Table of Contents
1.1 Types of waste.
1.4 Uses of Biogas.
1.5 Advantages of Biogas Utilisation.
1.6 Potentials of Waste to Energy in Nigeria.
2.1 Digester Design and Construction.
2.1.1 Construction material
2.1.2 Digester Size and Operating Volume.
2.1.3 Total Digester Volume.
2.1.4 Gas Holder Volume (Vg).
2.1.5 Force on Gas Holder.
2.1.6 Loading of Substrate.
2.1.7 Daily Measurement Parameters.
2.2 Response Surface Methodology.
2.3 Financial Analysis.
2.3.1 Estimation of Net Present Value of biogas.
3.1 Technical Performance and Production of Biogas.
3.2 Results of Temperature and pH Optimisation with RSM...
3.3 Net Present Value.
A small scale floating drum biodigester was constructed with low cost, recyclable materials and the biogas generated was optimised with an aim to get the best possible combination of parameters to yield optimum biogas through the aid of a Mini-tab software version 17.0. The volume of biogas produced was 0.098 cubic meters, while the optimisation temperature was 28.4 degrees Celsius, being the highest temperature recorded and pH was 7.1. The initial capital invested took 17 years to breakeven.
Waste to Energy refers to the generation of energy from waste materials such as sewage waste, landfill waste, agricultural vestiges, biomass, and wood-related wastes, which when subjected to various processes can be used in generating energy, particularly electricity or biogas. Humanity over the millennia has utilised environmental resources as the foundation for its developmental drive. The underside to the development drive of the human race is the enormous amount of waste generated by industrialisation and urbanisation which seems inevitably on the rise (Graziano and Matteo, 2010).
Environment can be described as the totality of our surroundings. It consists of both the natural sphere, which encompasses land, air, water, fauna, and flora, as well as the anthropogenic sphere consisting of cities, settlements and other human-induced structures. Furthermore, Odafivwotu and Godwin, (2015) also defined environment as a complex of physical, chemical, and biotic factors such as climate, soil, and living things that act upon an organism or an ecological community and ultimately determines its form and survival . The Bruntland commission in 1987 succinctly defined Sustainable development as “the development that meets the needs of the present without compromising the ability of future generations to meet their own needs”, this definition contains within it two key concepts; that of “needs” and “limitations”. This is captured in the definition stated above where the present generation has to concentrate in meeting its needs whilst remaining within set limitations regarding use and consumption of environmental resources in order to ensure future generations will have access to environmental resources needed to meet their developmental needs.
1.1 Types of waste
Agricultural Residue: These are residues gotten from harvesting of crops e.g. shafts from grains and animal droppings that have high organic content (Diji, 2013). They are mostly left as useless by farmers and in some cases are burned to make way for the next planting season. When these wastes are burned they emit carbon dioxide which has dire effects on the environment.
Forestry Residues: These are wood fuels produced from existing lumbering operations in established forestry such as wood chips, forestry trimmings, sawdust and bark (Diji, 2013).They could also appear naturally from the falling of trees due to natural causes such as rain, erosion and strong winds.
Animal Waste : These are waste gotten from livestock as well as humans. Livestock waste can be a daunting task to dispose especially for large scale dairy/poultry farms. Findings by Edirin and Nosa, (2012) revealed that in 1985 the number of cattle, sheep, goats, horses and pigs as well as poultry in Nigeria was about 166 million this also amounts to about 227,500 tonnes of waste. Furthermore, Dayo (2008) and Illoeje (2004) estimated that Nigeria produces about 61 million tonnes of animal waste. These could also be further estimated to be about 2.93 X 109 KWh and 7.85 X 1011 respectively (Sambo, 2005)
Municipal Waste : These are wastes gotten from household, industrial and commercial sources. This waste can be raw, i.e. unsegregated or segregated (glass, metal paper etc.). (Diji, 2013).
Biogas is a combustible mixture of gases gotten from anaerobic decomposition of organic compounds and constituting mainly of methane, carbon dioxide and traces of water, ammonia, hydrogen sulphide and nitrogen (Meshach, 2010). Biogas has a heat content of 7.5GJ/t and density of 1.15kg/m3. Biogas composition is largely dependent on the organic material/compound being decomposed. When the fatty content of the organic material is high, the methane yield would be equally high. However, when it contains more of glucose, cellulose or semi-cellulosic materials the methane yield is mostly lower (Peter, 2009). Biogas is naturally found in swamps, lakes, tundra, oceans, bowel of ruminant animals and termites. Although, temperature increment could facilitate the emission of biogas from its natural sequesters into the atmosphere.
Methane is a major constituent of biogas which is colourless and odourless. It is the first and simplest member of the paraffin hydrocarbon group, it has a boiling point of -162oC and density of approximately 0.75. Methane gas has a global warming potential 23 times that of carbon dioxide. This makes it a major greenhouse gas, as it contributes about 20% of total greenhouse effect caused by anthropogenic activities.
Table 1.1: Sources of methane and content
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(Source: Ludwig Sasse 1988)
1.4 Uses of Biogas
Biogas like many other fuels has several uses both domestically and industrially as a source of energy in the form of heat or other secondary forms of energy such as electricity etc. The possible uses of biogas are elaborated below:
Cooking: Biogas is mostly used as cooking gas especially in developing countries where low income earners find it difficult to cope with persistent increase in the price of conventional cooking fuels due to market volatility of the sources of conventional fuels. Biogas as a cooking gas is being encouraged in developing countries as a means of discouraging the unsustainable use of wood as cooking fuel. In addition, biogas is a clean fuel and poses no hazardous treat when used indoors compared to wood fuels. Biogas is about 60 percent more efficient than wood, this in turn frees more time for the cooks especially women to indulge in other duties of choice.
Lighting: Biogas can be used as fuel for lighting in rural areas and in areas with poor quality of electricity. There are specialised household gauze mantle lamps consuming 0.07 to 0.14m3 of gas (Meshach, 2010).
Refrigeration Biogas can also be utilized for refrigeration, especially on automatic thermo-siphon machines operating on ammonia and water. Also, Refrigerators that run on kerosene flame could be converted to run on biogas (Meshach, 2010).
Biogas as Mechanical Fuel
Biogas can be used to run mechanical engines such as industrial machines, vehicles, auto-rickshaws, vehicles etc. Biogas is compactable with four stroke diesel and ignition spark engines. Nonetheless, biogas has some impurities such as hydrogen sulphide and water molecules that could damage engines. Hence, for biogas to be utilised, it has to be purified by passing the gas through a wire gauze. Meshach (2010) also discovered that biogas is being used as a mechanical fuel in Nepal to power irrigation pumps.
Meshach, (2010) resolved that the use of biogas for electricity is a much more efficient use compared to using the resource for lighting from an energy standpoint. Biogas to electricity is mostly utilised in rural electrification and to power farms where cow dungs are used as anaerobic substrates. The gas consumption is about 0.75m3 kw/hr with which 25-40 watt lamps can be lighted for one hour.
Biogas can be used for space heating in cold areas. This could be achieved with the help of a heat converter/exchanger that takes the warm air from the biogas combustion chamber to the homes or area of need.
1.5 Advantages of Biogas Utilisation
Biogas has diverse advantages both at the microeconomic and macroeconomic level, impacting the lives of individuals, communities within reach for good. Below are various advantages of biogas utilisation on a small/large scale.
Quality Sanitation and Hygiene: Biogas could be seen as a by-product of a quality/integrated waste management system. Furthermore, anaerobic digestion involves the use of waste, which when left unattended to could result to the development of pathogens and diseases which could have serious consequences on the individual or communities harbouring them.
Social Empowerment: Women are mostly responsible for cooking in most homes in developing countries. In addition, biogas utilisation for cooking is known not to produce sooth and more efficient than conventional kerosene stoves. Furthermore, domestic biogas utilisation makes available time for women of which they could invest in more productive activities such as paid work, education, recreation etc.
Environmental Improvement: In developing countries wood fuel is mostly used for cooking, this practice has resulted in the decimation of forests reserves that act as carbon sinks and protects the soil against erosion etc. Biogas utilisation has tremendous impact on the environment by preserving forest reserves that could have been cut down for the sake of cooking fuel.
Microeconomic Income Generation: The use of biogas is not only advantageous to women by freeing up time for women that engage in cooking but also men/women that are involved in the construction of biodigesters. The skill gained in biogas construction could be put into good use by further construction and maintenance of subsequent biogas plants, providing income for the apprentice and worker.
Macroeconomic Value Addition: Energy gotten from biogas generation would reflect in the economy as value is added in sectors such as agriculture by increasing the marginal productivity of farms by the sludge fertilizer and energy generation. Also, biogas construction skills would create skilled employment for youths which would reflect on the economy.
1.6 Potentials of Waste to Energy in Nigeria
The repressor defined waste as any substance which constitutes a scrap material or an effluent or any unwanted surplus substance from the application of any process. The conversion of garbage into useful materials involves the use of other useful materials. An example to further illustrate this point is the recycling of newspaper which would always require a lot of energy. Adejobi and Olorunnimbe (2012) established in a survey conducted in Lagos metropolis on the composition of household waste revealed as follows: 56% food waste, 12% paper, 10% plastic, 7% glass, 5%metal, 6% textiles and 4% miscellaneous. Food waste being the highest constituent of waste decays and when disposed inappropriately can provide a conducive environment for bacterial growth hence, a haven for germs and diseases of all sorts.
Recycling of biodegradable organic waste is crucial to meeting the requirements of the landfill directives. However Adejobi and Olorunnimbe, (2012) also stated that by utilising land fill waste disposal system we are limiting the potential for reuse and recycling of valuable resources, thereby increasing demand for new resources and generating more greenhouse gases into the atmosphere which apparently is not sustainable. Olokesusi, (1994) research on ring road refuse disposal system in Ibadan discovered that waste disposal facilities are often perceived to have a negative social impact by host communities. Adesina, (1983) described household waste as one of the easiest to monitor and reduce. It is observed that the rate of waste collection and disposal lag behind the rate at which this wastes are generated, this is the reason why waste are littered around cities (Uwadiegwu and Chukwu, 2013). Uchegbu, (1988) noted that big cities like Enugu, Lagos, Kano and Port Harcourt produce an average of 46kg of waste per individual daily. Adejobi and Olorunnimbe, (2012) stated that an average Lagos inhabitant generates 1 kg of waste daily. As living standards rise, the volume of waste also is bound to increase, Rosenbaum, (1974) established that waste production has often been seen unofficially to reflect economic prosperity as wealthier nations produce more waste (Uwadiegwu and Chukwu, 2013)
Table 1.2: Major Nigerian cities and their waste production
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(Source: Ogwueleka, 2009)
2.1 Digester Design and Construction
A floating drum biodigester design was adopted for anaerobic digestion because of its ease of technicality and the efficiency in measuring daily gas production. It would has a feed-in inlet, an outlet that extends downwards for slurry collection and a drain for routine maintenance when necessary. In conformity to conventional floating drum designs, the gas container is incorporated into the digester and protrudes upwards as gas is produced subsequently. A shaft was incorporated to stir the digestate in the digester so as ensure maximum result by maintaining an even digestate mixture.