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Concerns over sustainable growth have been on the increase for some time. It is linked to a variety of causes, including environmentally unfriendly manufacturing practices in the manufacture of chemicals and fuels. To find an alternative, experiments and analysis have been undertaken to develop environmentally sustainable biofuels and biodegradable materials from organic sources. In order to accomplish this, the US National Renewable Energy Laboratory introduced a biorefinery plan that incorporates processes and equipment that generate fuels, chemicals, and electricity from organic sources. It is at the main focus of this paper to discuss ethanol production from rice straw as one of the biorefinery processes. Being mostly abundant renewable material, this lignocellulosic plant is considered much more attractive for the production of ethanol.
Key words: biorefinary, biofuels, ethanol, rice straw.
Bioethanol Production from Straw Rice
Rice straw is worldly considered to be among the lignocellulosic waste materials posing a lot of threat to development. It is connected with the large production of rice to feed the ever-increasing human population which in return leaves back millions of tons of rice straw, some of which are used as animal feeds and a large portion being wasted (Binod et al. 2010). The limitation options to dispose rice straw waste makes the farmers opt for field burning since the left wastes degrade slowly in the soil and the number of diseases harbored in their stems. Burning significantly affects health due to the increased air population. Bioethanol production is a technological process that bridges the gap in the wise use of rice straw while at the same time providing a lot of benefits for humanity (Singh, Srivastava and Shukla 2016). The production process of this biofuel product is a step by step treatment process that largely considers the availability factor of rice straw.
“Rice straw is a lignocellulosic crop residue that is worldly abundant” with annual production of about 731 tones distributed in Africa, Europe, the U.S.,” and Asia. Besides straw, the other primary residue from rice farming that is used in energy production is husks. The very significant features behind rice straw being preferred as the” potential feedstock for ethanol production is the high content of cellulose and hemicelluloses that can readily be hydrolyzed into fermentable sugars (Binod et al. 2010). “Chemically, the straw is composed of lignin, hemicelluloses, and cellulose. This chemical composition majorly influences the efficiency of bioenergy production.”
Another significant factor considered in bioenergy fuel production is the quality of the feedstock. This is determined by the content of ash and silica in ash. As a feedstock, the advantage of rice straw is its relatively low alkali content total. The straw quality, however, varies within seasons and regionally. When exposed to the field, which normally happens, a lot of alkalies and alkaline compounds leaching takes place thereby improving the quality of the feedstock.
Ethanol Production Process
The technology for converting cellulose, hemicelluloses, and lignin components of rice straw as feedstock involves two platforms-sugar and synthesis gas platform. In sugar platform, there is the first conversion of hemicelluloses and cellulose to fermentable sugars (xylose, arabinose, glucose, mannose, and galactose) which are further fermented for ethanol production. To generate these sugars, cellulose and hemicelluloses are hydrolyzed using either enzymes or acids (Binod et al. 2010).
Synthesis/syngas platform involves the subjection of the biomass through gasification process where complete combustion without or little oxygen takes place. The gaseous products of this step (carbon monoxide and hydrogen) are either catalytically converted to produce ethanol or fermented by specific microorganisms to ethanol. The difference in the platforms is the utilization of the rice straw components to produce ethanol. Only carbohydrates fractions are utilized to produce ethanol whereas all the three biomass components are utilized. The whole process involves various treatments of the biomass, hydrolysis, and fermentation.
The pretreatment step is to lower cellulose crystallinity and increase the surface area of biomass, hemicelluloses removal and the breaking of lignin seal. The heterogeneous complexity of carbohydrate composition involving lignin layer is a great hindrance in the enzymatic hydrolysis step. Exposure of cellulose and hemicelluloses by removing lignin seal is thus necessary for enzymatic action. Pretreatment methods are chemical, physical and thermal, and the combination of these methods.
Physical pretreatment aims to increase the pore sizes and the surface area accessibility as well as decrease the crystallinity and cellulose polymerization degrees. In degrading lignocellulosic residue, the most used pretreatment physical methods include grinding, milling, irradiation, steaming, temperature, and pressure.
Chemicals for rice straw pretreatment are ammonia and alkali. The application of alkaline solutions (Sodium hydroxide/potassium hydroxide) removes a fraction of hemicelluloses and lignin. This effectively increases the ease of access of cellulose by enzymes resulting in a sharp saccharification yield increase. It is the most effective method compared to acid or oxidative reagents to break ester bonds between lignin, hemicelluloses and cellulose (Binod et al. 2010). The pretreatment is performed under low temperature, high base concentration and with relatively long time.
The desirable features of ammonia as a pretreatment reagent is the effective action in the swelling of lignocellulosic materials, high selective reaction with lignin over those with carbohydrates, non-corrosive and non-pollutant.
Oxidative pretreatment using oxidizing agents like hydrogen peroxide removes lignin and hemicelluloses thereby increasing cellulose accessibility. The agent detach and solubilize lignin by delignification while at the same time loosening lignocellulosic matrix hence the digestibility of enzyme.
Pretreatment using acid minerals like hydrochloric and sulphuric acids improves anaerobic digestibility at appropriate temperature. Acid pretreatment solubilize hemicelluloses thus making cellulose be better accessible to enzymes (Weerasai et al. 2014). Enzymatic digestibility is also enhanced through organosolv pretreatment where hemicelluloses are removed with cellulose left as rich-residue.
The pretreatment method is significant in providing low energy and chemical use as well as safe and environmental-friendly in delignification. White-rot fungi of Basidiomycete class are the microorganism considered for the biological pretreatment. The method induces the loosening of the cell structures with simultaneous porosity increase.
Combined pretreatment involves combined physical, chemical and biological pretreatment methods. Microwave as a physical pretreatment method, for example, is much efficient when applied with other methods in delignification of rice straw than when applied alone or with alkali alone.
This is an enzymatic reaction process that is significantly dependent on rice straw pretreatment, substrate concentration and the cell loading for higher glucose production output. The hydrolysis of lignocellulosic biomass efficiency increases with the combined use of xylanases, pectinases, and cellulases under optimum conditions of temperature and pH (Banoth et al. 2017). Cellulose contains glucans whereas hemicelluloses have several sugar polymers (Binod et al. 2010). The enzymes cleave the cellulose and hemicelluloses together with the consequent hydrolysis product of cellulose being glucose whereas that of hemicelluloses giving rises to several pentoses and hexoses.
“Producing ethanol from rice straw biomass is highly dependent on the microorganisms that ferment the end products of hydrolysis-glucose, pentoses, and hexoses. Ethanol conversion from hemicelluloses and cellulose can either be through” simultaneous saccharification and fermentation (SSF) or separate enzymatic hydrolysis and fermentation (SHF) processes” (Siti Norfariha, 2015). The optimum temperature for fermenting microorganisms and the hydrolyzing enzymes that result in high production of ethanol is a considered factor in the choice of the fermentation process. Most study reports state 40-50 degrees Celsius as the optimum temperature for enzymatic activity (Binod et al. 2010). This high temperature is not tolerated by the microorganisms with good ethanol productivity and yield. Research studies have identified a seemingly good strain of microbes for ethanol production from rice straw.”
The fermentation process is thus inoculated with microorganisms that can either be yeast or bacteria that acts by digesting glucose and xylose sugars with the production of ethanol and other components that can be used as biofuels (Banoth et al. 2017). Ethanol is separated by distillation. Longer chain molecules can also be gotten through the fermentation or chemical conversion of these sugars, and processed into renewable diesel and gasoline. Water is then removed from the stillage through a series of steps after which centrifugation is applied to recover the lignin-rich residue and processed to generate electricity and steam.
Production of ethanol from rice straw proves to be a viable technology in the production of multispectral biofuels intermediates that are safe and environmentally friendly. This is in line with the sustainable development agenda in the conservation of the environment. Much research is still being conducted for alternative production of biofuels and biodegradable products from organic environmental friendly sources and the production methodologies requiring least input with significantly large output. Research in genetics dedicated to the ways of production strains of microbes with good ethanol production that can tolerate high temperatures for high ethanol and other intermediates from the chemical synthesis of rice straw has been underway with promising outcomes. Considering the worldly abundance of rice straw, it is one of the best choices of organic sources that will save the world from environmental pollution caused by other industrial processes producing fuels and other environment-unfriendly intermediates.
Banoth, C., Sunkar, B., Tondamanati, P., and Bhukya, B. (2017). Improved physicochemical pretreatment and enzymatic hydrolysis of rice straw for bioethanol production by yeast fermentation. 3 Biotech, 7(5): 334.
Binod, P., Sindhu, R., Singhania, R., Vikram, S., Devi, L., Nagalakshmi, S., Kurien, N., Sukumaran, R., and Pandey, A. (2010). Bioethanol production from rice straw: an overview. Bioresource Technology, 101(13): 4767-74,
Ekman, A., Wallberg, O., Joelsson, E., and Börjesson, P. (2013). Possibilities for sustainable biorefineries based on agricultural residues – a case study of potential straw-based ethanol production in Sweden. Applied Energy, 102: 299-308.
Singh, R., Srivastava, M., and Shukla, A. (2016). Environmental sustainability of bioethanol production from rice straw in India: A review. Renewable and Sustainable Energy Reviews, 54: 202-216
Siti Norfariha, M. (2015). Fermentation of rice straw by Vermiwash for Bioethanol Production. Iranica Journal of energy and environment, 6(1): 13-19.
Weerasai, K., Suriyachai, N., Poonsrisawat, A., Arnthong, J., Unrean, P., Laosiripojana, N., and Champreda, V. (2014). Sequential Acid and Alkaline Pretreatment of Rice Straw for Bioethanol Fermentation. BioResources, 9(4): 5988-6001.
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