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Description of technology

The process of natural anaerobic decomposition occurs during the storage of solid household waste in the MSW landfills with oxygen shortage, high temperature and humidity levels. One of the by-products of this process is landfill gas, also known as biogas, - a mixture of methane and carbon dioxide in the average concentration of 50-75 and 25-50% respectively, with a small amount of impurities (nitrogen, silicon, sulfur, hydrogen sulfide). The landfill gas may also include dozens of different organic compounds. The presence of various components in biogas depends on the waste composition stored in the landfill.

Table 1 – Estimated biogas composition (according to US EPA, www.epa.gov )



Component

Contents, %

1

Methane

50-75

2

Carbon dioxide

25-50

3

Hydrogen

0-1,0

4

Hydrogen sulphide

0-3,0

5

Nitrogen

0-10,0

6

Oxygen

0-2,0

The decomposing waste mass continues to generate gas for 10-50 years in average, with the specific output of 140-280 m3 / t MSW. Gasification reaction process can be described by the following stoichiometric equation:

C6H10O5 + H2O → 3CH4 + 3CO2

The reaction is followed by heat release.

The speed and gas production of this process is determined by the environmental conditions within the polygon (humidity, temperature, pH, percentages of organic fractions).

In natural conditions biogas partly leaches into the atmosphere from the surface, as well as from the slopes of the landfill, some of it also burns out during spontaneous combustions inside the waste layers due to high temperatures.

Biogas collection systems are built to extract biogas from the body of solid waste landfills. The systems include:

- A network of specially equipped vertical wells;

- Horizontal type 1 pipelines for biogas transportation from wells to the gas gathering points;

- Gas gathering points;

- Long-distance pipelines to move the biogas from the gathering points to the biogas utilization facilities (cogenerators);

The wells are constructed by drilling the area on the landfill, creating wells with a distance of 25-30 meters between them. The vertical depth of wells is ¾ of landfill's depth. Each well drains specified blocks of solid waste, has a cylindrical form with a radius of 15 m. Preliminary calculations and computer simulations of gas production inside the waste layers are made in order to make the the biogas collection as effective as possible. This preliminary activities then determine the optimum number and positioning of wells at the site.

The engeneering and installation of wells is divided into several stages.

The first stage - the well casing is put inside the drilled hole to prevent landslides. Then a special perforated steel or plastic pipe with a sealed bottom and a fitted flange connection in the collar part is placed inside the well. The lower part of the well, 0.5 m high, is filled with gravel (fraction size 40 - 70 mm). The space between the pipe and the casing is then filled with a porous material (such as gravel or crushed stone, fraction size 20 - 40 mm) sealing it layer by layer to a depth of 1 - 2 m from the wellhead. At the last stage the well's casing is removed and a 1-2 m thick clay layer is placed around the well to prevent the air to reach into the well. Then the non-perforated part of the pipe, 0,6-0,7 m heigh, is brought to the surface. Wellheads can be protected from mechanical damage with reinforced concrete rings, with a diameter of 1 - 1,5 m.

It is recommended to use perforated polymeric pipes 100-150 mm in diameter for gas wells. Perforation is conducted with a drill 18 mm around the circle, every 60°, the distance between holes is 50 mm. The upper part of the pipe should be 1 - 2 m long and solid, without perforations.

Gas pipes made of low pressure polyethylene marked "gas" type "C" are commonly used for the gas wells and gas transportation. The pipes are connected by welding and must be tested by hydraulic pressure of at least 0,6 MPa, or have a specific certificate corresponding to the valid requirements, standards and specifications for this specific type of pipes.

When using valves,it is essential to take the pressure and temperature conditions, as well as the presence of aggressive environment, into account. Gate circuits, valves, dampers and other similar supplies must comply with the gas industry standards.

In case the amount of methane in biogas, produced by one or more wells is reduced, the wells can be retracted from the system using the valves and thus the quality of gas entering the cogeneration plant is improved.

Figure. 1 — Gas well vertical section

The temperature of biogas inside the waste layer can reach 40 - 50 Co, with humidity up to 5 - 7% vol. When the landfill gas reaches the pipelines, a sharp decrease in temperature occurs and it leads to the formation of condensate, which can be released in significant quantities. To drain the condensate the pipelines are laid under a slope of 3o. Special steam traps are placed in the lower points of the sloping pipeline to ensure the removal of moisture from the system and return it into the body of the polygon (waste layers). Gas pipelines are laid in trenches at a depth of 1 m to prevent freezing in winter. Pipelines should be laid on top of a layer of solid waste the layer should be at least 6 month old.

Pipes are placed on metal or concrete lining (40 - 50 cm long) in steps with 2,5 - 3 m space between them.

The compressor installation with a vacuum pump creates a dilution inside the “type 1” horizontal gas pipelines, pushing biogas to the gas gathering points, where a larger diameter trunk pipeline transports it to the utilization point. Special quality control devices for checking the quality of the biogas are situated in the gas-collecting points.

Before entering the utilization point, biogas undergoes a filtering process (to get rid of any moisture drops) to prevent the equipment corrosion, and in case of high content of hydrogen sulphide and silicon- an additional purification. Biogas is also cleaned from fine and coarse dust.

The utilization of biogas to obtain electricity and heat is conducted in the engines of the cogeneration plants. The electricity produced by the cogenerator can be forwarded to the distribution networks, thermal energy can be sold to nearby consumers (industrial enterprises, farms, greenhouses, etc.). An alternative way of recycling thermal energy is the installation of adsorption machines (the so-called trigeneration) and transforming heat into cold to be forwarded to nearby businesses.

 

Figure. 2 – Cogeneration plant

 

For the first stage of gas burning trials and for the cases of accidental failure, maintenance or required repair of the cogeneration plants, in addition to the cogenerators, a flare with gas combustion efficiency of about 90% is installed at the site.

Fig. 3 - Flares for burning excess biogas

For continuous control over the quantity and quality of extracted and utilized biogas automated monitoring systems are installed on flares and cogeneration plants, including the following devices:

- Flow meters for measuring the volumetric flow of gas that passes through the system;

- Pressure and temperature sensors to calculate the mass flow rate of biogas;

- Stationary gas analyzers that capture the quality of gas (the content of methane, carbon dioxide, oxygen and nitrogen), served into the cogenerators and flares;

- Portable gas analyzers that monitor the quality of gas (the content of methane, carbon dioxide, oxygen and nitrogen) inside the pipeline;

- Thermocouple, which is used for monitoring the temperature of the flame in the flare set;

- Automatic data registration system.

Landfills continue to generate biogas for 20-25 years after their closure, whereas the cost-effective period for an installation lasts for about 15 years.

Figure.4 –  MSW landfill biogas collection and utilization scheme

Utilizing biogas in the cogenerators is not the only possible way to use it. Purified from traces of sulfur and carbon dioxide, biogas can be used as a fuel in combustion engines for cars, or it can be used in thermal plants without the purification.

 

 

OUR PARTNERS

Mariupol municipal council





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