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Biogas can be produced by different types of feedstocks, different feedstocks do release different amount of methane. Figure 2.1 below shows the comparison chart of methane emission between different feedstocks.
Figure 2.1 Comparison of methane emission of different feedstocks (Amon et al., 2007; Chynowerth et al., 1993; Foster - Carneiro et al., 2007).
The data in the chart above shows that POME has a higher methane emission rate when compare to manure, biomass and municipal solid waste (MSW). There is a great potential for POME to be utilized to generate high commercial return and produce renewable energy in a more sustainable way.
Methane yield ranging from 0.47 to 0.92 m3/kg BODadded was achievable in the biomethanation of POME for reaction temperature between 35Â°C to 55Â°C (Yeoh, 2004). The methane produced can be harnessed for the generation of either thermal or electric energy and for land application of digester effluent.
2.2 Process Techniques
In general, there are two main approaches for using plants for energy production, which are, growing plants specifically for energy use and using the residues from plants or wastes produced. Biogas can be produced through anaerobic digestion, which is a biological process, from oil palm biomass. Anaerobic digestion has become a promising technology particularly for recovery of energy from organic fraction of solid wastes.
Anaerobic digestion is a process in which the microorganisms break down biodegradable material in the absence of oxygen. There are four main biological and chemical steps of anaerobic digestion process, which are hydrolysis, acidogenesis, acetogenesis and methanogenesis. In the beginning of the process, bacteria hydrolyze the input materials to break down both insoluble organic polymers and high molecular weight compounds into soluble organic substances and make them available for other bacteria. After that, acidogenic bacteria will convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, organic acids, hydrogen sulphide and other by-products. In the third step, acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide. The methanogenesis produces methane by two groups of methanogenic bacteria in which the first group splits acetate into methane and carbon dioxide and the second group uses hydrogen as electron donor and carbon dioxide as acceptor to produce methane. The digestion process will produce biogas which consists of methane, carbon dioxide and traces of the other contaminant gases.
Anaerobic digesters can be designed to operate under different process configurations, which are batch or continuous process, mesophilic or thermophilic, high solids or low solids, single stage or multistage. In a batch system, biomass is added to the reactor at the starting time of the process. The reactor is then sealed throughout the process. In continuous digestion process, organic matter is constantly added in stages into the reactor and the end products are constantly or periodically removed, resulting in constant production of biogas. A single or multiple digesters in sequence may be used.
There are two conventional operation temperature levels for anaerobic digesters, mesophilic digestion and thermophilic digestion, which are determined by the species of methanogens in the digesters. The mesophilic digestion takes place optimally around 30-38oC, or at ambient temperatures between 20oC and 45oC, where mesophiles are the primary microorganism present. For thermophilic digestion, it takes place optimally around 49-57oC, or at elevated temperatures up to 70oC, where thermophiles are the primary microorganisms present. At a temperature less than 10oC, anaerobic process is very slow, it takes more than three times the normal mesophilic time process.
However, the biodegradability of different kind of waste materials is different depending on their composition. The higher the lignin and cellulose content, the lower the biodegradability was obtained. During anaerobic condition, sugars and starch are easily degradable, lipids and proteins are intermediately while cellulose is not easily degradable. Therefore, to improve the biodegradability of lignocellulosic materials, a pre-treatment is required to open up the compact structure.
There are several methods to be involved in the pre-treatment process, which are mechanical, physical, thermal, chemical and biological methods. For mechanical pre-treatment, it will result in no inhibitors and by reducing the particle size of the substrates usually lead to increase in methane production. By using this method, the degrability of organic matters can be increased by disrupting the flocs and lysing the bacterial cells. Mechanical pre-treatment is proved to be suitable for the applications at full scale biogas plants and may increase the methane yield of lignocellulosic substrate by up to 25%. However, this method needs a high energy demands and is not economically attractive.
Furthermore, the steam pre-treatment has been proved by research that it can significantly improve the biodegradability and also enhance the biogas production. Steam pre-treatment with NaOH presoaking has been reported to increase biogas production from municipal wastes by 50%.
For thermal pre-treatment, anaerobic digestion may be carried out under psychrophilic, mesophilic and thermophilic condition, which is 55oC. Mesophilic anaerobic digestion is more widely used when compared to thermophilic digestion. This is because of the lower energy requirements and higher stability of the process. However, thermophilic digestion is more efficient in terms of organic matter removal and methane production. It also enhances the destruction of pathogens.
For the chemical pre-treatment method, treatments with alkaline have been proven to effectively improve the biological conversion of lignocelluloses. Pre-treatment with NaOH can be classified into two types, which are "high concentration" and "low concentration" processes. NaOH can hydrolyzes the bond linkages between lignin and cellulose as well as intra-lignin linkages, Î±-ether bonds, phenyl glycosidic linkages, acetal linkages and ester bonds can be cleaved by the added alkali. The "low concentration" NaOH pre-treatment needs a high temperature and high pressure condition to be efficient. After the pre-treatment process, no NaOH reuse is possible because the mechanism is a reactive destruction of lignocelluloses. On the other hand, the "high concentration" NaOH pre-treatment requires only at ambient pressure and relatively low temperature to dissolve the cellulose and regenerate it. This process is very effective for the reduction of cellulose crystallinity leading to improvement in biological conversion of lignocelluloses.
One of advantages of high concentration NaOH pre-treatment is that the NaOH solution is possible to be reused after the process and this is very important regarding the economy and environmental impact of the process. The pre-treatment process with phosphoric acid is the most efficient process which has been studied for the improvement of enzymatic hydrolysis of lignocellulosic materials. This pre-treatment is able to disrupt the lignocelluloses structure and eliminate the resistance of lignin and hemicelluloses. The main advantage of using this method is that the phosphoric acid is able to be reused after the pre-treatment process.
After the chemical pre-treatment process, the total solids (TS) and the volatile solids (VS) content of the treated sample are lower than the untreated sample. These low solid conditions bring several advantages on operating the anaerobic digester, which are lower energy input for pumping and mixing, better accessibility for the microorganisms to the substrates and higher productivity. The ratios of VS to TS increases when treated with NaOH while decreases when treated with phosphoric acid. Hence, the pre-treatment with NaOH is more efficient as it produce higher methane yields and also higher VS to TS ratio. The higher VS to TS ratio can decrease the reactor volume needed. On the other hand, the VS content in a treated sample may decrease as the treatment time increases. These show that more organic materials are lost during treatment with phosphoric acid while more inorganic materials are lost in the treatment with NaOH. However, when the treatment time with NaOH increases, the amount of organic materials lose also increased. Thus, a very long treatment time may not be suitable for the pre-treatment process.
For biological pre-treatment, Enzymic Hydrolysis process is use to enhance the degradation of waste by the use of microbial enzymes. Enzymic Hydrolysis was first used to kill pathogen but an enhancement in biogas production was observed during the anaerobic digestion. Besides that, anoxic gas flotation (AGF) process is also an anaerobic digestion process that has potential to enhance biogas production. This process uses anoxic gas to float, concentrate and return bacteria, organic acids, protein, enzymes and undigested substrate to the anaerobic digester for the rapid and complete conversion of waste to gas and soluble constituents. By virtue of greater solids destruction and gas scrubbing of AGF process, methane production can be enhanced and also the biogas quality can be improved.
Although all of the pre-treatment methods have their own merits on contributing to accelerate anaerobic digestion and enhancement of biogas production, they do have their own drawbacks. For thermal pre-treatment, it requires a considerable amount of heat to preheat the feedstock, so it is unavoidable to consume some of the biogas produced. For mechanical pre-treatment, ultrasonication is no doubt the most powerful method to disrupt cell walls but power consumption becomes a serious drawback. For the other mechanical pre-treatment methods, such as grinding and high pressure homogenization, are less effective than the other methods. Although mechanical pre-treatment does not require chemicals or heat, most of its techniques consume a lot of power.