Quality & Safety

Biogas instead of natural gas? Activated carbon is mandatory for the high requirements

Donau Carbon

Due to the current events, biogas is once again in focus and this time less because of the already known sustainability, but rather because of the security of supply. Accordingly, the processing of biogas for feeding into the natural gas network and the general capacities are to be intensified for two reasons.
Since the first beginnings of biogas production in Germany, there has been a real boom in the commissioning of new plants, especially in other European countries. In Germany alone, there are now almost 10,000 biogas plants, which together produce around ten billion cubic meters of gas with an energy content of around 100 terawatt hours (TWh) per year - which corresponds to around ten percent of German gas consumption.
More and more biogas plant operators use the financial advantage of feeding their product into the normal gas network. Currently, about 10 percent of the biogas in Germany is actually processed and fed into the gas network. The remaining 90% is converted into electricity and heat in block-type thermal power stations where it is produced – which is also directly related to natural gas consumption, since the electricity and heat generated does not have to be provided by natural gas power stations.
For both purposes, there is a prerequisite for a very pure biogas - almost free of hydrogen sulphide, which can be achieved by processing the raw biogas with activated carbon.
With a focus on reducing costs and increasing efficiency in the processing of biogas, we at Donau Carbon have developed new activated carbon for the highest demands, which have been used successfully for years even under fluctuating conditions. In this way we are trying to make our contribution to the ecological turnaround as well as the current security of supply.
 

Origin and the problems with sulphur

Landfill and biogas treatment are playing an increasingly important role in sustainability and self-sufficiency in energy supply. Because biogas is considered an environmentally friendly source of energy alongside wind and solar energy. Municipalities as well as farms are therefore increasingly deciding to set up biogas plants.
Vegetable and animal waste products from agriculture serve as the basis, from which digester gases are obtained. As a side effect, however, hydrogen sulfide is also produced, which must be removed.
The background to this is that there is no source of carbon that occurs without sulphur. Whether it is slaughterhouse waste that contains high concentrations of protein or substrate plants such as corn, grain or grass. There is sulfur everywhere.
While in Germany the majority of biogas production is based on the fermentation of agricultural waste, liquid manure and energy crops grown on site, the market is structured differently in other European countries. In France, Luxembourg, Sweden and Switzerland, gas production is dominated by municipal waste supply and household waste. The use of renewable raw materials plays a subordinate role there.
The raw biogas in so-called fermenters is created during the fermentation of substrates of biological origin. These contain microorganisms that produce biogas as a metabolic product. In addition to methane - the raw biogas consists of 45 to 70 percent methane (CH4) - the bacteria in the biogas plant unfortunately also produce hydrogen sulphide. Other sulfur compounds are considered in the total sulfur parameter. These have a corrosive effect and must therefore be removed from the biogas. Otherwise they endanger system components and gas-consuming devices, e.g. the engine oil becomes acidic, which means that it loses its lubricating properties and engine damage can occur.
 

Desulfurization techniques

With the available processes, a distinction is made between coarse and fine desulfurization according to the sulfur concentration in the clean gas. In the case of very high sulfur contents, e.g. far more than 500 ppm, we therefore recommend a coarse desulfurization first. For this purpose, a product based on iron chloride is used in advance, which was developed by our sister unit at Donau Chemie Wassertechnik.
In general, the rough desulfurization can be carried out both biologically and chemically, with the chemical variant with the addition of iron salts having proven its worth. Activated carbon is used most economically for fine desulfurization.
Due to decades of experience in using activated carbon in the desulfurization of natural gas (Oxorbon), nothing stood in the way of the successful use of activated carbon and it has long been state of the art.
In this process, H₂S is catalytically converted into elemental sulphur, which is adsorbed by the activated carbon.
Overall reaction: H2S + ½ O2 → S + H2O


Optimal operating parameters

Based on our many years of experience in using activated carbon, we recommend the following operating conditions for the optimal use of our high-quality activated carbon products. There is a lot to consider and to take into account when planning!
  • Contact time: At least 5 seconds ⇨ Sufficient contact time for the chemical reaction to take place during adsorption.
  • Gas speed:  5 - 40 cm/s ⇨ Creation of an even flow along the entire activated carbon bed.  
  • Temperature: 10 - 70 ° C ⇨ Below a temperature of 10 °C, reduced adsorption kinetics of chemisorption can lead to incomplete utilization of the adsorption capacity of the activated carbon. ⇨ Above a temperature of 70 °C, SO₂ can form from H₂S, which can lead to corrosion of the systems/equipment.
  • Relative gas humidity: Application range: 20 – 80% r.h. ⇨ Not all biogas plants are operated under rather higher or even gas humidities. As an essential difference and advantage for our customers, our Desorex® G 50 shows a very good performance even with low gas humidity and is therefore always ready for use.
    • constant performance with almost all gas humidities
    • flexible and uncomplicated application
 

Possibilities and limits

High H2S concentrations of e.g. well over 500 ppm make the use of activated carbon uneconomical, as the service life is very short and the change intervals are too frequent. The personnel and disposal costs are correspondingly high, which affects the price and thus the profitability of the biogas.

In these cases, it makes sense for the sulfur to be bound by means of a coarse desulfurization or already directly in the fermentation process by adding other additives, see explanations above. This significantly reduces the H2S concentration so that the activated carbon can take over the fine desulfurization to protect the other systems.

Furthermore, there are immense differences in the limits for the production of biogas in the substrates that can be used, since the biomethane yields are subject to immense fluctuations depending on the substrate. Liquid manure achieves a yield of approx. 20m³ biomethane per ton, while grass and maize achieve approx. five times as much.

 

High requirements before feeding into the natural gas network

The requirements of the Gas Network Access Ordinance (GasNZV), which are based on the Energy Industry Act, are decisive for the plant operator of biogas production. According to this, there is a priority connection obligation for biomethane if the feed-in is technically possible and not economically unreasonable (§34 GasNZV).
In addition to methane, biogas contains many gas components that have to be separated so that it meets the requirements for feeding into the gas network. In addition to separating carbon dioxide, the largest proportion at 25 - 50%, sulfur compounds, ammonia and other trace elements must also be removed and the gas must be dried. In the following, the processes for the necessary separation of the carbon dioxide will be considered in more detail. Separation in the raw biogas can take place in various ways. The methods differ significantly in their specific power and heat requirements and can basically be divided into physical processes such as pressure swing adsorption (PSA), pressurized water washing, membrane processes or low-temperature rectification; chemical separation, as in amine scrubbing, or chemical-physical techniques.
Based on the number of plants implemented so far, chemical scrubbing, pressure swing adsorption (PSA) and pressurized water scrubbing (PWW) dominate. Membrane processes have so far only been used on a large scale in the Netherlands, Austria, Great Britain and Germany. In some plants, cryogenic technology, which is still very new in the field of biogas feed, is also used.
Which method makes the most sense always depends on the boundary conditions. Considering the specific costs for smaller treatment capacities, amine scrubbing and membrane processes are the most economical methods for gas treatment.
After the processing, the feeding takes place essentially via a compressor, which raises the pressure level of the bio natural gas to that of the connected pressure gas line. A prerequisite for the feed-in is that the quality of the biogas to be fed in corresponds to the provisions of the gas class on site.
When feeding in biomethane, a distinction is made between replacement gas and additional gas for quality reasons. The replacement gas has the same quality as conventional natural gas and can therefore fully replace it. In contrast, the composition of the additional gas is not equivalent to that of natural gas and can therefore only be partially mixed with natural gas. The natural gas available in Germany differs according to geographical origin. As a result, the required degree of processing of the biomethane also differs from region to region.
Natural gas is divided into "natural gas L (low)" and "natural gas H (high)". Natural gas H has a higher calorific value than natural gas L and is mainly produced in the CIS countries and in the North Sea. Natural gas L contains about 89 percent combustible gases (mainly methane, but also small amounts of ethane, propane, butane, and pentane), while natural gas H consists of about 97 percent combustible gases.
Feeding into the grid offers the advantage that biogas can be used far away from the place of production.
 

Reactivation vs. necessary disposal

The used activated carbons are called spent activated carbons and after their use for desulfurization are usually very heavily loaded with sulphur. Values ​​of more than 50% are not uncommon here. As a result, the current reactivation technology clearly exceeds the limits that currently make reactivation of this highly sulfur-laden used activated carbon impossible.
Because for our reactivation plants there are certain limit values ​​for the spent activated carbon, such as for sulfur compounds. These must be observed in order to be able to ensure the protection of the environment and systems during the reactivation process (see https://blog.donau-chemie-group.com/blog-posts/Donau-Carbon/Activated-carbon-reuse-or-disposal​). Due to the classification as waste, all used activated carbon, regardless of the raw material basis and type of application, is exempt from the fertilizer ordinance (DüMV § 3, as well as Annexes 1 and 2) and may therefore not be used as fertilizer (DüV § 3).
Used activated carbon should be disposed of according to the waste code number assigned to the waste by the user. Proper disposal of used activated carbon requires a licensed disposal contractor.

  • Professional disposal of used/loaded activated carbon
  • Certified disposal company
  • Disposal usually as non-hazardous waste
  • Issuance of a corresponding disposal/recycling certificate

 

Summarized

Biogas, or more precisely biomethane, can indeed make a significant contribution to increasing security of supply and at the same time continuing on the path to sustainability and thus reducing the use of fossil fuels. The substrates are plentiful, insofar as the topic of cultivated biomass (energy crops) is taken into account. The techniques for processing the biogas into biomethane and thus feeding it into the gas network are state of the art and well established.
A small, obligatory component is the use of activated carbon for (fine) desulfurization of the gas and we would be happy to advise you on the correct use of our high-quality activated carbon qualities under the mostly very different conditions.
Our mobile filter systems, which are flexible in every respect, are also in great demand for the biogas plants: the small and medium-sized filters are completely replaced after use. For larger filters there are facilities for changing the loaded activated carbon. Donau Carbon offers logistics and changeover concepts that are tailored to the needs of customers and reduce personnel and operating costs. Ready-to-use part or complete solutions (i.e. activated carbon filters including all necessary components) are available to those customers who do not have their own logistics or personnel for the maintenance of the filter systems. There is also the option of just renting the filters - for temporary needs or to test the extent to which an adsorption process can be used economically.
 
Author: Head of Activated Carbon Application Technology Marco Müller
Together with his team, he is constantly working on high-quality activated carbon qualities and innovative application solutions. Application technology is the responsibility of Gabriele Neuroth, Head of Application Technology / Quality Assurance at Donau Carbon.

Donau Carbon

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