance for the selection of alternative and efficient clean coal fuels in rural areas for household purposes coupled with public health and safety.

1. Introduction

Coal has always been the main energy source for household heating and cooking in winter in northern China’s towns and villages. However, according to the “China Rural Bulk Coal Management Comprehensive Report 2022” statistics, scattered coal accounts for about 15% of the coal used in China every year, of which about 150–170 million tons/year are consumed by residential bulk coal, and most of them are accompanied by traditional inefficient coal-fired stoves. This use of untreated coal has caused serious pollution to both the air environment and human health [1,2,3].

Household coal combustion contributes significantly to air pollutants such as SO₂, nitrogen oxides (NO_x), and particulate matter with an aerodynamic diameter of fewer than 2.5 microns (PM_2.5) in the winter, accounting for 36.1%, 9.1%, and 16.6% of the overall contribution, respectively [1]. In addition, the impact of NMHCs on human health cannot be ignored [4]. These organic compounds are easily oxidized by OH, NO₃, and O₃ in the atmosphere into secondary organic aerosols—the precursors of atmospheric PM_2.5—affecting air quality, regional climate, and human health [5]. Additionally, their ozone generation potential is even higher than that of conventional harmful gases [6]. NMHCs emitted from incomplete combustion in furnaces due to inefficient combustion practices combining poor quality fuels with inefficient stoves [7,8,9].

To solve this problem, several effective measures have been implemented, such as “coal to gas,” “coal to electricity,” and “coal to biomass” to mitigate the harm caused by household coal combustion. However, due to expenses, energy availability, etc., these measures cannot be applied in less developed rural areas [10]. Therefore, in recent years, the focus of relevant research has steadily switched to improving the performance of stoves and the quality of fuels to control pollution from the uncontrolled burning of household scattered coal.

Wang et al. developed a new type of clean combustion stove with a double hearth structure for cooking and heating, and the thermal efficiency was increased from 50% of the traditional stove to 68%, while SO₂, NO_x, and carbonic oxide (CO) were significantly reduced [11]. Shen et al. also showed that emissions of CO, total suspended particles (TSP), organic carbon (OC), element carbon (EC), and PAHs were generally lower in improved coal-fired gasifier stoves compared to traditional stoves [12]. Further, some researchers classified the current novel coal-fired stoves according to the combustion methods, namely updraft stove, downdraft stove, and cross-draft stove [5,13,14]. They conducted relevant tests on the thermal performance and pollutant emissions characteristics of different household coal-fired stoves, both of which were determined by not only fuel properties but also combustion methods. Generally speaking, a better effect can be achieved by using high-rank coal fuels or improved stoves [15].

In the processing and upgrading of household coal fuels, promising treatments are processing raw powdered high-rank coal with low volatile content or carbonizing bituminous coal with high volatile content, respectively. However, the former not only suffers from defects in use such as slow ignition speed and incomplete combustion in a furnace [5], but also has no prospect of application in rural areas due to the scarcity of high-rank coal resources, which leads to its high price [16,17]. The latter is a porous solid fuel with low volatile content, which is formed by the process of pyrolysis of bituminous coal [18]. Due to carbonization, the content of volatile matter in carbonized solid fuel becomes much lower. Its large specific surface area enhances the efficiently gas-solid contact, as well as increasing the reaction rate during the combustion process, resulting in more complete combustion in the household furnace [19,20]. However, its processing is more complex and costly, it is only suitable for some areas with better economic conditions. For some less developed rural areas, the economic cost is the decisive factor limiting the selection of alternative fuels, so it is more difficult to promote the use of this fuel [21].

Some researchers suggested using processed fuels in optimized stoves to reduce emissions, improve efficiency, and reduce fuel consumption, including so-called ‘smokeless fuel’, ‘carbonized fuel’, ‘torrefied fuel’, ‘briquettes’, and ‘pellets’ [18]. Currently, a promising alternative coal fuel is the direct processing of low-cost bituminous coal into bituminous briquette with some low-cost additives to remove pollutants, which not only meets the traditional heating habits of the population but also significantly reduces the processing costs. Liang et al. indicated that making bituminous raw coal into briquette and matching it with a suitable improved household stove structure can significantly improve the combustion efficiency of the stove, and the VOCs emission can be reduced to a level comparable to that of low-volatile clean coals such as semi-coke and anthracite [5]. Das et al. added a small amount of binder with cow dung to bituminous coal, which reduced PM_2.5 emissions by about 47% compared to bituminous raw coal [18]. Jiao et al. also obtained similar conclusions that PM_2.5 and CO emissions were reduced by 83–90% and 61–76%, respectively, by processing bituminous coal into granular coal [22]. Li et al. conducted several measurements combining 35 coal samples with two stoves and found that the replacement of unprocessed raw coals by semi-coke briquettes can significantly reduce emission factors of primary PM_2.5, elemental carbon, and organic carbon [19]. Carbonized solid fuels exhibit better performance in reducing carcinogenic potency and pollutants resulting from their low volatile matter content [20]. Zhi et al. suggested that if all household scattered coal fuels were replaced with briquettes burned in improved stoves, PM, OC, and black carbon (BC) could be annually reduced by 63 ± 12%, 61 ± 10%, and 98 ± 1.7%, respectively [23].

Therefore, in this study, the bituminous raw coal was used as a baseline, and the combustion characteristics of bituminous briquette and semi-coke in the traditional and improved stoves are evaluated comprehensively in terms of thermal performance, pollutant emission, and slagging tendency. The results will provide a promising and optional solution for the clean and efficient combustion of household coal.

2. Experimental Method

2.1. Coal Fuels and Stoves

The bituminous raw coal (length × width: 4 cm × 2 cm) selected in this study is produced in Yulin, Shaanxi province, which has the characteristics of high volatile content and calorific value. The semi-coke (5.5 cm × 3 cm) is the solid product after the medium and low-temperature pyrolysis of bituminous coal, produced in Yulin, Shaanxi province. The bituminous briquette (4 cm × 2 cm) is made by crushing and mixing the bituminous raw coal and powdered calcium-based additives (5%) for sulfur retention. The proximate and ultimate analyses of the three coal fuels are shown in

Table 1. Proximate and ultimate analyses of three coal fuels.

Coal Fuels	Proximate Analysis (%)				Ultimate Analysis (%)				Lower Calorific Value Qnet,ar (MJ/kg)
	Mad	Ad	Vd	FCd *	Cd	Hd	Od *	Nd	Sd
Raw coal	2.9	9.8	34.8	55.4	73.6	4.5	10.2	1.3	0.63	28.1
Briquette	3.1	17.2	36.7	46.1	62.8	4.3	13.9	1.2	0.63	25.9
Semi-coke	6.5	11.8	7.2	81.0	80.3	1.9	4.8	0.9	0.28	26.1

Notes: ad is “air dried”, d is “dried”; “*” means estimation by difference.

. It can be seen that compared to bituminous raw coal, the volatile and sulfur content of semi-coke are significantly reduced, while the ash content is elevated, and the calorific value is slightly reduced due to the addition of additives in the bituminous briquette.

Table 2. Ash composition analysis of three coal fuels (unit: %).

Coal Fuels	CaO	SiO2	SiO2	MgO	Na2 O	Al2O3	Fe2O3	Cl	P2O5	TiO2	MnO
Raw coal	9.3	41.1	0.3	4.2	0.7	34.7	7.6	0.4	0.1	1.3	0.1
Briquette	45.3	23.4	0.5	2.1	0.3	15.7	11.4	0.3	0.1	0.7	0.1
Semi-coke	14.4	57.3	0.3	1.1	0.4	11.3	13.3	0.1	0.5	1.2	0.1

shows the results of the ash composition analysis of the three coal fuels analyzed using X-ray fluorescence (Thermo Fisher Scientific Company, Waltham, MA, USA).

The stoves measured in this article are the two most common commercial stoves with natural ventilation, i.e., traditional stoves and improved stoves, of which the main difference is the air distribution required for combustion. The former set the primary and secondary air at the bottom and top of the furnace, respectively, and feeds coal in the same direction as the flue gas, while the latter sets the primary and secondary air positions opposite to the former.

2.2. Experimental Test Systems

The schematic of the experimental system is shown in Figure 1. The two stoves used for the experiments were both heating stoves rated at 10 kW and equipped with flue gas analyzing and sampling devices. The combustion air volume is natural ventilation, about 20–25 m³/h, and the fuel is about 2.5–3 kg/h. The experimental process starts with adding fuel coal to the furnace chamber, after which the fuel is ignited from below the grate by propane gas, and the test starts when the furnace rises to the rated load, and the rate control is achieved by the PLC system, the turbine flowmeter is selected for flow measurement to regulate the inlet water flow, which can be calculated by the following equation:

$$q=c\times m\times \left(T_2-T_1\right)$$

(1)

where c is the specific heat capacity of water (4.186 kJ/(kg·°C)); m is the water flow rate measured by a turbine flow meter (kg/s); and $T_1$ and $T_2$ are the inlet and return water temperatures (°C), respectively. All experiments were conducted at more than 80% of the rated load to simulate the normal usage habits of residents during cooking and heating. Although the pollutant emissions from the ignition stage were much higher than those from the steady combustion stage [24,25], the effects of emissions from the ignition stage were not considered in this paper, because the ignition stage has a very small share in the daily use of residents compared to the steady combustion stage. The sampling and testing of pollutants in this paper were performed directly in the chimney. Till the stable operation of the stoves, the thermal efficiency and cooking performance of the stoves were tested according to the method of The Water boiling Test (Version 4.2.3, 2014) to evaluate the thermal performance of different coal types burned in both stoves.

2.3. Sampling and Analysis Methods

In this paper, the real-time concentrations of O₂, CO, CO₂, NO_x, and SO₂ in the flue gas were tested using an online Fourier infrared flue gas analyzer (Madur photon S). The Modified Combustion Efficiency (MCE, %) of the stove can be calculated from the measured CO and CO₂ concentrations as follows [26]:

$$MCE=\frac{C_{{\mathrm{CO}}_2}}{C_{\mathrm{CO}}+C_{{\mathrm{CO}}_2}}$$

(2)

For PM sampling, the A-2000 fully automatic isokinetic sampling system developed by Environmental Supply (ES), USA, was used to sample both filterable and condensable particulate matter (FPM and CPM), and the sampling process was carried out according to the methods provided by EPA-Method 201a and EPA-method 202, with a sampling time of 3 h of each test. Firstly, the 115 °C thermostatic flue gas probe was inserted into the chimney, and the sampling pump tracked the flue gas flow rate to adjust the sampling flow rate synchronously. The flue gas was collected in the tank at 120 °C by quartz filter membrane to collect the filterable particulate matter, then it was cooled by a 30 °C water bath to collect the condensable particulate matter, and the remaining condensable particulate matter was filtered off by Teflon filter membrane, and finally, the remaining flue gas was recovered by ice water bath and dried silica gel. The recovery of the sample includes all the materials captured by the two filter membranes and the glassware in between. The glassware is rinsed with distilled water, acetone, and hexane in turn, and the particulate matter in the three rinsing solutions is extracted and filtered for recovery and analysis.

In this study, the concentration of non-methane total hydrocarbons (NMHCs) was tested. Firstly, the flue gas was extracted into a gas bag at a constant flow rate of 500 L/min, after which the NMHCs were analyzed by subtracting methane determined by a flame ionization detector (FID) from the total hydrocarbons, using Agilent 6890 gas chromatography developed by Agilent according to the method of HJ/T 38–2017.

Since the O₂ concentration varies with operating conditions, it is uniformly converted to 9% for treatment in this paper, i.e.,

$$C_{i,9{\%\mathrm{O}}_2}=\frac{\left(20.95-9\right)}{\left(20.95-{\phi }_{{\mathrm{O}}_2}\right)}\times C_i$$

(3)

In the above equation where $C_i$ is the average mass concentration of i gas component (mg/Nm³), ${\phi }_{{\mathrm{O}}_2}$ is the volume fraction of O₂ in the flue gas (%).

2.4. Thermodynamic Equilibrium Calculation

To investigate the slagging tendency of the three coal fuels in the household stoves, factsage 7.1 based on the principle of Gibbs free energy minimization is used to calculate the thermodynamic equilibrium composition of the combustion products. To simulate the combustion reaction conditions of the three coal fuels in the household stoves, the reaction conditions of the calculation process are set as follows: the reaction temperature is 800~1600 °C, the temperature step is set to 50 °C, the atmospheric pressure is set, and the excess air coefficient is 2.0. The ash composition data of the three coal fuels were input as shown in Table 2, and the database was selected from FactPS and FToxid.

3. Results and Discussion

3.1. Thermal Efficiency

To evaluate the thermal performance of two clean coals in different stoves, this study compared the cooking ability, thermal efficiency, and combustion efficiency of two clean coals with bituminous raw coal in both traditional and improved stoves. Since the improved stove does not have a cooking function, the comparison of the cooking ability of different coals was carried out with the traditional stove, as shown in Figure 2. All experimental data are shown as a bar graph with error bars that designate standard deviation. It can be seen that the cooking ability of both bituminous briquette and semi-coke is higher than that of bituminous raw coal, the former is due to the regular and uniform shape of the bituminous briquette with large internal porosity after being processed from raw coal, which is conducive to the uniform distribution in the furnace during the combustion process. The latter is because, in the process of preparing semi-coke, the release of volatile matter makes the solid product form a porous structure with a large specific surface area, which is conducive to gas-solid contact, promoting the combustion condition [20,21]. According to the standard GB/T 16155-2018, Firepower (P_c) and Thermal Efficiency ($\eta$) can be calculated as follows, respectively:

$$P_c=\frac{\left(G_{c2}-G_{c1}\right)r}{3600}$$

(4)

where $G_{c1}$ is the initial mass of water in the pot, kg; $G_{c2}$ is the remaining mass of water in the pot after evaporation for 1 h, kg; $r$ is the average latent heat of vaporization at the average evaporation temperature, kJ/kg; 3600 is evaporation time, s.

$$\eta =\frac{G_z\left(t_{cp}-t_{jp}\right)\times 4.18}{B\times Q_{net,v,ar}}\times 100$$

(5)

where $\eta$ is thermal efficiency, %; $B$ is coal consumption, kg; $Q_{net,v,ar}$ is the low calorific value of coal at constant volume, kJ/kg.

Figure 3 compares the thermal efficiency of three kinds of coals in two kinds of stoves. The thermal efficiency of the improved stove is always higher than that of the traditional stove, which is due to the staged combustion design of the improved stove making the distribution of gas and solid phases more reasonable. The char layer at the bottom of the furnace provides a strong heat storage capacity at the same time. When the flue gas passes through the char layer, not only does homogeneous combustion occur between the flue gas and hot air, but also heterogeneous combustion occurs between the flue gas and hot char layer. This prolongs the residence time of the secondary air in the furnace, thereby enhancing thermal efficiency. It can also be seen from the figure that for different coal types, the thermal efficiencies are, in descending order, bituminous briquette > semi-coke > bituminous raw coal. The reason why the thermal efficiencies of the two clean coals are higher than that of raw coal is consistent with their cooking ability. The thermal efficiency of bituminous coal is higher than that of semi-coke because of the higher volatile content, which promotes combustion in the furnace. The carbon content of the bottom ash of bituminous briquette and semi-coke is 0.8% and 7.1%, respectively, which indicates that the burnout rate of bituminous briquette is higher.

In addition, it can be seen from Figure 4 that the combustion efficiency of bituminous briquette is also higher than the other two coal fuels, indicating that its incomplete combustion heat loss in the gas phase is lower. Therefore, in terms of thermal performance, the thermal efficiency and combustion efficiency of the bituminous briquette are the highest in both stoves, which inherits the combustibility of the bituminous raw coal and improves the structure of the bituminous raw coal, making the gas-solid contact more uniform and the combustion more completely.

3.2. Real-Time Estimation of Air Pollutants

To evaluate the pollutant reduction effect of two clean coals relative to the bituminous raw coal, this study conducted online real-time tests of the main gaseous pollutants generated by the combustion of three coals in traditional and improved stoves, and the results are shown in Figure 5 and Figure 6.

Comparing Figure 5 and Figure 6, it can be found that the pollutant emissions in traditional stoves fluctuate sharply, especially for bituminous raw coal and briquette. As mentioned above, this is because there is no stable char layer in the bottom of the furnace of the traditional stove to provide the residence time and heat required for the combustion of volatile matter, the release of volatile matter and char combustion occur at the same time and space. When coal feeding and bottom ash removal are carried out, the pollutant emissions fluctuate sharply. The improved stove, on the other hand, separates the release of volatile matters from the combustion of char due to the formation of a stable char layer at the bottom of the furnace. The pollutant emission fluctuation of semi-coke in the traditional stove is less because most of the volatile matter has been released during the preparation process of semi-coke. The combustion process is dominated by the burning of char, while gaseous combustion has less influence on pollutant fluctuations.

Therefore, it can be seen from Figure 5 that in traditional stoves, the pollutant emissions of semi-coke are always lower than those of bituminous raw coal and briquette. While the bituminous briquette has lower CO and SO₂ emissions than raw coal due to its higher specific surface area and rich pore structure, as well as the addition of sulfur retention additives. The Ca-based sulfur retention additives have a catalytic promotion effect on NO_x generation, so the NO_x of briquette is higher than that of raw coal [27,28].

In the improved stove, as shown in Figure 6, the pollutant emissions of all three coals are much lower than those of the traditional stove. The combustion is more complete, and the average CO emission concentration is also all lower than 300 mg/Nm³. For the briquette, the SO₂ concentration is significantly reduced, from about 600 mg/Nm³ to within 200 mg/Nm³. This is because the reaction temperature range of the sulfur retention additives is between 600~1000 °C [28,29], and the temperature inside the furnace of the improved stove is distributed in a gradient, which is conducive to the sulfur retention reaction, and the sufficient O₂ is also conducive to the sulfur retention of Ca-based additives. In contrast, in the traditional stoves, although the primary air volume is high, the concentrated release and combustion of volatile matter and char are in the same space, making it relatively oxygen-depleted in the furnace at the early stage of volatile matter release, while all the above reactions require a relatively oxygen-rich atmosphere. In the same trend of NO_x emission as traditional stoves, the bituminous briquette is also affected by a Ca-based sulfur retention agent causing elevated NO_x emission. Calcium-base additives have an adverse effect on NOx emission since the limestone is a catalyst to promote Nox formation. This result has been confirmed by previous studies [30,31].

Figure 7a compares the filterable and condensable particulate matter emissions of different coal types in the two stoves. It can be seen that the filterable particulate matter accounts for a higher percentage in the traditional stove, especially for bituminous raw coal, where the filterable particulate matter exceeds 400 mg/Nm³. Although the source of the briquette is bituminous raw coal, its higher compress strength allows some of the PM to be captured in the bottom residue during combustion. The filterable particulate matter of semi-coke is lower than 100 mg/Nm³, which is due to its lower volatile matter.

The ratio of filterable and condensable particulate matter between the three coals in the improved stoves changed significantly, with the filterable and condensable particulate matter being basically the same, and even the latter being higher than the former, which resulted in the key role of the improved stoves in the reduction of filterable particulate matter. As mentioned above, the staged combustion of the improved stove makes the coal fuel’s combustion more completely in the furnace, and most of the carbon black particles or tar molecules are cracked into small molecule gas phase products, reducing the emission of particulate matter. However, the condensable particulate matter of the improved stove is only slightly reduced compared to the traditional stove, which is due to the mechanism of condensable particulate matter generation. The condensable particulate matter is mainly composed of SO₄²⁻ and NO₃⁻, which are converted from gas-phase precursors such as SO₂ and NO_x [32,33]. Residential coal combustion and its PM are an important primary source of atmospheric sulfate. These gas-phase products are mainly determined by the elemental content of the coal fuels, and the influence of the stove is small [34].

Therefore, the impact of improved stoves on particulate matter acts mainly on filterable particulate matter. In addition, it can be seen in Figure 7b that the emission concentrations of filterable and condensable particulate matter from the bituminous briquette in the improved stove are almost comparable to those from semi-coke, indicating that making the bituminous raw coal into briquette, with a suitable stove, can yield essentially the same particulate matter emissions as the high-quality, low-volatile-content clean coal, semi-coke.

Another pollutant formed during combustion can reflect the combustion condition in the furnace. The trend of NMHCs is similar to that of filterable particulate matter, and the results are depicted in Figure 8. Similarly, both the stove structure and the coal type determine the emissions of NMHCs. The combination of bituminous briquette and improved stove are almost equivalent to those of semi-coke.

3.3. Slagging Parameters

The slagging in the furnace during the combustion process will block the air inlet of the grate, making the combustion air unevenly distributed, resulting in deteriorative gas-solid contact and affecting the normal combustion situation. At the same time, the fouling of the heat exchanger tubes caused by ash-related problems during the combustion process can also lead to low heat utilization efficiency, and the heat provided by the combustion process cannot be effectively converted into room temperature, thus affecting the heating ability. This ash-related problem is usually related to the fuel properties and the combustion conditions in the furnace; therefore, in this study, the combustion conditions in the household stoves were simulated using thermodynamic equilibrium calculation software, and the slagging tendency of bituminous raw coal, briquette, and semi-coke is evaluated.

In the factsage software, the calculated products include three types of substances: gas phase, liquid phase, and solution phase, where the solution phase is divided into a solid and liquid solution, and the proportion of the liquid solution Slag A in the total solution phase determines the slagging tendency in the furnace. Figure 9 depicts the variation of the proportion of Slag A in the total solution phase of the three coals under the reaction condition of 800~1600 °C excess air coefficient of 2, and atmospheric pressure, respectively. It can be seen that, compared with the bituminous raw coal, the starting generation temperature of Slag A increases after the addition of different types of Ca-based sulfur retention additives to the briquette, which results from the increase of Ca content in the briquette, and will react with Si and Al to form Ca/Al silicates with high melting temperature. It is beneficial to alleviate the slagging tendency [35]. On the contrary, the onset of Slag A generation temperature of semi-coke is greatly advanced, with liquid slag generation below 1100 °C, and the solution phase generated before 1200 °C is changed from a mixture of solid and liquid slag to completely liquid slag. It can be seen from Table 2 that the elemental composition of semi-coke is mainly SiO₂, and the contents of Fe₂O₃ are also higher than that of bituminous raw coal and briquette. Therefore, the compounds in the ash formed during thermal conversion are mainly composed of Si, Al, and Fe, as shown in Figure 10. The presence of these compounds in ash will lead to a lower melting point and easier formation of substances that promote slagging. Since the combustion temperature in the household stove is usually lower than 1200 °C, bituminous raw coal and briquette will not slag in this temperature range, while the semi-coke will have different degrees of slagging tendency, therefore, from the aspect of daily maintenance and service life of the stove, bituminous raw coal and briquette are more suitable for the residential field.

4. Conclusions and Summary

This study evaluated the feasibility of using bituminous briquette and semi-coke as alternative coal fuels to bituminous raw coal in the residential field in terms of thermal performance, pollutant emission characteristics, and slagging tendency. To verify the combustion effects of the two clean coals in traditional and improved stoves, the bituminous raw coal, a kind of heating fuel most commonly used by residents, for making bituminous briquette was selected as a benchmark, and the following conclusions were obtained:

(1)

The cooking ability, thermal efficiency, and combustion efficiency of bituminous briquette and semi-coke are higher than those of bituminous raw coal because the specific surface area of the original fuel is increased and the internal pore structure is improved during the processing and preparation, both of which is conducive to the gas-solid contact between the air and the fuel required for combustion. The thermal efficiency and combustion efficiency of bituminous briquette are the highest due to its high volatile content, which enables the gas and solid incomplete combustion products to be completely combusted. Meanwhile, combined with the improved stove with a reasonable staged combustion design, the heat storage capacity provided by the stable char layer in the furnace further improves the combustion of different coal types.

(2)

In the traditional stove, the lowest SO₂, NO_x, CO, and PM emissions were found in the semi-coke, and no significant fluctuations were observed during the test, which was because most of the S/N-riched volatile matter had been released during the preparation. The pollutants of both bituminous raw coal and briquette show significant fluctuations because the release and combustion of volatile matter and the combustion of char in the traditional stove occur at the same time and space. The resulting local oxygen depletion and short residence time lead to incomplete combustion in the stove.

However, the improved stove designed based on the principle of air/fuel-staged combustion resulted in a significant reduction in CO, NMHCs, and filterable particulate emissions for the three coals due to their complete combustion in the furnace. The SO₂ emissions of the bituminous briquette were lower than those of the semi-coke due to the sulfur retention additives. The elevated NO_x emissions of bituminous briquette is resulted from the contribution of Ca-based sulfur retention additives to NO_x generation. Stove improvements have no significant effect on the condensable particulate matter since it is largely dependent on the fuel properties.

(3)

Thermodynamic equilibrium calculations show that compared to the bituminous raw coal and briquette, slagging is more likely to occur in the furnace for semi-coke due to the formation of some low melting point compounds during the conversion of ash in the combustion process.

Hence, this study identified clean coal replacement as a feasible clean combustion technology for residential winter cooking and heating.