Biogas collection and utilization system
Project "The placement of a biogas collection and disposal system for electricity generation at a solid waste landfill"
The biogas collection and disposal system is designed to remove and use biogas (approximate composition: methane – 40...60%, carbon dioxide – 30.45%, nitrogen, oxygen, hydrogen and other gases – 5. 10%) in order to generate electricity and reduce the negative impact of the MSW landfill on the environment. It consists of the following elements:
- Gas collection wells – 26 units;
- Biogas pipelines – 26 units;
- Main gas pipeline – 1 unit;
- Gas collection points – 4 units;
- Compressor unit – 1 unit;
- Biogas preparation Unit – 1 unit;
- Candle for burning biogas – 1 unit;
- Cogeneration plant – 1 unit.
Wells of the biogas collection and disposal system are located on the area of the Solid Waste Landfill at a distance of 45...50 m from each other. An individual biogas pipeline is laid from each well towards the gas collection point. The pipeline is laid in trenches at a depth of at least 1 m. In the places of gas collection points, biogas pipelines are connected to the main gas pipeline using Control and control valves. Since gas collection points reach the surface of the Solid Waste Landfill, the pipeline elements in them are made of steel.
Along the gas collection points to the site for technological equipment, a main gas pipeline is laid in the trench, which transports biogas to the compressor unit. The latter includes a centrifugal gas blower, as well as a biogas preparation unit and a candle for burning it, which will be used in case of stopping cogeneration modules and/or in the presence of excessive gas formation at the landfill. The condensate accumulated in the main pipeline and the biogas preparation unit is drained into the drainage well, which is located next to the compressor unit.
After the compressor unit, the prepared biogas is fed to a spark plug for combustion, or to a cogeneration unit, where it is disposed of in generator engines with electricity generation. The generated electricity, with the exception of its own needs, is supplied to the general power grid through a transformer and a power transmission line.
Waste taken to the landfill is stored within conditional working maps, the operation of which often depends on weather conditions. Four T-171 bulldozers are used for waste storage, which work alternately. Further compaction of waste with heavier equipment is not used due to its absence. Under such operating conditions, the average waste density at the solid waste landfill is estimated at 750 kg/m33.
Upon completion of waste storage, most of it remains open and in contact with atmospheric air. Filling of waste or isolation with layers of soil in a solid waste landfill is used quite rarely, depending on the available materials. Usually, the thickness of this filling does not exceed 0.3 m. This practice can lead to fires at the solid waste landfill during warm periods of the year and to precipitation entering its body, which is the result of a decrease in gas formation and an increase in the amount of filtrate.
According to official data, the Solid Waste Landfill is filled with waste by more than 90% and has almost exhausted its design capacity. Its operation will last until 2018. However, there are still no plans or places for the construction of a new landfill in the Rivne City Council. Therefore, it can be predicted that the operation of this facility and waste disposal may continue until at least 2026
Equation of the gas formation model
The Solid Waste Landfill is a source of biogas formation (approximate composition: methane – 40...60%, carbon dioxide – 30.45%, nitrogen, oxygen, hydrogen and other gases – 5. 10%). To predict the amount of biogas generated in a solid waste landfill, the National multicomponent model of gas formation developed for the National cadastre of anthropogenic emissions from sources and absorption by greenhouse gas sinks in Ukraine is used.
The update of the national model is based on the well-known first-order attenuation method, which takes into account the composition, properties and practice of waste storage, the capacity and service life of the solid waste landfill, as well as the climatic conditions of the area where the Solid Waste Landfill is located. In accordance with this method, the calculation of the expected amount of biogas generated in the Solid Waste Landfill is performed using the formula:
where Q(t) is the estimated amount of biogas produced during the period t, m3;
- normalizing multiplier that corrects summation;
MWSi - total mass of solid waste stored at the landfill per year, i, t/year;
MWSj,i - content of the category j in solid waste for the year i, mass %;
kj - constant rate of biogas formation, 1 / year;
L0j,i - potential of biogas formation per year i, tbg/t;
t - billing period, year;
x - sum of years for the previous period, year.
The potential of biogas formation is determined by the formula:
where DOCj - total mass of organic carbon decomposed in the waste category j, tS/t;
DOCF - decomposing organic matter content in 1 ton of waste (DOCF=0,5);
MCFi - correction factor for methane generated per year i.
The total amount of biogas generated in the MSW landfill during the period t is obtained by summing the calculation results for each waste category j. Taking into account unforeseen circumstances, the total amount of biogas that can be collected from the MSW landfill during the period t is determined by the formula:
where there are Q'(t) the amount of biogas that can be collected during the period t, m3;
KC - efficiency coefficient of the biogas collection system (KC=0,5);
K - coefficient of Correction for unforeseen circumstances (K=0,7).
When calculating the amount of biogas, the methane content in it is assumed to be 55%, at which the calorific value of biogas is 5,000 kcal (~21 MJ/M3).
