**1. Introduction**

The increasing scarcity of conventional fossil fuels has led to diversification of energy resources. In addition, the combustion process of fossil fuels for electricity generation emits greenhouse gasses and criteria pollutants, which are harmful to both living organisms and the planet. The global carbon emission has been increasing at an alarming rate. Average annual global carbon dioxide emission from burning of fossil fuels was 3.1 GtC per year in the 1960s. Recently, the rate has recorded an increment higher than threefold where 9.4 GtC per year was emitted during 2008–2017 [1]. Combustion of finite non-renewable fossil fuels for energy production in various sectors such as transportation and industrial activities has been reported to be the main perpetrator to this worrying situation. This dire

situation prompts for cooperative and collective effort at a global scale as manifested by the Kyoto Protocol and Paris Agreement. Many countries around the world are phasing out and rendering non-renewable fossil fuels as an obsolete option for energy production.

Reducing our reliance on finite fossil fuels and exploration of potential renewable resources for energy generation have become a focus at the global scale, and Malaysia is not left behind in this worldwide trend. The initiative has gained support at the governmental level as evidenced by the introduction of the Five-Fuel Diversification Policy. Under this policy, renewable energy is included as the fifth fuel in the supply mix where utilization of abundant biomass is one of the strategies being encouraged [2,3]. According to the Malaysia Energy Commission, 100,721 ktoe of energy was supplied in 2015 where 95.5% of the energy was generated from non-renewable resources, mainly natural gas (61.7%) and crude oil (32.2%). Biomass, on the other hand, contributed a small fraction of 0.2% to the total energy supply for that year [4]. Biomass is derived from living organisms through the photosynthesis process where solar energy is converted into carbohydrates. A wide range of biomass is available that entails significant variation in their properties, characteristics and chemical compositions. In general, major constituents of biomass consist of oxygen, carbon and hydrogen. The use of biomass for energy production is considered carbon neutral due to carbon fixation process during photosynthesis [5].

Conversion processes (physical, biological, thermochemical, etc.) of the ample biomass produce renewable syngas, which provides alleviation for both energy security and global warming issues. Additionally, various types of value-added products can be produced from the conversion processes. For this purpose, a range of thermochemical conversion processes is available such as pyrolysis, gasification, liquefaction and direct combustion. The main difference between thermochemical technologies is the availability of oxygen during the process. In some applications, more than one thermochemical conversion process is combined to enhance the quality of producer gas as conducted by Alipour Moghadam, et al. [6] where both pyrolysis and air-steam gasification processes are integrated together. Four possible biomass thermochemical conversion routes for renewable energy production have been discussed and compared by Mohammed, Salmiaton, Wan Azlina, Mohammad Amran, Fakhru'l–Razi and Taufiq–Yap [5].

Thermochemical conversion of biomass produces syngas with half energy density of natural gas. The reactions involved during biomass conversion process are summarized in Table 1 [7].


**Table 1.** Chain of reactions involved in the biomass thermochemical process.

In Malaysia, many work related to the thermochemical conversion of biomass has been concentrated on palm oil derived biomass due to its abundancy and wide availability [8,9]. In order to broaden the range of biomass utilized for renewable energy generation, which directly supports the fuel diversification policy of Malaysia, new potential renewable energy resources are being explored. Napier grass (NG) has gained considerable attention in recent years due to its desirable characteristics as potential renewable fuel. This energy crop of African origin is highly productive with low establishment

cost [10]. The annual yield is 40 tonnes per hectare with multiple harvest frequency. There is limited information on the potential of producing green energy from Napier grass reported in the literature where the works have been concentrated on using the pyrolysis conversion process [11–13].

Fluidized bed gasification has been reported to be a versatile technology for biomass conversion. Intensive mixing in the bed enhances heat and mass transfer that leads to a high reaction rate [14]. Abdoulmoumine, et al. [15] reported that operation parameters have a major influence on the kinetics of reactions involved, which directly affect yield and the quality of producer gas. To our knowledge, the potential of generating renewable fuel from gasification of Napier grass has never been conducted. It is the aim of this study to evaluate the feasibility of syngas production from Napier grass via the bench-scale gasifier system at varying operating conditions.

## **2. Materials and Methods**

#### *2.1. Sample Preparation*

Mature Napier grass was sourced from Crops for the Future Research Centre (CFFRC), Semenyih, Selangor, Malaysia. The biomass was dried in an oven at 105 ◦C according to BS EN12048 standard prior to size reduction by using the Retsch rotor beater mill. The sample size was reduced to 0.2 and 2 mm and kept in air-tight plastic bags for further analysis.

#### *2.2. Proximate Analysis of Mature Napier Grass*

Proximate analysis was conducted on the shredded form of Napier grass by using a thermogravimetric analyzer (TGA; TGA/SDTA851, Mettler Toledo, Columbus, OH, USA) to determine fixed carbon, volatile matter, moisture and ash contents in Napier grass. The details of the experimental procedure can be found elsewhere [16].

## *2.3. Ultimate Analysis of Mature Napier Grass*

An ultimate analysis was conducted to determine elemental composition of mature Napier grass by using the CHNS/O analyzer (model LECO CHN628 and 628S, St. Joseph, MI, USA) according to the ASTM D-5291 standard method.

### *2.4. Measurement of the Higher Heating Value of Napier Grass*

The gross calorific value of mature Napier grass was measured by using the Parr 6100 oxygen bomb calorimeter (Moline, IL, USA) according to BS EN 14918.

#### *2.5. Gasification of Napier Grass for Syngas Production*

The gasification of the shredded Napier grass was conducted in a fluidized bed gasifier. The reactor was cylindrical with 370 mm high and 54 mm wide, made of stainless steel. The schematic of the experimental rig is shown in Figure 1.

The procedure began with charging the reactor with 20 g of sand as the bed material to obtain good temperature distribution, to stabilize the fluidization and to prevent coking inside the reactor. Air stream and biomass feedstock were introduced from the bottom and top of the reactor respectively as the bed temperature achieved the steady state condition. The experiment was carried out at five different temperatures between 650 ◦C and 850 ◦C at 50 ◦C temperature increment and three different equivalence ratio (ER; 0.2, 0.3 and 0.4).

**Figure 1.** Process flow diagram of lab scale gasifier setup.
