Standardization of the Electricity and Economic Potentials of Landfill Gas (LFG) in Lagos, Nigeria.

Globally, various practical data and scholarly estimations of the electricity potentials of landfill gas (LFG) have been forwarded and these can be juxtaposed for estimations in the megacity called Lagos. The calculated values were between 63.22- 700MW of derivable electricity. However, in order to limit observable disparities and ambiguities in these derivations and thus allow for more accurate projections, these estimations can be gauged using as template; -stoichiometry, establishing 50% of landfill gas as methane, assuming 50% of this volume as recoverable, and using a proposed engine efficiency of 30%. This standardization projects a theoretical mean achievable electrical power of 121.69 MW for the Lagos area from a population of about 21 million with a generation per capita (GPC) of 0.63kg with biodegradable content of about 60%. The yearly electrical energy was placed at 1,066,004.4 MWh with tariff revenue in excess of US$ 106.6 million /yr. An accruing carbon credit of about US$75.59 million /yr is expected from certified emission reduction (CER). The projected derivations can be used as models for evaluation of the landfill gas and electricity potentials in many parts of the world.


INTRODUCTION
Anaerobic degradation of biomass in the Lagos landfills in the presence of excess water, bacteria and a pH of about 7 produces methane which can be harnessed for electricity. Globally, several studies have been done to estimate methane production out of municipal landfills. These are mostly based on Monod first order decay equations which are called first order decay models. First order models have a linear relation with maximum potential of methane production per weight unit of waste as well as an exponential relation with decay rate and time. A few models are classified as zero order models in which methane production is assumed to be fairly constant against time (Kamalan, et al., 2011 ). However, in order to limit observable disparities and ambiguities in these derivations and allow for a more accurate projection, these estimations can be standardized using as template: stoichiometry, establishing 50% of landfill gas as methane, assuming 50% of this volume as recoverable, and by using engine efficiencies of 30%. Methane is lighter than air and can accumulate beneath structures and buildings, resulting in vegetational stress and toxicity due to H2S and volatile organic compounds (VOCs), corrosion due to CO2-created acidity, greenhouse gases and air emissions. It is explosive above 5 to 15% by volume and causes subsurface migration offsite (up to 150 m).
Lagos has about 27.4% of the country's urban population but which has had its electric power allocation plummeting from over 800MW ( of the country's 4,000MW) in the last decade to below 300MW in 2011 (Suberu et al., 2012) is in dire need of alternative energy sources . An added incentive for methane exploitation schemes in landfills is that it attracts carbon credits based on the Kyoto Protocol. The environmental benefits of methane capture are expected to be exponentially higher in terms of green house gas mitigation. Therefore, the accurate estimation of this resource is important in view of exploitation designs and projections in the Lagos area.
The thrust of this study is to standardize some previous attempts at the estimation of the green energy potential & environmental benefits of the landfill gas (LFG) from around the globe for estimations in the Lagos area. It also attempts to establish a benchmark and threshold for these estimations in the Lagos context using stoichiometry and gas laws.

MATERIALS AND METHODS
The study was undertaken after a reconnaissance geological assessment of all the government operated landfills areas in Lagos to ascertain their suitability for landfill gas capture. The Lagos Waste Management Authority (LAWMA) is the Government Agency statutorily charged with Solid Waste Management in Lagos State. It is saddled with the responsibility of collecting, transportation and general handling of all the solid waste generated from different sources within Lagos. Given its associated successes in the current political dispensation, the LAWMA's model is being replicated in such places as: Federal Capital territory (Abuja) Ogun State, Plateau State, Ekiti State, Cross Rivers State, Osun State, Banjul, Gambia, Accra Metropolitan Authority (Ghana), City of Freetown (Sierra Leone), Addis Ababa, Ethiopia,etc. Its size is about 3,577 km 2 , has about 180km coastline and is about 4.6m above sea level (A.S.L.) . It has about 22% water coverage consisting of rivers, lagoons, creeks and streams (LAWMA, 2011).
The population is currently estimated at 21 million with a population density of about 6,030/km 2 . It has 20 LGAs & 37 LCDAs with over 2,600 communities. The megacity is home to trans-national corporations and national conglomerates headquarters and has the largest Stock Exchange in West Africa. It is home to 200 firms (out of 250) listed on NSE . It has over 2000 industrial complexes (65% of country total), 10,000 commercial ventures (70% of country total) and 22 industrial estates & several business districts. Also known with the sobriquet '' Centre of Excellence'', it contributes about 31.89% to national gross domestic product GDP -(2004), 60% of nation's value added manufacturing and 65% of nation's value added tax VAT.

GEOLOGICAL SETTINGS
The geology of the landfill areas suitable for landfill gas exploitation is essentially that of the Oligocene to Pleistocene Coastal Plain Sands. The name Coastal Plains Sands (now also called Benin Formation) was introduced by Tattam (1943) to describe the extensive red earths and loose, ill-sorted sands underlying the Recent deposits of the Niger Delta. The nomenclature is well established in the stratigraphy of the Delta and it has been retained in the south-western coastal sedimentary basin, although the abundance of clays in the Formation in this area do not make it entirely appropriate (Jones and Hockey, 1964).
The Coastal Plains Sands consist of soft, very poorly sorted, clayey sands, pebbly sands, sandy clays, pockets of shale, and rare, thin lignites. They are indistinguishable in the field from much of the Ilaro Formation and from the basal continental beds of the Abeokuta Formation (

ENVIRONMENTAL BENEFITS OF METHANE CAPTURE
Methane has 23 times the globe warming potential of carbon dioxide (IPCC, Third Assessment Report TAR 2001). According to Scheehle et al. (2006), an estimated 12% of methane emissions are caused by landfilling of wastes. Another study by Melack et al.( 2004) and Ramos et al. (2006) noted that about 4% of global warming scenario is being induced by methane emissions from man-made waste dams. Furthermore, a study by Lindberg et al found methyl mercury in the water vapour that condensed out of the gas emanating from a Florida landfill. Although even mercury in its elemental form is toxic, its most poisonous embodiment is methyl mercury, the result of a chemical modification by bacteria.This collaborates previous finding in wetlands, where researchers had previously identified certain bacteria that methylate natural, inorganic mercury derived from minerals. This same family of microbes resides in landfill. (Science News, 2001).
The IPCC (2013) Report asserted with about 95% certainty that humans are the dominant factor in view of climate change since the 1950s. Therefore, the ability to capture methane from the Lagos landfills will help to utilise this waste to energy (WTE) potential and mitigate contributory effects to global warming and its attendant consequences.
Landfill gas (LFG) exploitation is one of the critical areas of a modern integrated landfill. In this regard, portions of the Olusosun landfill have been fitted with underground LFG collection pipes for eventual exploitation (Fig. 6). All the landfills except Epe have recurring incidences of spontaneous fire outbreaks owing to the ignition of the released over-pressured-methane by solar insolation and/ or electromagnetic sparks during lightning.
Discharge of Green House Gases (GHGs) and the management of municipal solid waste (MSW) continue to be a major challenge particularly in growing economies. However, these are resources which can be converted to green energy (Yedla S, et al (2001). Landfill gas is continuously generated due to the anaerobic degradation of the organic fraction of solid waste. Therefore, in a landfill in which an extracting system is not installed, there will be an over-pressure that will force the biogas to be released into the atmosphere (Surroop and Mohee, 2011) In the Lagos landfills, explosions often occur with attendant risks on surrounding buildings and manholes where the gas accumulated may be accidentally ignited by use of open fire and electrical sparks. To reduce explosion hazard at landfill and to reduce emissions of methane, landfill operators can either collect and flare the gas or harness it to recover energy. Both options address local air quality and safety concern but the second option can capitalise on the energy value of landfill gas and displace the use of fossil fuel. Offsetting the use of fossil fuel to generate electricity reduces further greenhouse gas emissions and pollutants, including sulphur dioxide, which is a major contributor to acid rain (Yip and Chuan (2008). ∞ = ( standardized using 50% of LFG as methane, recoverable methane set as 50%, engine capacity= 30%)

THEORY AND METHODS Estimation of the landfill gas and Electricity Po of Lagos Using stoichiometry
Biomass materials which include paper, fo yard wastes, wood, leather, cotton and wool, co about 60% of the Lagos municipal solid waste LAWMA, 2011). The rest are petrochemical plastics), inorganic materials such as metals, glass, According to Themelis and Ulloa (2006), by us ultimate atomic analysis of various types of was the atomic weights of the respective elements, possible to derive the composite molecular fo corresponding to mixed food wastes and paper. Th Mixed food and green wastes: C6H9.6O3.5 N0.28 S0.2 Mixed paper: C6H9.6O4.6N0.036S0.01 . If the minor elements are excluded, the average mo structure of organic compounds in municipal solid (MSW) can be approximated by the mo composition C6H10O4 . It is interesting to note th composition corresponds to the structural formu least ten organic compounds, such as ethyl buta acid, succinic acid, adipic acid, ethylene glycol di etc.
Shortly after MSW is landfilled, th components start to undergo biochemical reacti the presence of atmospheric air (that is near the of the landfill), the natural organic compoun oxidized aerobically, a reaction that is sim combustion because the products are carbon diox water vapour.
The black colour of the leachate in conjunctio the lack of sunlight inside the leachate reservoir the oxygen levels to decrease markedly leading appeasement of anaerobic conditions. (Junqueira 2000). This anaerobic process is the essential rea the landfill and takes place in three stages. Firs complex organic matters are hydrolyzed by ferme

STANDARDIZATION OF THE LAGOS LFG ELECTRICITY POTENTIAL
In 2006, Themelis and Ulloa gave the following theoretical estimation for methane generation from municipal solid wastes (MSW) in USA: 1 tonnes CH4 → 1,400Nm 3 CH4 ...(16) (This is 100% of theoretical/ stoichiometry) (Recall: 1 tonne MSW → 0.417tonne C6H10O4 → 0.149tonne CH4 → 208.6 Nm 3 CH4 .) By juxtaposition, at 100% recovery of generated methane, in the Lagos area the derivable maximum electricity using internal combustion engines of 30% will be: 243.36MW However in practice, in many of the US landfills: 1 tonne MSW → 100 Nm 3 CH4 ... (17) (i.e. about 48 % of theoretical ). This is supported by the estimation that about 50% of produced methane is recoverable. This means that an electricity of 116.81 MW is achievable.
(2) Yip & Chuan (2008) Akujieze and Idehai (2013) made series of comparisons with some of the aforementioned authors. However, the results obtained were not subjected to the present standardization procedures (Fig. 4).

The Lagos LFG Economic Potential.
Johari, et al (2012) estimated that in Malaysia: 8,196

CONCLUSION
Estimations of the landfill gas (LFG) and electricity potential of Lagos by comparisons with those of previous workers in other parts of the world gave values between 63.22-700MW. The existing disparities and ambiguities in values were significantly reduced by a consideration of stoichiometry and actuality. The standardization here attempted can be further developed to create a credible model for the eventual exploitation of this resource in Lagos and other parts of the world. By subjection to this suggested standardization, the projected electricity is placed at 121.69 MW and this can power over 121,690 homes and provide electricity to a human population of over 0.73 million. Expected annual yield from electricity tariff is about US$ 106.6 million with an accruable US$ 75.59 million/ year from carbon credit. The concomitant effects are expected to be exponentially higher in terms of the reduction of green house gases and the mitigation of other environmental hazards.