1. Introduction
Agricultural residues are potentially an attractive feedstock for producing energy as their use contributes little or no net carbon dioxide to the atmosphere. Agricultural production in Malaysia continued to record positive growth from 2000 to 2005 and is expected to expand more in 2010 consistent with the government policies [
1]. In Malaysia, the major agricultural products are palm oil, sawlogs, paddy and tropical fruits of which more than 70 million tones are collected annually. Significant amount of wastes and residues also called agricultural wastes are produced from the post-processing of these products. The processing of this waste has become a technological issue that has attracted the attention of numerous researchers. Presently, the most conventional way of handling these waste streams is to burn them with energy recovery or for landfilling. However, both combustion and landfill use cause secondary pollution problems. Novel disposal technologies are in high demand to provide for more energy efficient and environmentally and economically sound solutions. An alternative to these combustion and landfill uses is gasification. Thermochemical gasification of biomass is a well-known technology that seems to be a feasible application and has been developed for industrial applications [
2,
3,
4,
5]. This technology has been identified as a possible system for producing renewable hydrogen. The transformation of biomass into hydrogen-rich gas is a suitable source for energy production due to the advantages typical biomass (renewable character, net below zero CO
2 emissions, possibility of using residues generated from agriculture activities). Most of the research spurred by this interest has been techno-economic in nature, based on gasifier performance data acquired during system proof of concept testing. However, less emphasis has been given to experimental investigation of hydrogen production via biomass gasification, the focus of this paper.
Most of the researchers have employed oxygen and steam as a gasifying agent and operated under a high temperature slagging mode. This was due to the fact that high carbon conversion can be achieved in a short residence time, and tar production should be low. However, the slag may be corrosive to the gasifier and can cause smelt-water explosions if improperly handled [
4]. A fluidized bed reactor operating under medium temperature (around 900-1,000°C), is an alternative to agricultural waste gasification with air as gasifying agent. The air gasification process can convert these solid agricultural residues into a synthesis gas that is suitable for use in electricity production or for the manufacture of chemicals, hydrogen or transportation fuels. The gas produced from biomass gasification can be directly used as fuel in internal combustion engines or gas turbines [
3]. An advantage of air gasification over other gasifying agents is to simplify the gasification process and reduce operating and maintenance cost because air separation to obtain oxygen is a more complex and expensive process. On the other hand, in the case of air gasification, its nitrogen content remains as an inert component in the produced gas that dilutes the fuel gas and leads to a lower calorific value fuel [
4]. Although the feasibility of the air gasification process has been demonstrated with different feedstocks, less emphasis has been given to experimental investigation of hydrogen production via agricultural wastes (biomass) gasification, the focus of this paper [
3,
4,
5,
6,
7,
8,
9,
10].The transformation of biomass into hydrogen-rich gas is a suitable source for energy production due to the typical advantages of biomass (renewable character, net below zero CO
2 emissions, etc). The production of hydrogen from biomass was described as listed reactions below [
8]:
Oxidation reaction: | C + O2 → CO2 | (1) |
| C + ½ O2 → CO | (2) |
Boudouard: | C + CO2 → 2CO | (3) |
Water gas: | C + H2O → CO + H2 | (4) |
Methanation: | C + 2H2 → CH4 | (5) |
Water gas shift: | CO + 2H2O → CO2 + H2 | (6) |
The quality of the gas produced (composition, production of CO, H
2, CO
2 and CH
4 and energy content) and the gasification performance (gas yield) depend upon feedstock origin, gasifier design and operating parameters such as temperature, static bed height, fluidizing velocity, equivalence ratio, gasifying agent, catalyst and others, which are explain elsewhere [
5,
7,
9,
10,
11,
12,
13,
14,
15,
16]. Warnacke demonstrated that the gas composition was a function of gasifier design whereby the same fuel may give different calorific values with different gasifiers [
11]. Among all designs, the fluidized bed gasifier has been shown to be a versatile technology capable of burning practically any waste combination with high efficiency. The significant advantages of fluidized bed combustors over conventional combustors include their compact furnaces, simple design, effective burning of a wide variety of fuels, relatively uniform temperature, and the ability to reduce emissions such as nitrogen oxide and sulphur dioxide gas [
17].
In this work, a laboratory scale fluidized bed was used to investigate the characteristics of gasification of agricultural wastes. The effect of gasification temperatures, fluidization ratio, static bed height and equivalence ratio (ER) on gas composition, gas yield and gas heating value were studied.
4. Conclusions
Air gasification of agricultural wastes was successfully performed in a lab scale fluidized bed gasifier, producing a fuel gas with a lower heating value in the range of 85 and 2384 kJ/NM3 and 1482 and 5578 kJ/NM3 for coconut shell and palm kernel shell, respectively, which could be used in many end use applications. Among the gasification parameters tested, the equivalence ratio appeared to have the most pronounced effect on the reactor temperature, the gas composition, the gas yield, and the gas heating value. The selection of suitable equivalence ratio would depend on the final use of the gas produced. As a higher equivalence ratio (ER) had complex effects on tests results and there existed an optimal value for this factor, which was different according to different operating parameters. The influence of equivalence ratio on the performance of a gasifer could be regarded as the effect of reactor temperature as the reactor was found to be ER dependent. The fluidizing velocity and static bed height would only show minor effect during the gasification process. The fluidization velocity was observed to have an influence on the gasification process to some extent because it will result in the carryover of fine chars from reactor. The bed height would affect the residence time of gases in the high temperature dense bed. Hence, the rise of the bed height favored tars and hydrocarbon cracking reactions, but too high a bed height showed a negative effect due to formation of large bubbles.