Globally, approximately 600 million tons of rice paddies are produced each year. On the average, 20% of the rice paddy is husk, giving an annual total production of 120 million tonnes. On the other hand, rice straw produced in the Philippines is estimated to be 10.68 Mt in which 95% is subjected to open-field burning. The concern about the environmental effects of burning these residues in the field. The idea that these residues can be utilized as energy source as biogas. Using this as biogas would lessen rice farm emission.
There are already various energy conversion processes for these rice residues. These include physicochemical, biochemical, and thermochemical processes. Physicochemical processes are not applicable to these rice residues since there is nothing that can be extracted from them. Biochemical process, like anaerobic digestion is more suitable and feasible to farm level. On thermochemical processes, gasification is also suitable as it produces syn gas, bio char and bio-oil. Other thermochemical process such as combustion is also suitable however; its only use is to provide heat. Other processes that yield bioethanol and biodiesel are also suitable. However, these processes are expensive.
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Environment and damaging issues
The common practice of farmers to get rid of rice residues is burning. This practice is greatly discouraged due to its environmental effects, specifically global warming. According to Mussoline (2014), anaerobic digestion of biomass occurs naturally in the environment and if methane from these sources can be captured, then not only could it be used as clean energy source but it also reduces greenhouse gas emissions that contribute to global warming.
She further added that rice straw is commonly tilled back into the soil. It can also be used as fertilizer for the crops. That rice straw in the soil has a half-life of two years and approximately 80 to 90% is decomposed within the first year. She also said that rice fields are often flooded in dormant periods creating anoxic conditions, and the decaying organic matter releases gases such as nitrogen, hydrogen, methane, ammonia, and hydrogen sulphide. Thus, she concluded that the practice of tilling the straw back into the field results in increased methane emissions from rice fields.
Rice hull, has been a predominant problem in terms of its abundance and storage difficulties. Rice hull is usually left outside the rice mill facility. While immature rice paddy (also resembles like a rice hull) is left on the field after threshing. The accumulated amounts of such wastes lead to open burning to clear the field for next planting.
The future of rice hull for energy production
The utilization of rice residues for biogas and gasification will address the growing concern about global warming. Capturing energy from these residues will not only help mitigate greenhouse gas emissions. It will also help obtain fuel which can be readily used (Mussoline, 2014). The product of gasification such as the carbonized rice residues has various uses. According to PhilRice (2015), these can be good sources of organic fertilizer. Rice hull are good soil conditioners, which replenish nutrients lost. It can also serve as waste water filter and water purifier. This can also be a good base material for microorganism inoculants.
Hence, utilization of rice residues would serve as environmental conservation. It could help in energy recovery approach. The production of gas, char and oil out of these rice residues is a pivotal approach to farming. Where energy can be generated after rice harvesting season. Bio gas produced can also serve as fuel that is beneficial to household. Which can be used as cooking and providing heat.