The emission reduction bonus is awarded to those who can prove in a measurement report from an accredited measurement institute that all their emission sources at their biogas plant fall below the emission limits for the emission reduction bonus and whose plant was connected to the grid before 01.01.2012 (BImSCH) or before 01.01.2009 (building law). The emission reduction bonus applies from the day of measurement.

Good ignition ensures better combustion of the biogas, which is generally very willing to ignite. This leads to better efficiency and lower methane slip – in other words, lower biogas consumption. If you save on spark plugs and maintenance of the ignition system, you pay for the money you save twice or three times over in the form of the methane slip – which is initially only indirectly visible via the gas consumption. The high methane slip is also extremely harmful to the environment and to the catalytic converter. This is because if the methane slip is ignited via the catalytic converter at high exhaust gas temperatures, the catalytic converter melts through within a few minutes.

Formaldehyde is simply pre-oxidized methane, i.e. not yet completely burned to carbon dioxide and water. Formaldehyde is therefore the result of incomplete methane combustion, which occurs in particular at the edge of the piston. The diagram shows an exhaust gas measurement on a MAN 12-cylinder engine with turbocharging and charge air cooling. The correlation between methane slip in ppm (green line) and formaldehyde emissions in mg/Nm³ (red line) is clearly visible. Over the course of the flue 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 motors. In our example, the MAN engine had a 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 undercut even without catalytic converter technology and that the use of a catalytic converter could be dispensed with in certain situations – i.e. during the exhaust gas measurement.

Good gas treatment must be considered independently of the use of catalytic converter technology. The biogas saturated with water would cool the combustion process in the engine and thus reduce nitrogen oxides. However, since the biogas already 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. Downstream desulphurization can significantly increase the service life of spark plugs, engine oil, moving engine components and, ultimately, the catalytic converter. With desulphurization, it is not so much the desulphurization technology that is important, but rather 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 that is fed into 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. This makes the fuel gas significantly more ignitable, increases the service life of the entire engine, reduces methane slip and emissions and consequently increases efficiency.

The NOₓ limit value is set within the engine via the fuel-air mixture, and the CO limit value is still well below even with heavily deactivated catalytic converters.
The decisive factor for receiving the emission reduction bonus is compliance with the formaldehyde limit value of 20 mg/Nm³ including measurement tolerance. In order to increase the service life of the catalytic converter, it is important to know that the formaldehyde measurement result is significantly influenced by two factors:

– The formaldehyde raw emissions, which correlate directly with the level of methane slip
– The activity of the built-in catalytic converter

In order to achieve the best possible formaldehyde measurement result with an already installed catalytic converter, it is therefore very important to work with very good ignition (spark plugs, ignition coils, ignition timing setting) and to burn biogas with a high methane concentration. (see also “Why is good ignition so important?”)
The activity of a fresh catalyst is irrelevant. All catalytic converters are good when fresh. It is crucial that the catalyst technology is tuned in such a way that it is deactivated as slowly as possible by the catalyst poisons in the biogas.
The sulphur-resistant catalytic converter developed by Emission Partner is a technology that has been specially produced for biogas applications.
In addition to the sulphur in the biogas, the engine oil ash deactivates the catalytic converter the most. Engine oils that are as zinc-free as possible and whose ash can be blown off the catalytic converter surface have proven to be advantageous. (Mobil Pegasus 610/705/1005 and Tectrol Methaflexx HC Plus). We recommend removing oil ash residues from the catalytic converter every 6 months. For this reason, Emission Partner only offers catalytic converters that are very easy to remove and install in order to facilitate maintenance or simple replacement.
If the above-mentioned points are taken into account, the emission Partner catalytic converters still achieve formaldehyde values below the limit value even in second and now even in third measurements.

The question about the conversion of a catalyst is actually the right question. A catalytic converter is not “broken” if it no longer falls below the formaldehyde limit value. Its conversion is just no longer sufficient. There can be a number of reasons for this. In addition to the reasons already discussed, catalytic converter ageing due to oil ash deposits and sulphur poisoning, poor air flow is the most important reason for inadequate conversion. The best conversion is achieved when the catalytic converter is exposed to the flow at a 90° angle, as is the case when installed in the exhaust gas heat exchanger. If the catalytic converter is flowed against from the front, the catalytic converter diameter should be less than twice the pipe diameter, as the outermost areas of the catalytic converter are only insufficiently flowed against or not at all.
Particularly with heat exchanger catalytic converters and also with catalytic converters in catalytic chambers, care must be taken to ensure optimum sealing. Even small gaps, through which the gas can escape past the catalytic converter, lead to slippage of up to 10 %. If the catalytic converter is sufficiently aged, slippage alone can be responsible for failing an emissions test.

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

– The ignition must always function optimally
– Change the spark plugs before misfiring occurs
– The turbochargers must not lose any oil (observe maintenance and replacement intervals)
– The ignition nozzles of the ignition jet motors must be replaced in good time (before they are burnt out)

To protect the catalytic converter from burning out, 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 combusted in the engine to SO₂ and almost completely oxidized to SO₃ via the catalytic converter. The SO₃ reacts with the water contained in the exhaust gas to form H₂SO₄, also known as sulphuric acid. If the temperature falls below the dew point, i.e. if the sulphuric acid condenses in the heat exchanger, it condenses 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 desulphurization, which is installed downstream of desulphurization by air injection and possible gas scrubbing. The absence of sulphur in the exhaust gas means that sulphuric acid cannot be produced in the first place. The higher the sulphuric acid in the exhaust gas, the higher the dew point, and the more likely it is that the temperature in the exhaust tract will fall below the dew point. If necessary, you should leave the exhaust gas temperature above 200°C when using oxidation catalytic converters to ensure that the temperature does not fall below the dew point.
The pressure and temperature monitoring system developed by Emission Partner signals to the operator as soon as the exhaust gas downstream of the heat exchanger falls below a threshold value and there is a risk of sulphuric acid condensation.

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