“We can do in ten minutes what nature took millions of years to complete,” said Henning Bockhorn from the Engler-Bunte Institute at the Karlsruhe Institute of Technology (KIT) referring to a method which his team developed for the energy supplier Energie Baden-Württemberg (EnBW). The patented method enables biomass to be converted into a material similar to brown coal. The method is known as “biomass steam processing” (BSP) and is currently being optimised in a pilot plant operated by Energie Baden-Württemberg.
The researchers hope that by mid-2014 the process will be sufficiently mature to go into series production in 2016. Henning Bockhorn’s initial calculations show that the new biomass conversion method offers added value if it is operated with biomass subject to disposal requirements. The goal is to produce mobile carbonisation plants that enable coal to be produced locally from waste biomass.
Around four years ago, Bockhorn and his team started investigating hydrothermal methods in the context of growing interest in processes that enabled the carbonisation of waste biomass. However, standard processes such as pyrolysis and hydrothermal methods are associated with long reaction times. Pyrolysis is a process in which biomass is kept at around 450°C for a number of days during which pyrolysis gases and charcoal are produced. In comparison, hydrothermal carbonisation processes have reaction times of less than 24 hours, but the carbonisation of the liquid, heterogeneous reaction mixture is technically more complex and also requires the application of high pressure.
So it was no great surprise that Bockhorn and his team managed to find hydrothermal processes that were much easier to apply and did not require any use of pressure. The inventors of the hydrothermal process at the Max Planck Institute in Potsdam initially thought about using the method for the production of valuable carbon materials whose specific surface structure makes them excellent catalysts.
The group of researchers at the KIT tested the BSP method with straw, wood, grass, orange peel and sewage sludge both theoretically and experimentally in laboratory reactors and pilot plants in the gramme to kilogramme scale. The moisture content of the residual biomass used contains 60 percent chemically bound water, which nevertheless does not compromise the process. In contrast, the drawback of hydrothermal carbonisation is that water in the near-critical state is highly corrosive to the reactor material.
The method is relatively simple: residual biomass is transported on an auger conveyor from storage container to reactor. The auger conveyor is equipped with a device that prevents the gas from flowing back. The reaction during which cellulose-containing biomass is converted into coal takes place in the twin screw that is heated externally.
Organic, gaseous components that contain carbon are formed during this process. These components are returned via the coal, which causes them to disintegrate again with the result that far more carbon is retained in the solid phase than in the gaseous phase.
The biochar produced using the BSP method is odourless, can be easily ground and can be treated further, for example it can be thermally treated to transform it into active coal or into catalytically active carbon materials. The coal can also be combusted.
The pilot plant in Karlsruhe is designed for a biomass throughput of 50 kg/h. The plan now is to scale it up to an hourly throughput of one ton of biomass. However, the researchers still need to optimise the gas cycle which can potentially increase the solid phase carbon even further.
Biomass steam processing can potentially be technically implemented quite rapidly. Bockhorn has calculated that the process would be cost-free if biomass requiring disposal is used, thus saving on disposal costs. He also believes that series production would be feasible with reactors with a throughput of 10,000 tons of biomass per year.
Henning Bockhorn sees several advantages of the new method over current ones. On the one hand, it increases the energy density of the biogenic energy from 10-15 MJ/kg to 25-30 MJ/kg. This significantly facilitates the management of the fuel as its mass is reduced by 60 percent. The biochar can also be used for carbon sequestration and be integrated into soil. Its potential to increase soil fertility is currently being examined by researchers from the University of Hohenheim.
Even the combustion of the coal does not sustainably compromise the carbon cycle as the carbon is released at the same speed as it is assimilated. Moreover, biochar has the potential to be used as starting material for the carbon industry (coal hydrogenation, synthesis gas, methanisation). In principle, it is also suitable for use with different biorefinery concepts.
The biomass steam processing method has a positive energy balance. The conversion of cellulose to carbon releases energy. “Essentially, the process consumes little energy,” Bockhorn said. In fact, energy is released and returned to the process (vapour recovery). Around three percent of the total chemical energy introduced is required for heating the biomass to reaction temperature.”
In contrast, regular charcoal production is associated with huge energy loss. Specific pretreatment (e.g. the torrefaction of wood) can reduce the amount of energy required, but is associated with the loss of relatively large amounts of carbon to the gaseous phase. Bockhorn: “We want to lose as little carbon as possible to the gaseous phase. We want to keep all the carbon in the solid fuel and release as little as possible into the environment. This helps us close the carbon cycle.”
In cooperation with the Institute of Technology Assessment and Systems Analysis (Ludwig Leible), Henning Bockhorn is carrying out an ecobalance assessment with the aim of gathering information on potential environmental impacts associated with the carbonisation of biomass using BSP. The research project is funded by the foundation Energieforschung Baden-Württemberg.