Biogas waste management

The decomposition of organic matter by biological activity produces gases (on average between 100 and 400 m3 per tonne of household waste and at a rate of between 5 and 15 m3 per tonne and per year, for US-type landfills) which can have harmful effects on the environment and the health of the population.

They must therefore be collected and processed. Once collected, the gases produced must be burned or used for energy purposes. When the depth of the landfill exceeds 8 meters, it is necessary to pump the gases (e.g.: centrifugal pumps from 150 to 500 m3/h) in order to limit the accumulation within the wells and also increase their radius of action.

Regulating pumping at each well, by controlling the opening of the valve at the wellhead, limits the entry of air into the waste mass (risk of flammability) and maintains a relatively low methane supply. constant at the level of the biogas treatment devices (maximum 20% variation compared to the optimal operating conditions).

When energy recovery is not possible due to insufficient flow or unsatisfactory quality (i.e. when the methane content in the biogas is less than 35%), it is simply burned in the air. free in a flare. The flare is a device for eliminating biogas with an excess of air of 10 to 15% automatically regulated. The particular design isolating the flame in a cylindrical combustion chamber avoids disturbances linked to atmospheric conditions and the formation of colder zones.

Ideally, the operating temperature is 150 to 250°C higher than the ignition temperature of the gas (i.e. 600°C for methane). In general, the flare operates at more than 1000°C with a peak at 1200°C for 6 seconds or at 1200°C with a residence time greater than 0.3 seconds allowing the decomposition of complex and toxic organic molecules ( aromatic, chlorinated, sulphurous, etc.). Visit here to read Philadelphia Recycling Blueprint.

It is also equipped with a self-ignition system and continuous monitoring of its operation. It should be noted that, even if the biogas is recovered for energy, the presence of a flare is essential on the site, in particular to supplement the recovery systems in the event of a stoppage or overload.

Knowing that biogas rich in methane has a calorific value of approximately 4 to 5 kW per cubic meter, equivalent to that of half a liter of heating fuel, it seems interesting to be able to valorize it. Three possibilities are generally possible: combustion in a boiler or use as fuel in a gas engine or a gas-steam turbine. The first possibility is by far the least expensive and the easiest to implement if there are local opportunities to use the calories produced (in the form of steam).

Transporting biogas is in fact not very profitable because it is very corrosive and its calorific value is low (it often has to be mixed with natural gas for the burners to work properly).

To enable biogas fueling, internal combustion engines most often, or sometimes diesel or multi-fuel, (of the order of 400 kW to 2 MW) have been adapted in order to compensate for the lack of lubricating properties of the mixture and the risks of corrosion. Using a gas containing molecules that can be toxic, particular attention must be paid to engine tuning in order to guarantee a certain quality of the exhaust gases (absence of sulfur compounds, dioxins, etc.).

It is the same for the turbines, even if the temperatures reached in the combustion chamber are normally higher than 1800°C. The advantages of gas turbines are in terms of weight and the frequency of machinery maintenance operations. It is important to note that the recovery cannot cover all of the biogas produced within the landfill.

In general, the effective recovery rate of methane is estimated at between 40 and 60%. The rest is biogas, the methane content of which is too low to allow the operation of an engine or a boiler without additional fuel.

That said, an optimum can be defined between the energy loss at the flare during the incineration of lean biogas and the benefit to be generated by recovering it in a mixture with another fuel such as natural gas. Added to this are also the prospects for cogeneration by diversifying energy flows: steam, electricity, mechanical energy and hot water by recovering heat from combustion gases.