Using rice straw as raw material
Adequate and uninterrupted supply of energy is the basic demand for economic growth and development of any country. Earth's energy crisis and global warming from energy sector, day by day, enhance the demand for renewable energy. Although several options of renewable energy have been discovered, Biogas and Solar are the most prominent renewable energy sector still now. Biogas technology is most popular throughout the world because it not only produces energy but also ensures better and hygienic environment, good health and other socioeconomic benefit as the technology convert the biodegradable organic waste from agriculture, industries, municipalities, livestock etc. to energy and supplies the slurry as a by product that can be used as organic fertilizer.
Biogas is a mixture of 50 to 70% of Methane (CH4), 30 to 40% of Carbon di oxide (CO2), Hydrogen Sulphide (H2S) and moisture generated after the fermentation of biodegradable organic wastes in the absence of oxygen which is known as anaerobic digestion process. Biogas is about 20 percent lighter than air and has an ignition temperature in the range of 650 degrees to 750 degrees C. It is an odourless and colourless gas that burns with clear blue flame similar to that of LPG (Sathianathan, 1975). Its calorific value is 20 Mega Joules (MJ) per m3 and burns with 60% efficiency in a conventional biogas stove.
Biogas can be used to transform its energy content into different forms like mechanical energy and heat energy. Cooking, lighting, refrigeration and production of electricity by running internal combustion engine are the common uses of biogas. A variety of organic wastes are used as raw materials for biogas production i.e. manure of pig, cow, horse, sheep and poultry; grass, elephant grass, vegetable residuals, water hyacinth, municipal solid waste, sewage sludge, rice straw etc. In Bangladesh most of the biogas plants are based on the cow dung for its availability. Cow manure have a biogas yield of 150 350 lit/kg of VS and rice straw have 170 280 lit/kg of VS.
Rice straw was not used as raw material for biogas production last time because bacteria cannot easily break down its cellulose due to complex physical and chemical structure of lignocellulosic biomass as well as gas production yield was low compared to other available raw materials although rice straw is the most available material in the country. In the recent time researchers have found out the technology to use rice straw as a raw material for biogas production with increased gas production yield.
Rice straws are lignocellulosic materials with a low bulk density and relatively high silica content. The main chemical components on a dry basis are cellulose, hemicelluloses, lignin and ash. Lignin is found in the middle lamella and adjacent primary cell walls of the residue tissue, and as such it encapsulates the cellulose and hemicellulose fractions found primarily in the secondary cell walls. Rice straws have relatively high proportions of silica-rich ash. Very recently researchers in China have developed a solid-state sodium hydroxide (NaOH) pretreatment process that can boost the production of biogas from rice straw by almost 65% by increasing its biodegradability (Xiujin Li. et. al.).
They also observed degradation of 16.4% cellulose, 36.8% hemicellulose, and 28.4% lignin, while water-soluble substances were increased by 122.5%. The ester bond of lignin-carbohydrate complexes (LCCs) was destroyed through the hydrolysis reaction, releasing more cellulose for biogas production. The linkages of inter units and the functional groups of lignin, cellulose, and hemicellulose were either broken down or destroyed, leading to significant changes of chemical structures. The original lignin with a large molecular weight and three-dimensional network structure became one with a small molecular weight and linear structure after NaOH pre-treatment. The cellulosic crystal style was not obviously changed, but the crystallinity of cellulose increased.
The changes of chemical compositions, chemical structures and physical characteristics made rice straw more biodegradable and thus responsible for enhancement of the biogas yield.
In another study it was found that pre-treatment of rice straws reduce the retention time in anaerobic digestion process (F. Wenjie. & Xiujin Li.). Ruihong Zhang and Zhiqin Zhang investigated the effects of different pretreatment methods, physical (mechanical), thermal and chemical (ammonia) treatment, on the digestion of rice straw at the mesophilic temperature of 35°C. A combination of grinding (10-mm length), heating (110°C), and ammonia treatment (2%) resulted in the highest biogas yield, 0.47 l g-1 VS-1 fed, which is 17.5% higher than the biogas yield of untreated whole straw. Pre-treatment temperature has a significant effect on the digestibility of straw.
For proper anaerobic digestion, the raw material should have a carbon to nitrogen ratio of approximately 30:1. Rice straw has a carbon to nitrogen ratio of 80 -100:1. Therefore to attain the correct ratio a source of nitrogen such as animal manure, night soil, ammonia etc must be added. Using the formula we can easily find out that how much manure or other nitrogen rich substrate we need to add to attain the preferable C/N ratio. The fibrous nature of rice residues may pose a problem, as residues tend to float and form a hard scum on the surface of digester. In this case, special measures must be taken -- maceration of the slurry before feeding into the digester and intermittent or continuous mechanical mixing during digestion (Barreveld, 1989; Marchaim, 1992).
Mixing of nitrogen rich manure i.e. cow dung with rice straw will increase the specific gravity of the mixed substrate, which will also reduce the floatation tendency of the rice straw in the reactor. In addition we can introduce UASB (Up-flow Anaerobic Sludge Blanket) type reactor in place of conventional fixed dome and floating type reactor. It will increase the loading rate, eventually reducing the volume in reactor. The three phase separation system in UASB reactor will also help to reduce the floatation of rice straw as well as formation of hard scum.
Bangladesh is an agricultural country and rice is the main agricultural product which is cultivated almost throughout the year resulting huge amount of rice straw left in the agricultural field. The common practice with this waste straw is to burn in the field which cause air pollution and other is to plough in the field as according to agricultural experts straws are rich source of organic manure. But rice straw requires a lot of time to decompose while undecomposed straw cause problem during sowing.
On the other hand if we use rice straw as a raw material by setting up rice straw based biogas plant we can reduce the air pollution as well as decomposed slurry can be used as organic fertilizer in the paddy field. Biogas produced from the plant can be used for running an internal combustion engine to produce electricity which can be used for running electrical pumps as well as biogas can be directly used in modified diesel engine to run shallow pump or low lift pump for irrigation in rural areas where electricity supply is not adequate.
Surplus electricity and biogas can be distributed to the adjacent farmer families for lighting and cooking purposes. If it can be done total socio economic scenario will be changed dramatically.
In India, Punjab has taken a project to use rice straw for the generation of electricity. Punjab roughly produces 100 million tonnes rice straw which would yield 108 MW of electricity. The first plant will be operational within this year. So why should not we, when we are facing huge energy crisis? This is the high time to think about the alternative way to produce energy.
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