When it comes to acting with foresight, biogas plant operators have for a long time generally seemed shy of addressing the issue of external sulphur filters. This can be costly.
Author: Dittmar Koop
Photos: Works images
IN BRIEF
- Experts recommend using external desulphurisation in addition to coarse desulphurisation.
- The less hydrogen sulphide to be found in the methane, the fewer the repair costs that operators need to reckon on.
- Biogas can also be desulphurised without pre-drying using pellets made from recycled material.
- To avoid having to replace their activated carbon too early, operators should perform a regular gas measurement to monitor the uptake status of the carbon.
The cheapest process for coarse desulphurisation of biogas is air injection into the fermenter.
But is this sufficient? It reduces the hydrogen sulphide (H2S) content of the gas to around 500 to 100 parts per million (ppm). However, manufacturers of catalytic converters accept only 5 bis 10 ppm for operation, and gas engine manufacturers generally accept around 200 ppm for the warranty.
Inject more air then, in order to stay on the safe side in terms of H2S content? If air injection is more than 1 vol. %, the oxygen added to the gas has a negative effect on methane formation and therefore reduces the efficiency of the fermenter.
An additional possibility for coarse desulphurisation is to use iron salts. But this approach too is to be treated with caution. "Iron salts are strong acids, which unfortunately also inhibit methane formation and are a hazardous substance," says Toralf Goetze, Managing Director of the filter manufacturer Necatec AG in Essen. "When dosing iron compounds, an additional, excessive dose must be added for the removal of the last parts per million, which makes the process expensive and laborious," says Silvana Rossow of the activated carbon and filter manufacturer AdFiS products GmbH in Teterow.
Matthias Effenberger of the Institut für Landtechnik und Tierhaltung (Institute for Agricultural Engineering and Animal Husbandry) at the Bayerische Landesanstalt für Landwirtschaft (LfL) (Bavarian State Research Centre for Agriculture) also regards the difficulties with controlling this desulphurisation process as a significant disadvantage. The LfL records the desulphurisation performance of plants as part of its biogas monitoring. He reports that: "Internal biological desulphurisation usually causes high variability in the measured hydrogen sulphide content in the biogas, although chemical desulphurisation is frequently performed in addition." Effenberger recommends additional external desulphurisation. Although they are initially significantly more expensive to procure, he says, external desulphurisation systems can be accurately sized and precisely controlled.
Worthwhile investment
It is an investment that pays off, because maintenance and repair costs are reduced with every ppm less H2S in the gas. "Hydrogen sulphide is an acidic and therefore highly corrosive trace gas. It attacks engine parts such as valves and cylinder liners and damages downstream catalytic converters," says Goetze. Heat exchangers become clogged due to the formation of calcium sulphate. On the other hand, the oil in the engine is resistant thanks to the base buffer, which has the effect of reducing the damaging effects on engine parts. "But the base buffer is used up more quickly and the engine oil needs to be replaced sooner," says Goetze. Removing H2S from the combustion gas, he explains, could increase the life of the engine oil by 3 to 5 times. This saving on operating costs can be quantified in figures. "In our discussions with engine manufacturers and service providers, we are finding that hydrogen sulphide in biogas incurs costs of around 0.1 to 0.2 ct/kWh," says Wolfgang Doczyck, Managing Director of Siloxa Engineering AG in Essen. "For a typical 500 kW plant, this adds up to € 4,300 to € 8,600 per year." Goetze adds a comparative calculation: "The typical (average) consumption of activated carbon at a 500 kW plant is around 500 kg per year with a merchandise value of around € 1,750. An adequate filter costs around € 15,000 and lasts approximately 10 to 15 years. There are no significant maintenance costs," he says.
Activated carbon filters have proven reliable in practice. "Fine desulphurisation pays off in the case of peaks and highly fluctuating H2S values or values below 500 ppm," says Goetze as a rule of thumb. You can, he says, reduce the H2S content to technically zero. Renowned activated carbon filter manufacturers and activated carbon suppliers include, for example, Necatec, Züblin Umwelttechnik, Siloxa, AdFiS and Umwelttechnik Rausch. Their reference projects vary between 200 and over 1,000 installed filters and/or activated carbon deliveries to biogas plants.
The filters are offered in all sizes. Before the gas flows through the filter, it must first be cooled and dehumidified. "The biogas is cooled from an inlet temperature of 35 °C and 100 % relative humidity to an outlet temperature of around 7 to 10 °C and 100 % relative humidity in suitably sized heat exchangers and cooling units," explains Timo Rausch, Managing Director of Umwelttechnik Rausch in Olfen. By cooling the gas to below the dew point, the water vapour is condensed out and the biogas dehumidified. "For 300 m³ biogas, this corresponds to a condensate volume of 16.2 l water, which no longer needs to be passed through the engine. No additional energy needs to be used for vaporisation either," he explains. The dehumidified gas is then reheated in a heat exchanger by around 15 °C. "This prevents the biogas from being cooled below the gas's dew point and additionally condensed further along the pipeline network or in the activated carbon filter," explains Rausch. The relative humidity of the reheated gas is around 50 %.
What about pellet filters instead of carbon?
UGN-Umwelttechnik in Gera uses another method for the external desulphurisation of biogas. UGN filters hydrogen sulphide out of the untreated, hot and humid gas using pellets made from recycled material, i.e. without pre-drying. This offers an economic advantage: It reduces the energy consumption required for the operation of a biogas plant by eliminating the need for electrical power for gas cooling/gas drying. No investment costs are then incurred for gas cooling/drying and reheating. The raw gas flows through a module containing the filter bed. At the end, the saturated filter pellets predominantly contain pure sulphur and can be used as fertiliser, according to UGN. The company has fitted around 50 biogas plants with this system since 2011. The systems are found in Nawaro biogas plants, waste fermentation plants and sewage gas and landfill gas processing facilities.
In a project funded by the German Federal Ministry of Economics, the Deutsche Biomasseforschungszentrum (DBFZ) is focusing on the development of a new type of adsorber system for the desulphurisation of biogenic gases with the aid of xerogel moulds. A xerogel is a porous solid with a reticular structure, formed by drying a gel. The project centres around suitability tests for the removal of hazardous gases such as hydrogen sulphide in the laboratory and on an industrial scale. Project partners include Clausthal University of Technology, Bergakademie Freiberg and the Institut für Keramik, Glas- und Baustofftechnik (Institute of Ceramic, Glass and Construction Materials).
The project began at the start of May and is planned to run for two years. H2S measurements indicate when the material is saturated. "Since the H2S content is rarely absolutely constant, you cannot simply calculate the filter service life and define it on a fixed basis. To avoid replacing the carbon too soon or overloading the filter, operators must check the uptake status of the activated carbon with a regular gas measurement," says Wolfgang Doczyck.
Siloxa, for example, does this using fixed gas analysers or mobile measuring devices. The filters have nozzles where gas can be extracted and analysed at various heights in the activated carbon packed bed (50/80/100 %). Twin-chamber systems offer the advantage that material with a really full contaminant load can be extracted from the first chamber. These filter systems therefore have lower specific activated carbon consumption. They are more expensive than single-chamber systems or replaceable filters, however.
Coarse desulphurisation is nonetheless still to be recommended. This is because pre-reducing the H2S content of the biogas in the fermenter increases the time until the saturated filter material needs to be replaced. "As a rule, the activated carbon needs to be replaced at least once a year – provided it is high-quality. We often achieve much higher filter service lives, however, such as two years or more," says Claus Bogenrieder of the biogas/exhaust air purification department of the filter manufacturer Züblin Umwelttechnik in Stuttgart. (rz)
What operators need to bear in mind when using activated carbon
Wolfgang Doczyck, Managing Director of Siloxa Engineering AG, Essen, describes what biogas plant operators need to bear mind when using activated carbon: "Selecting the right activated carbon is the first important decision. There are big differences in performance. High-performance carbon lasts twice as long as standard carbon." Activated carbon or an activated carbon filter for desulphurisation is actually a very simple system. There are only a few operating parameters, but these should be kept as close to ideal as possible."
- The gas must contain enough oxygen. If there is too little oxygen, the uptake capacity decreases dramatically. The reading on the measuring device should ideally be > 0.5 vol. %.
- The uptake capacity of the activated carbon is at its highest when the relative gas humidity is between 40 and 60 %. In practice, this means that the gas should be heated by 10 to 12 °C from its coldest temperature (dew point). Example 1: At the rear of the gas cooling system, the gas has a temperature of 10 °C. It must then be heated to 20 °C and the relative humidity is 52 %. Example 2: If an underground gas pipe is used for cooling, there is no defined dew point. The gas temperature fluctuates depending on the season and gas flow rate. In this case too, the gas must be heated by 10 to 12 °C after the cooling section.
- Very important: At a rel. humidity of < 30 % or > 90 %, the activated carbon performs much worse – this range must be avoided at all costs.
- Hydrogen in the gas falsifies the H2S measurement. This means that the measuring device shows H2S, but it is actually only measuring the hydrogen.
- Trace gases and constituents such as siloxanes and long-chain hydrocarbons inhibit the effectiveness of the carbon on the H2S. However, this actually only applies for co-fermentation plants or biogas plants that process waste.