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The limit values for the emission reduction bonus are based on TA Luft. These are 500 mg/Nm3 for nitrogen oxides (NOx), 1000 mg/Nm³ for carbon monoxide (CO) and 40 mg/Nm³ for formaldehyde (HCHO) for gas engines. The limit values for NOx and CO are twice as high for spark ignition engines. Every three years, a BImSCH-approved plant must additionally verify the sulphur oxide limit value (SOx) of 311 mg/Nm³ and for igniters additionally the dust limit value of 20 mg/Nm³.

Emission reduction bonus is awarded to anyone who proves through a measurement report from an accredited measuring institute that all of their emission sources on their biogas plant fall below the emission limit values for the emission reduction bonus and whose plant went on line before January 1, 2012 (BImSCH) or January 1, 2009 (building right). The emission reduction bonus is valid from the day of measurement.

A good ignition ensures a better combustion of the biogas, which is generally very unwilling to ignite. This leads to better efficiency and lower methane slip – i.e. lower biogas consumption. Anyone who saves on spark plugs and maintenance of the ignition system pays double and triple the money saved via the methane slip – which is first of all only indirectly visible via the gas consumption. The high methane slip is also extremely harmful for the environment but also for the catalyst. If the methane slip is ignited via a catalyst at high exhaust gas temperatures, the catalyst melts through within a few minutes.

Formaldehyde is simply oxidized methane, i.e. not yet completely burned to carbon dioxide and water. Formaldehyde is therefore the result of incomplete methane combustion, especially at the piston edge. The diagram shows an exhaust gas measurement on a MAN 12 cylinder engine with turbocharging and charge air cooling. The connection between methane slip in ppm (green line) and formaldehyde emissions in mg/Nm³ (red line) is very nice to see. Over the course of the exhaust gas measurement, the formaldehyde emissions are about 5% of the methane emissions (400 ppm *0.05 = 20 ppm = approx. 27 mg/Nm³). This rule of thumb also applies to other engines. In our example, the MAN engine had very low methane slip and, as a result, very low formaldehyde emissions below the limit value for the emission reduction bonus. By recording the formaldehyde emissions during the exhaust gas measurement, Emission Partner was able to inform the operator that the limit values for the emission reduction bonus were not reached even without catalyst technology and that the use of a catalyst could be dispensed with depending on the situation – i.e. during the exhaust gas measurement.

Good gas treatment must be considered independently of the use of catalyst technology. The biogas saturated with water would cool the combustion in the engine and thus reduce the nitrogen oxides. However, since biogas ignites very poorly (almost 50% of the biogas is inert CO2), all moisture should be removed from the biogas by cooling in order to increase the ignitability of the mixture and the efficiency of the combustion engine. The service life of spark plugs, engine oil, moving engine components and at the end also of the catalytic converter can be significantly increased by downstream desulfurization. Desulphurisation is not so much a matter of desulphurisation technology, but rather of the fact that a large proportion of the hydrogen sulphide dissolved in the biogas does not reach the engine. The decisive factor is the total amount of sulphur supplied to the engine over the year, and this should be as low as possible. The third stage of gas treatment – and currently still the future – is the concentration of the biogas by removing the carbon dioxide. As a result, the fuel gas becomes significantly more ignitable, the service life of the entire engine increases, methane slip and emissions decrease, and efficiency increases as a result.

The NOx limit value is adjusted within the engine via the fuel-air mixture, the CO limit value is still well below even with strongly deactivated catalytic converters. In order to increase the service life of the catalyst, it is important to know that the formaldehyde measurement result is significantly influenced by two factors:

– The formaldehyde emissions, which correlate directly with the level of methane slip
– The activity of the built-in catalyst.

In order to achieve the best possible formaldehyde measurement results with an already installed catalyst, it is therefore very important to work with a very good ignition (spark plugs, ignition coils, ignition timing setting) as well as to burn biogas in high methane concentrations. (see also “Why is a good ignition so important?”) The activity of a fresh catalyst is insignificant. All fresh catalysts are good. The decisive factor is that the catalyst technology is coordinated in such a way that it is

deactivated as slowly as possible by the catalyst poisons of the biogas (see also Technology. The sulphur-resistant catalyst developed by Emission Partner is a technology that has been specially developed for biogas applications: in addition to the sulphur in biogas, the engine oil ash deactivates the catalyst most strongly. Zinc- free engine oils, whose ashes can be blown off the catalyst surface, have proved to be advantageous. (Mobil Pegasus 610/705/1005 and Tectrol Methaflexx HC Plus (download specification here, see appendix). We recommend that the catalytic converter is cleaned of oil residue every 6 months.

Emission Partner therefore only offers catalysts that are very easy to install and remove for maintenance or easy replacement, and if the above points are taken into account, Emission Partner catalysts also reach formaldehyde values below the limit for second and now even for third party measurements.

The question of converting a catalyst is actually the right one. A catalyst is not “broken” if it does not fall below the formaldehyde limit. Its conversion is just no longer sufficient. This can have a lot of reasons. Besides the reasons already discussed for catalyst aging due to oil flake deposition and sulphur poisoning, poor flow is the most important reason for insufficient conversion. The best conversion is achieved when the catalyst flows at an angle of 90°, as is the case with installation in the exhaust heat exchanger. If the catalytic converter is flown to the front, the diameter of the catalytic converter should be less than twice as large as the diameter of the pipe, since the outermost areas of the catalytic converter are only insufficiently or not at all flown to. Even small gaps, through which the gas can escape past the catalytic converter, lead to slip of up to 10%. If the catalytic converter prematures accordingly, the slip alone can be responsible for the fact that an exhaust gas measurement is not passed.

A burnt-out catalytic converter is a clear indication that too much fuel (methane slip, oil from turbocharger, crankcase ventilation etc. but also ignition oil from the pilot injection engines) was burned over the catalytic converter. Anyone operating a biogas cogeneration plant with catalyst technology must ensure that

– The ignition always works optimally
– Change the spark plugs before misfiring occurs
– Do not lose oil (observe maintenance and replacement intervals)
– Change the ignition nozzles of the jet engines in good time (before they are burned)

In order to protect the catalytic converter from burn-through, Emission Partner has developed a pressure and temperature monitoring system that switches off the combined heat and power plant if the exhaust gas temperature is too high.

The H₂S contained in the biogas is completely burnt in the engine to SO₃ and almost completely oxidized to SO₃ via catalyst. The SO₃ reacts with the water contained in the exhaust gas to form H₂SO₄, also known as sulfuric acid. If the temperature falls below the dew point, i.e. if the sulphuric acid in the heat exchanger condenses, it settles on the stainless steel tubes of the heat exchanger and eats through them within a few days or weeks. This can be remedied by effective ultra-fine desulphurisation downstream of desulphurisation by air injection and possible gas scrubbing. The absence of sulphur in the exhaust gas prevents the formation of sulphuric acid in the first place. The higher the sulphuric acid in the exhaust gas, the higher the dew point, and the more likely it is to fall below the dew point in the exhaust tract. If necessary, you should leave the exhaust gas temperature above 200°C when using oxidation catalysts to reliably prevent it from falling below the dew point. The pressure and temperature monitoring developed by Emission Partner signals the operator as soon as the exhaust gas falls below a threshold value after the heat exchanger and there is a risk of condensation of sulfuric acid.

Instead of an oxidation catalyst, a reduction catalyst reduces nitrogen oxide (NOx) emissions. The nitrogen oxide catalyst developed by Emission Partner further reduces formaldehyde and, by using this new generation of catalysts for biogas engines, methane slip can be prevented and fuel consumption reduced by up to 5%.