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Projektinfo – Detailed information on energy research

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© Suncoal Industries
Biogas for cogeneration
Projektinfo 04/2017

Total process chain from biomass to biocoal. During treatment and conversion, waste gas, wastewater and steam-saturated exhaust air (fumes) are produced, which need to be additionally treated.
© Deutsches Biomasseforschungszentrum DBFZ / Stadtwerke Halle (SWH)

Sectional view of the gasifier in which the biomass is converted
© Technical University of Munich
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Syngas from biocoals

Organic waste, thinning wood, straw and landscape management waste products are still little used for generating energy. This is because such biogenic residues have poor combustion properties owing to their inhomogeneous composition or high moisture contents. However, researchers from the Technical University of Munich and SunCoal Industries are now producing biocoal from such material using a new process. Subsequently they are converting biocoal in an entrained-flow gasifier especially designed for smaller plants. The syngas produced in this way can be used for the generation of electricity and heat in a gas-engine-driven CHP unit.

Biomass and residual materials are primarily used to generate energy by direct combustion or fermentation in combination with a gas engine. However, household waste from organic waste bins, garden waste such as grass and foliage, horticultural and agricultural residues, sewage sludge as well as residues from the food industry are only suitable to a limited extent. They are often fibrous, very differently composed and usually have a very high water content – which makes it very difficult to process and utilise them. However, treating these previously unused plant residues to form biocoal enables them to be used to produce CO2-neutral energy.

Using the hydrothermal carbonisation (HTC) process, the project partner SunCoal is producing a high-quality biocoal whose calorific value is comparable to that of brown coal. This biocoal is dried and ground. An entrained-flow gasifier developed by the Technical University of Munich converts the biocoal dust into a high calorific syngas that can be used to drive the gas engine of a combined heat and power unit. In order to produce energy from biocoal in the CHP unit, the researchers have adapted the large-scale entrained-flow gasifier technology used in coal-fired power plants for smaller decentralised systems.

The researchers at the Institute for Energy Systems at the Technical University of Munich have investigated the entire technology chain from the supply and treatment of biomass to its gasification. They have developed an autothermal entrained-flow gasifier with a thermal input of 100 kW and tested the gasification behaviour of biocoal dust in this laboratory entrained-flow reactor.

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Hydrothermal carbonisation of organic waste

SunCoal has set up a pilot-scale HTC plant in Ludwigsfelde. They have used this to produce biocoal for the gasification investigations. An industrial plant can generate approximately 17,600 tonnes of biocoal per year from an input of 60,000 tonnes of biogenic residues. Its fine particle size and the associated fluidisability makes it highly suited as feedstock for entrained-flow gasifiers.

The hydrothermal carbonisation process uses heat and pressure to replicate the natural carbonisation process of biomass. Here, plant residues with 20 to 75% water content and a low calorific value are converted into hydrophobic biocoal that can be mechanically dewatered.

In the HTC plant, the supplied organic waste and biogenic residues are crushed to a maximum grain size of 60 mm and inorganic contaminants such as rock, metal or plastic are removed. The processed biomass is converted in an aqueous solution at a temperature of about 220 °C and at a pressure of 25 bar in three to four hours. In the first step, the carbohydrates are hydrolysed releasing water; in the second step, these small molecules are polymerised to produce coal precursors and resins, whereby further water is released. Short reaction times result in humus or peat-like products; the result is comparable to brown coal after a period of up to six hours.

The hydrothermal carbonisation is exothermic and heat is released. The process for producing the biocoal consumes up to 30% of the chemical energy of the biomass used as well as external energy amounting to approximately 7% of the energy content of the final product. Only a small part of the thermal energy released could be utilized as useful heat.

During the carbonisation process, the material loses its underlying fibrous structure and is homogenised. The calorific value increases by up to 70% relative to the raw material. The biocoal produced is hygienically clean and can be further dried with low energy consumption. The water content of the resulting sludge is initially reduced to 50% in a press. The sludge is then dried in a thermal drying plant to further reduce the water content to around 5%. The resulting water is treated on site or sent to a sewage treatment plant.

Biocoal has good combustion and gasification properties. It is very easy to grind, which is particularly important for dust combustion and entrained-flow processes, for which average particle sizes of 50-100 μm are required.

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Addresses

Project management, HTC plant
SunCoal Industries GmbH

Entrained-flow gasification, plant optimisation
TU München, ES