Producing Energy with Biogas

Depending on the origin, i.e. where biogas is produced, we can distinguish between:

	Sewage gas (waste water) Landfill gas (waste disposal sites)
	Biogas (plant fester)
	Fermentation gas (animals, lakes, … )

All of these gases have in common that they undergo a fermenting process of organic substances utilizing bacteria thus producing usable energy.

The advantages of biogas are that:

	The production is CO2 neutral.
	The raw material for energy production can be stored.
	Unlike sun and wind energy it is available at all times.

The above facts explain the great interest which is shown in this form of energy carrier. All over the world companies begin to invest in biogas applications. EON Ruhrgas, for example, is going to build 25 biogas plants in Germany within the next 3 years. Vattenfall Europe is going to invest more than 1 billion Euros in biogas plants over the next 5 years and LMS in Australia is going to build 50 plants using landfill gases.

The use of energy produced by biomass has changed over the past few years. Traditionally, biogas was used in thermal power stations or private electricity production, but power suppliers plan the following for the future:

	Connecting the produced electrical power into the public electricity networks.
	Feeding biogas in to supplement natural gas supply.
	Using biogas as the fuel for gas driven vehicles.

What all operators have in common is that the requirements on measuring technology have increased. On the one hand, operators have the natural interest to supervise the economical use and efficiency of their investment. And on the other hand, local authorities offer subsidies if the legal limit values are adhered to.

Calculating the energy produced and the efficiency is complicated since they strongly depend on the composition of the biogas, which in turn depends on the composition of the product to be fermented. As this composition is not stable the formation of the biogas can change within a few hours.

As a result, the methane content of biogas sways between 45 and 70%.

This is very decisive because the methane content directly corresponds to the energy content or calorific value of the biogas and is therefore the actually usable part of the gas. Up to now a fixed methane content of, e.g. 60% was used for the energy calculation leading, of course, to a high uncertainty in the result of the energy calculation. In newer biogas plants, gas analyzers are used to ascertain the actual methane content within the gas.

Application Manager RMM621

Endress+Hauser have developed a solution for such installations with which the energy produced can be determined easily and reliably. The heart of this solution is the Application Manager RMM621 or the Data Manager Memograph M containing tailored formulae for the energy calculation of biogas applications. The calculation delivers an energy value which far exceeds the expected precision of 95% for a complete installation. Thereby, a reliable statement about the efficiency of the plant is possible. The reason for the higher accuracy is that the measured methane contents are used to calculate not only the calorific value for the biogas but also the process and later the reference density.

The biogas package is completed with the necessary sensors for flow, pressure and temperature. Using a DP flow sensor is recommended for the application, but Vortex or Thermal are also possible if the conditions of the produced biogas allow it. An additional advantage is that all parts of the package are aligned to each other, leading to a minimized total system error.