The Problem of Emissions in Biogas Engines

Authors

Dr. Günther Herdin, PGES, Jenbach/Steyr

Year

2007

Summary

As a result of the political intentions concerning the fulfilment of the EU specifications aimed at increasing the amount of “renewable power”, the biogas branch has experienced a boom in the number of presently operable biogas plants. For example, in Germany the number of such plants jumped from 850 in the year 1999 to 3280 in the year 2006. While before the turn of the millennium the majority of these plants were small (depending on their location, mid-range outputs of 30 up to 150 kW), industrial plants currently dominate with an average output of 350 kW). Regarding emissions, smaller biogas plants are being run in a so-called uncontrolled operating mode on account of the higher cost pressure. This means that the plant is initially set and then later “left to its own resources”. The primary target for the operator is the maximization of the operation time of the plant in order to achieve a “return on investment” at some point. The emissions of these plants are far from the state of the art or what is potentially achievable. In the case of plants >200 kW as far as expenditures involving control system technology are concerned, already a greater application of technology can be financed, and intensive developmental work is also possible. Given an adequate quality of biogas, such a plant size is also able to comply with the NOx (500 mg@5%O2) and CO limit values (1000 mg@5% O2) specified by TA-Luft. Nevertheless, a number of influencing variables can cause surprises that are able to be analyzed in part only at very high cost. Relevant examples here are the influence of humidity (gas humidity) on the NOx emissions, the influence of trace gases in the biogas (such as NH3 on NOx emissions), as well as recently also the topic of formaldehyde emissions. Furthermore, there are also effects stemming from the running time that have a negative effect on the CO emissions and the VOCs. Depending on the emission limits specified in the future, research work will necessarily shift its focus from internal combustion measures to secondary treatment of exhaust gas.

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