An overall scheme of conversion routes from biomass to bioenergy including their stage of development offers the report Renewable Energy Sources and Climate Change Mitigation written by the IPCC in 2012 or the report Biomass for heat and power by the International Energy Agency (IEA) from 2015.
Processes relevant for LCMW biomass conversion to bioenergy are the following:
Table 1: Processes relevant for LCMW biomass conversion to bioenergy
A1: Biochemical conversion
Anaerobic digestion is a biological decomposition process, where breakdown of organic matter occurs in absence of oxygen. It proceeds in four stages involving four different groups of microorganisms. The final product of the decomposition is biogas. The residues from the digestion can be used, after being stabilized, as fertiliser depending on the composition of the input material.
A practical trial showed that it is possible to process grass in an anaerobic single-stage, semi-continuously fed reactor. The grass is chopped, then compacted and ensiled. Before being fed into the digester, the feedstock is soaked in water in order to dilute it.
Roadside grass is considered as good material regarding the biogas yields and no problems with the digestion occurred (Delafield, 2006). The option to partially replace maize in biogas plants by grassy material from cultural landscape conservation was investigated by a stakeholder alliance in two model regions in Germany. Landscape conservation biomass was confirmed to be interesting for the use in digestion as it creates more than 50 % of the feedstock for the biogas fermenter in one of the model regions (Pehlken, et al., 2015).
A2: Thermochemical conversion
Combustion is a process of oxidation, where carbon and hydrogen contained in cellulose, hemicellulose, lignin or other molecules like methane react with excess oxygen, releasing CO2, water and heat. When biomass or biogas is combusted for electricity production, the recovery of excess heat is desirable. The integrated systems of combined heat and power generation (CHP) utilize the excess heat for heating, cooling, dehumidification, or process applications. The optimal size for a biomass CHP plant is supposed to be around 20 MWe, with ideal biomass sourcing distance of maximum 50 km.
Waste wood biomass from pruning of urban green chopped to wood chips can be used as feedstock for combustion in a wood-chip boiler for heat production. Good practice examples are five wood-chip boiler plants (200 kW each) which have been chosen for the heating of five big public buildings in Viterbo Town, Italy (Carlini, et al., 2013).
Gasification of biomass takes place when the material is treated by high temperature (800 – 900 °C) under limited presence of an oxidising agent. The product of this process is called synthetic gas or syngas, a mixture of CO, CO2, CH4, H2 and water. The energy content of the gas is given by the biomass type and the gasification agent (air, oxygen, steam or hydrogen). Air or oxygen produces syngas with low to medium energy content, which is used in combustion for generating heat and electricity.
Five students from University of Oldenburg in Germany have developed a small-scale power plant (30kWel and 60kWth) which is able to convert solid biomass fuels – like woodchips and other agricultural or landscape maintenance residues by the thermo-chemical gasification into high quality syngas to generate CHP electricity via usual combustion engine and generator (LiPRO Energy).
B1: Torrefaction and HTC
Torrefaction is a mild pyrolysis carried out by 200 – 300 °C, where the solid fraction represents the main product. It offers the possibility of making torrefied pellets representing an even more densified form of an energy carrier.
Wood residues which compromise a largely available biomass resource are in the focus as feedstock for torrefaction as well as roadside grass and woody roadside biomass.
Hydrothermal carbonisation (HTC) is conducted in the presence of subcritical liquid water under temperatures between 180 – 250 °C. It converts the moist input material into carbonaceous solids without the need of previous drying. The water is kept liquid during the process by letting the pressure to come up with the steam pressure in a pressure reactor. Charcoal creates the main fraction among the products.
The SunCoal® CarboREN® technology (based on hydrothermal carbonization) is able to use LCMW biomass as feedstock. Almost all plant-based biomass can serve as a source of biocoal. Whole plants can be used, even lignite or wood-based materials that are not applicable to fermentation. Biomass with high water content can be used, for example grass cuttings, which would not be applicable for direct burning. Even impure biomass can be used, as the process involves a washing step. The company AVA-CO2 Schweiz AG and a new HTC facility in Halle Germany operated by the Hallische Wasser and Stadtwirtschaft GmbH use urban green residual biomass, respectively green communal- and garden residues as feedstock for HTC.
B2: Pelletizing and briquetting
Pelletizing and briquetting processes use mechanical compaction for bulky biomass, usually with screw or piston presses. Pellets and briquettes offer the advantage of consistent quality and size, better thermal efficiency and higher density than loose biomass, and allow a higher transport distance.
Different LCMW feedstock types can be used for the production of pellets and briquettes like grass, woody residues, roadside biomass or leaf-fall. Because fresh grass from first cuts has a high protein content and therefore high ash content, it is more beneficial to use the matured grass from last cuts which, above that, cannot be used as animal feed.
Table 2: Overview of possible conversion technology and matching LCMW feedstock type
For more detailed information and examples please see the greenGain Deliverable D4.1 “Report on the state of the art of the occurrence and use of LCMW material for energy consumption in Europe and examples of best practice”.
Authors: Kathrin Ludewig, Aline Clalüna