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ENERGY PRODUCTION FROM AGRIWASTE

Background information of Study

Biomass is the culmination of organic materials such as plant matter and manure that have not become fossilized and are used as a fuel or energy source. (Dictionary of the English Language, 2016). Plant biomass is a renewable source of energy that is produced through photosynthesis during Carbon IV Oxide assimilation to form carbohydrates and oxygen under the influence of sunlight. The most abundant source of plant biomass on the earth is lignocellulose. It is composed of carbohydrate polymers, cellulose and hemicellulose, and an aromatic polymer, lignin. All three polymers form constituents of the plant cell wall and are collectively called lignocellulosic biomass. Upon being broken down, these polymers yield chemical components that can be used to make biofuels. After all, cellulose is a homopolymer of D-glucose which if extracted can be fermented to produce biofuels. Hemicellulose is a polymer of a variety of sugars and lignin has a polymer backbone made from phenolic groups.

A barrier to the production of biofuels from biomass is that the sugars necessary for fermentation are trapped inside the lignocellulose. Lignocellulose has undergone evolution to make it resistant to biodegradation due to the rigid and compact plant cell wall structure. This resistance to degradation referred to as, recalcitrance, is due to the crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin via ester and ether linkages. The glucose polymer chains in cellulose are largely insoluble and exist in crystalline microfibrils that make the sugars difficult to access. The cellulose microfibrils are attached to hemicellulose, which contains a variety of sugars, making it more complicated to convert it into a single product e.g. ethanol. Surrounding these two is lignin which is a highly insoluble complex branched polymer of substituted phenylpropane units that are held together by strong ether and carbon-carbon linkages making it difficult to break down. The composition of lignin also varies from plant to plant and its true structure remains unknown. In addition, cellulose is difficult to hydrolyse due to the fact that its D-glucose units are held together by ?-1,4-glycosidic bonds that can solely be broken down by the enzyme cellulase which is majorly present in microorganisms. (Sanderson, 2018)

During the course of evolution, microorganisms have developed physiological mechanisms in order to survive. One of these is the development of cellular mechanisms in order to take energy from plant biomass during saprophytism. These microorganisms have developed the ability to secrete enzymes that degrade the plant cell wall, releasing sugar monomers that can be used as substrates for the metabolism of the microorganism. The capacity to degrade lignocellulose is mainly distributed among fungi and bacteria. In this way, microorganisms are able to recycle cellulose, hemicellulose and lignin. (Souza, 2013)

There include a variety of enzymes produced by different fungi species that break down lignocellulose. Cellulose is broken down by three different cellulases. Hemicellulose is hydrolysed by hemicellulases such as xylanase and lignin is degraded by enzymes such as laccases, peroxidases and manganese peroxidase.

Cellulases are mainly of three types: Endoglucanases that randomly cleave the internal bonds at amorphous sites to create new chain ends, exoglucanases (cellobiohydralases) that cleave 2-4 units from the newly exposed chain ends formed from the endoglucanase activity and cellobiases (?-glucosidases) that hydrolyse the exocellulase products into the final monomers. These monosaccharides produced, namely D-glucose could then be fermented to produce biofuels such as bioethanol.

Biofuels have several advantages over fossil fuels. Their raw materials i.e. biomass, are abundantly available globally thus they are a renewable source of energy as opposed to fossil fuels that will eventually run out. They generally emit less greenhouse gases than fossil fuels do thus reducing global warming. Biofuels are environment-friendly and minimally result in air and land pollution as compared to fossil fuels that when mishandled lead to oil spills and when burnt produce toxic fumes. In addition, biofuels do not contain the enormous amount of sulphur that fossil fuels contain which usually results in acid rain upon their combustion. (Advantages of biofuels, 2015)

Problem Statement and Justification

Fossil fuel use is in the process of being abolished in Kenya and all over the world. This is to fulfil the goal of boosting the world’s economy, environment and energy security. (Wynn, 2010) Fossil fuels in themselves are already being depleted due to the fact that they are non-renewable. In addition to that, the amount of greenhouse gases especially Carbon IV Oxide that they emit has already lead to climate change through global warming. This has resulted in the growing demand for biofuels as a clean, affordable and reliable source of energy. However, diversion of crops to produce biofuel instead of food has been found to cause more harm than good. This is due to the fact that it competes with the food supply causing more hunger among a nation’s people all in an effort to reduce oil insecurity. Food starch therefore cannot be used to produce biofuels. (Tenenbaum, 2008) An alternative source of lignocellulose biomass would be agricultural residues such as maize cobs, sugarcane bagasse, rice husks, fruits, vegetables and wheat straw.

The high cost of commercially produced cellulase enzyme is one of the major barriers of bioethanol production from lignocellulose biomass. Using the minimum ethanol selling price as the indicator to show the impacts of varying enzyme prices, enzyme supply modes and process parameters, it was revealed that the enzyme cost drives the cellulosic ethanol price below the minimum profit point. It is thus necessary to explore local sources of cellulases to degrade lignocellulose e.g. on-site enzyme production. (Gang Liu, 2015)

CHAPTER TWO

LITERATURE REVIEW

Energy Status in Kenya

There are three main sources of energy in Kenya: Biomass (69%), Petroleum (22%) and Electricity (9%). Biomass, in the form of wood fuel and charcoal, is widely used in the rural areas, mostly for cooking and heating. It is estimated that 83% of the population relies on biomass as a source of energy and this overreliance is most likely due to the lack of electrification in rural areas. (Energy 2018 Kenya, 2018) As a result, there has been exerted pressure on the forest and vegetation stocks to produce biomass which has led to the acceleration of land degradation processes. (Kiplagat, 2011)

Currently, Kenya imports 100% of her petroleum needs. The oil import bill in 2008 consumed 55% of the country’s foreign exchange earnings from exports. (Kiplagat, 2011) However, economically exploitable oil deposits were discovered in the north-western region of Kenya in 2012. It was expected that Africa Oil and its partner Tullow Oil, who made the discovery, would start small-scale production of crude oil to be transported by road and rail to the Kenyan port of Mombasa in 2017. Unfortunately, this has not happened. (Energy 2018 Kenya, 2018)

Electricity in Kenya is generated from geothermal (47%), hydropower (39%), thermal (13%) and wind (0.4%). The consumption of electricity has increased by an extraordinary 73% from 2007-2008. (Energy 2018 Kenya, 2018) Despite increasing installation capacity, the demand for electricity in the country is rising faster than the supply. This has been attributed to increased economic growth. Thus, in general, the country faces hardship in meeting her energy demand due to low supply.

Kenya Vision 2030 and the Second Medium Plan 2013-2017 identify energy as one of the infrastructure enablers for Kenya’s transformation into a “newly-industrialising, middle–income country providing a high quality of life to all its citizens in a clean and secure environment”

Access to a reliable, safe, clean and sustainable energy source such as agricultural waste as a raw material of biomass for biofuel production, is key for achievement of this vision.

Benefits of Biofuels

Biofuels are produced from biomass that is abundantly found all over the world. This makes them a renewable source of energy unlike their counterpart, fossil fuels, that pose the threat of eventually running out. Since biofuels are not composed of hydrocarbons, they generally produce lower levels of greenhouse gases upon combustion and thus do not contribute to the “greenhouse effect” that has led to global warming. In addition, they do not release sulphur and have much lower toxic emissions. The primary use of biofuels could lead to economic independence in matters of energy demands. This would be due to the fact that they are produced from a range of organic matter which is available in infinite amounts and is inexpensive to produce. It would alleviate the need for petroleum importation. The major biofuels, bioethanol and biodiesel, can be used in current automobile designs with minimal to no changes to the engine. This reduces cost of machine maintenance. The production of biofuels from agricultural waste offers an excellent route to the disposal of such problematic lignocellulosic residues such as stalks, leaves, husks, cobs etc. These would otherwise be damaging to the aesthetics of the surrounding environment. (Advantages and Disadvantages of Biofuel They Never Told You)

Biological Conversion of Cellulosic Biomass to Energy.

The Eastern African region generates huge amounts of agro-waste from agricultural activities that is typically underutilized, untreated and more often than not, it is disposed of in environmentally unfriendly manners that play a role in the emission of greenhouse gases and therefore climate change. Bio-refining of waste biomass involving the production of bio-energy such as biofuels and biogas is an excellent alternative for adding value to this waste as a bio-resource. (Sustainable utilization of agro-industrial wastes through integration of bio-energy and mushroom production. )

Biogas is a clean and renewable energy that can be produced from agricultural waste through anaerobic digestion. This involves a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. The resultant gaseous mixture is composed of methane and Carbon IV Oxide which burns in a clean way that does not result in environmental pollution. It can be used for cooking, heating and lighting homes. (Joseph, 2018)

Production of biofuels such as bioethanol from lignocellulose biomass involves a series of reactions. The biomass, e.g. sugarcane bagasse, wheat and rice husks, maize cobs etc. are first mechanically broken down by grinding to increase the surface area for enzymatic action. They are then treated with Sulphuric acid which removes the hemicellulose-lignin network present thus allowing efficient enzymatic degradation of cellulose. This acid treatment is usually alongside steam treatment that breaks the cell wall to avoid enzyme biodegradation interruptions. Cellulose solubilisation then follows. It is made possible by the use of cellulase. Once reducing sugars have been produced, the lignocellulose biomass undergoes fermentation with yeast under anaerobic conditions to produce ethanol. The produced bioethanol is then distilled for purification and lastly undergoes dehydration to remove any water traces. (Onuki, 2006)

A large variety of microorganisms, mostly fungi and bacteria, are capable of producing lignocellulose degrading enzymes such as cellulases. With the daily evolution of biotechnology, there exists great potential to develop new enzyme sources that offer higher specificity, better resistance to various inhibitors and better thermal and pH stability in order to maximize sugar yields at favourable costs. (Omwenga, 2017)

Solid-State Fermentation

Solid-state fermentation basically uses substrates that are solid in nature e.g. maize cobs, wheat and rice husks, sugarcane bagasse, woody biomass and paper pulp. It has been successfully used for the production of enzymes and secondary metabolites. There are several advantages of using SSF over submerged fermentation (SmF) that utilizes liquid substrates such as molasses and broths. These advantages include higher fermentation productivity, high end-concentration of products, higher product stability and lower demand for sterility due to the low water activity used. (U, 2004) The substrates are utilized very slowly and steadily such that the same substrate can be used for long fermentation periods. SSF is best suited for fermentation techniques involving microorganisms such as fungi that require less moisture. Bacteria cannot be used due to their need for high water activity. (Omwenga, 2017)

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