Karlsruhe centre advances gas-from-biomass process

Ludwigshafen, Germany -- The Institute for Technical Chemistry at Forschungszentrum Karlsruhe – one of the biggest science and engineering research institutions in Europe – is developing a process to produce synthetic fuels or chemical feedstocks from dry residual biomass.

The development work is focused on relatively cheap and virtually unused residual biomass such as excess cereal straw, set-aside hay, wood residues, bark and even paper/cardboard waste. These materials contain more ash and heteroatoms than, for example, bark-free wood and so require the development of dedicated processes.

In the Karlsruhe process, the biomass is first converted via an intermediate product to synthesis gas from which fuels and other chemical feedstocks can then be produced by known chemical syntheses.

The process starts with the production of pyrolysis oil and coke from decentrally generated biomass by rapid pyrolysis at about 500 °C. The air-dry biomass is shredded and, for rapid pyrolysis at ambient pressure with exclusion of air, is mixed in a twin-screw mixing reactor with hot, mechanically fluidized sand as heat transfer medium.

The actual pyrolytic conversion of the biomass particles at about 500 °C and condensation of the pyrolytic vapors occur within a few seconds. This results in about 40-70% organic condensate (pyrolysis oil) and about 15-40% pyrolysis coke. The remainder comprises a non-condensable pyrolysis gas whose heat of combustion can be used to heat the sand or dry and preheat the products.

The brittle pyrolytic coke is finely ground and suspended in the pyrolysis oil. Since the oil is not used directly as a liquid heating or propulsion fuel, processing and purification are not required at this stage. The content by weight of coke should be as high as possible (up to 40%) and the material must be capable of being pumped, stored and transported without hazard.

The oil/coke slurries are transferred from the pyrolysis plants to a centralised entrained flow gasifier facility and converted to synthesis gas at pressures of up to 80 bar and temperatures above 1200 °C. The slurries are converted completely and rapidly to tar-free gas. Compatibility with high-ash fuels is made possible by the use of a cooling screen on which the slag precipitates in liquid form and can run off. The synthesis gas is purified, conditioned and processed into synthetic fuel.

Purification and conditioning of the crude synthesis gas is performed at pressures above the synthesis pressure. This eliminates the need for energy intensive intermediate compression of the purified synthesis gas prior to fuel or chemical synthesis. However, the crude synthesis gas has to be cleaned of particles, condensable tars, alkali salts, H2S, COS, CS2, HCl, NH3 and HCN to prevent poisoning of the catalysts in the downstream synthesis.

Using established chemical processes, such as the Fischer-Tropsch method, the synthesis gas is converted into hydrocarbons from which all types of gasoline and diesel fuels can be produced. Mainly diesel fuels, but also numerous other basic chemicals, can also be produced by further processing of methanol.

The Karlsruhe concept is designed especially to take account of the decentralised generation of biomass and its comparatively low energy content. Rapid pyrolysis, for example of straw, can be performed in regionally distributed, compact plants with a catchment area of up to 50 km; transportation of the energy condensed slurry is then economically feasible over greater distances, allowing gas generation and synthesis to be carried out in economically operating plants which may be of a similar size to petroleum refinery.

The cost effectiveness of the process can be optimized by maintaining a balance between the generation and availability of the biomass, the catchment area, the number of feasibly required pyrolysis plants and the transportation distances to the central gas producer. This is a major advantage of the slurry concept. This approach also offers high flexibility in terms of usable slurry, which can be produced from a range of feedstocks and with admixtures of other materials. Utilization of the residual biomasses mentioned above could already satisfy more than 5 percent of current fuel consumption.

About 7.5 tonnes of biomass are required to produce a tonne of fuel. The costs of providing one litre of the synthetic fuel are already below €1 and thus within the range of taxed diesel fuel, with half of the costs arising from biomass provision.

Work in the Karlsruhe Research Center laboratory and pilot plant have demonstrated the technical feasibility of the key processes of the concept, namely rapid pyrolysis and high-pressure entrained flow gasification. Construction of a rapid pyrolysis pilot plant (500 kg/h) has begun there in cooperation with Lurgi AG.

A gas generation, gas purification and conditioning plant and a methanol synthesis plant will be added stepwise to these facilities. The pilot plant, meanwhile, is being used to further perfect and optimise the individual process steps and to demonstrate the integrated process prior to designing a demonstration plant as the first commercial scale facility.