Desulphurisation using activated carbon filters is now a standard, state-of-the-art technique used in the majority of German biogas plants. TA Luft (the Technical Instruction o Air Quality Control), already modified once, in 2002, are now due to be reissued in 2017, establishing even stricter statutory regulations.
IN BRIEF:
- Activated carbon filters for desulphurisaton are already in use in most Biogas plants.
- Packed bed adsorbers are the most commonly used.
- To ensure trouble-free desulphurisation, the preconditions must be right.
- The most common shortcomings can be attributed to measurement errors.
- Gas humidity, oxygen content and high-quality activted carbon, in combination with good specialist expertise on the part of the operator and care of the measurement instruments, will ensure efficient desulphurisation.
Authority: Joule September/October 2016
Author: Judith Schomaker
Photos: Works Images
The first measurement of the day is taken.
Until now, the measurement device has not indicated any hydrogen sulphide, but a weak concentration of H2S is now detected in the cleaned biogas. In technical jargon, the hydrogen sulphide is starting to break through. The time has come for the plant operator to organise a filter change.
The EEG (Renewable Energy Law) amendment of 2008 is to thank for what today is a routine feature of biogas plants. With this amendment, the issue of gas purification to reduce formaldehyde concentrations became extremely important, not least due to the financial incentive of the formaldehyde bonus. Separating out the formaldehyde from the combustion gas of a combined heat and power (CHP) plant requires the use of an upstream catalytic converter, which cleans the biogas of hydrogen sulphide (H2S). This is achieved using an activated carbon filter, which, of course, also led to an increase in the acceptance of and demand for these filters. The method known as fine desulphurisation guarantees that compliance with the maximum permissible concentration of formaldehyde in exhaust gas for stationary motors, as specified by TA Luft, is continuously achieved by the downstream catalytic converter, without any difficulty.
Activated carbon filters on the market
Roland Hengst, Head of Sales at Siloxa AG, points out that cleaning the biogas in this way also extends the service life of the motor oil and effectively prevents detrimental deposits from forming in the combustion chamber of the gas motor. Ultimately, this results in a reduction in operating costs for the gas motor and an increase in cost-efficiency.
An activated carbon filter or adsorber is a plastic or steel cylinder that is filled with activated carbon and allows the biogas to flow through. If plastic cylinders are used, then special attention must be paid to the safety aspects of the system. A further problem in the case of plastics is that the plasticizers in the material are released after years of exposure to UV radiation, causing embrittlement and a loss of structural strength of the cylinders when exposed to intense heat. These factors must be taken into account during planning.
A distinction is made here between packed bed adsorbers and flat bed adsorbers. In flat bed adsorbers, the gas flows into the adsorber from the side. The inflow area is large, which limits the pressure loss and thus results in a low flow velocity and low pressure losses. Packed bed adsorbers are most commonly used, since their working principle is simple and particularly reliable in operation. With this type of adsorber, the gas flows through vertically (from bottom to top), which requires a slightly higher pressure. In addition to single-chamber units, multi-chamber systems are also available. By switching between the various saturated activated carbon zones, these systems make it possible to adsorb more sulphur and thus to achieve lower specific operating costs in comparison to single-chamber systems. “With multi-chamber tanks, the initial construction costs are slightly higher, leading to additional costs of around 10 %”, explains Roland Hengst. Many biogas plants employ two or more adsorbers. “This is often made necessary by the design size (amount of activated carbon), but also has additional advantages”, points out Thomas Stifft, Director of AdFIS products GmbH.
If the adsorbers are connected in parallel, then one adsorber can easily be taken out of operation for a short period while the activated carbon is replaced, without having to shut down the motor. In most cases, the activated carbon filters are built permanently into the biogas plant, but replaceable adsorber systems are also available. In this case, instead of replacing the activated carbon, the entire filter element is replaced. The replacement process is easier and cheaper, although transport costs are slightly higher. When choosing from the wide range of activated carbon filter models available on the market, operators should pay particular attention to ease of service and maintenance. If, for example, a taller filter unit has no working platform, then additional costs will be incurred each time the activated carbon is exchanged. These costs are associated not only with ensuring compliance with the health and safety requirements (DIN EN ISO 14122) but also with the extra time taken to carry out the work.
Conditions for efficient desulphurisation
Several conditions must be met in advance if the desulphurisation process with activated carbon is to function well. The size of the adsorber (the amount of activated carbon used) should be generously dimensioned so that the gas has a contact time of at least three seconds within the activated carbon bed. Also, the gas must always contain sufficient oxygen, since the desulphurisation process will come to a halt if the oxygen content is too low. An oxygen content of less than 0.3 % should therefore be avoided. The ideal range is around 0.5 %. Moreover, the relative gas humidity should lie between 40 and 60 %, since the uptake capacity of the activated carbon is at its peak within this range. In order to achieve this humidity, the gas is preheated – this is achieved, in practice, by configuring the individual gas conditioning components appropriately.
Expedient monitoring of the desulphurisation process
Thomas Stifft also considers it prudent to use an upstream coarse desulphurisation process to reduce the concentration of H2S in general and to establish an efficiently and expediently designed fine desulphurisation process. In practice, various options are available for coarse desulphurisation (such as the introduction of oxygen). After a certain period of use, the saturation limit of an adsorber with elementary sulphur is reached and replacement of the filter material is necessary. To make sure that this crucial moment is not missed, fixed gas measurement devices are usually installed. “The larger and more industrial the biogas plant, the better the instrumentation”, says Toralf Goetze of Necatec AG. He points out that smaller plants often have no measurement devices at all. Changing the activated carbon according to guesswork, however, often leads to “overloading” of the filter. In the worst case, the consequences of this can negatively affect or even damage the CHP plant.
Avoiding problems in good time
Even the presence of measurement devices does not necessarily guarantee correct measurements, however. Goetze believes that a good 95 % of problems identified in the desulphurisation process can be attributed entirely to measurement errors. The most common causes of errors are infrequent servicing, far too frequent measurement cycles (e.g. every 20 minutes), or constant raw gas and clean gas measurements taken alternately. Measurement errors can also be caused by the presence of hydrogen in the biogas. With just 0.2 % hydrogen, the measurement system gives false readings of 15 to 20 ppm H2S.
Thomas Stifft includes the selection of substrate among the sources of errors: “Many plants use poultry manure, due to its high energy values. Poultry manure produces a high proportion of ammonia even in the fermentation phase”, says the expert. With specialist knowledge, good servicing and care of the instrumentation, proper monitoring of the system and the avoidance of short measurement cycles, desulphurisation problems can be addressed right from the start. If the gas cleaning system is also correctly designed and dimensioned, Toralf Goetze is sure that the operator has done everything he can to ensure smooth running of the desulphurisation process.
Roland Hengst also sees the three pillars of success that guarantee cost-effective desulphurisation as being an optimal gas humidity range, adequate oxygen in the gas, and high-quality activated carbon.
Never rely on guess work
The plant operator can remember the last time the activated carbon was replaced. When it comes to servicing his plant, he relies on the instrumentation – and not on his intuition. He can still postpone the filter replacement for a few days by setting the oxygen content a little higher. But then the professional service provider arrives and exchanges the activated carbon, so that the CHP plant can continue to operate without any hydrogen sulphide related damage or impairment. And there are immediately perceptible benefits too:
the service life of the oil in the CHP plant has doubled.