By: Sadah A. T. Muawad , Department of Mechanical Engineering, College of Engineering, Sudan University of Science and Technology, Khartoum, Sudan
Part 1 of this study can be accessed here.
Landfill Gas Assessment
To assess landfill gas (LFG) emissions from Tayba Al Hasanab landfill, two case scenarios will be investigated. The first one was discussed in the previous article and it’s about setting the forecasting period for 20 years, the landfill’s closing year is 2017, and the acceptance for each year from 2007 to 2017 is 770000 ton/year; Thus the model was set to calculate the amount of methane (CH4)from the starting year until ten years after landfill’s closure. The second scenario will be investigated in this article assuming the landfill’s full capacity is approximately 4,000,000 tons. 2007 is the starting year in which the landfill starts accepting waste; The model will calculate the closure year based on waste design full capacity; In this case the model was set to calculate the amount of CH4 from 2007 (the starting year) until ten years after landfill’s closure, taking into account that the landfill accepts 1500 tons of waste daily. LandGEM was used for both scenarios to calculate future prediction emission rates for CH4 from the landfill’s cells. For the estimation of CH4 generation from landfill cells, the Clean Air Act (CAA) defaults were used.
The following parameters were set in the prediction model for the second scenario;
- CH4 generation constant (k) is specified as 0.02 year -1
- CH4 generation potential (Lo) is specified as 170 m3/mg
- CH4 content in the LFG was specified as 50%
The model simulation terminates in 2014 as the closure year and the estimation of CH4 generation, emitted from the landfill cells is shown in Fig.1. In 2014, 2015, 2020 and 2024 the total CH4 was estimated as 7382 mg/year, 7578 mg/year, 6857 mg/year and 6330 mg/year respectively. The model result, conclude that the 2015 is the peak year for LFG generation and it is noticeable that the emissions value decreases after the peak year due to many reasons such as the landfill maturation. The emissions of greenhouse gases needs to be controlled and utilized; the LFG could be utilized for lighting, cooking and power generation.
Energy Recovery Potential
For both scenarios and based on LFG calculations from Tayba Al Hasanab landfill the potential energy could be recovered from LFG will be calculated, using the following model:
- Energy Recovery Potential = Recovered energy (MJ)
- LHVCH4= Methane lower heating value (MJ/m3)
- M= Methane mass (kg)
- D= Methane density (kg/m3)
- CE= LFG collection efficiency (%)
The following assumptions were considered for the energy calculations:
- CH4 mass for the second scenario (mg/year) was converted into the volume basis by taking the density of CH4 as 0.71 kg/m3
- Using gas engine unit for power generation due to good performance and the ability to generate constant output at various conditions 
- The efficiency of LFG collection system is 60% and the gas engine’s efficiency is 30%
- The lower heating value (LHV) of CH4 is 35 MJ/m3 [1, 3]
The previously predicted amount of LFG was used to calculate energy recovery potential that could be recovered. For the first scenario, as shown in Fig.2, the energy potential for 2017, 2020 and 2027 was 127823279.7 MJ and 120379431.4 MJ and 104652850 MJ respectively. The energy recovery potential for the second scenario, as shown in Fig.3, for 2014, 2015, 2020 and 2024 was 65503010.75 MJ, 67243379.71 MJ, 60844326.08 MJ, and 56166391.99 MJ respectively. 2017 and 2015 represent the peak years for the first and second scenarios respectively, thus the energy recovery potential declines afterward.
Conclusion and Recommendations
Municipal waste in Sudan is currently improperly managed as most of disposal sites and are poorly engineered. In addition, the practice of landfilling municipal waste at open dumpsites or uncontrolled sites has a severe impact on the environment due to; the decomposing of organic waste which produces LFG. LFG mainly consists of CH4 and CO2, both of which are greenhouse gas and contribute to global warming. This study recommends; implementing monitoring procedures for landfill sites for LFG migration. Furthermore, waste to energy technologies should be considered as an alternative source of energy and a waste management solution.
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