For conversion of woody biomass, the Netherlands based Biomass Technology Group (BTG) has designed a fast pyrolysis plant based on a rotating cone reactor developed by the University of Twente.
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. Waste woody biomass from riverside and dike maintenance, from pruning or from urban green chopped to wood chips can be used as feedstock for combustion in a wood-chip boiler for heat production.
Pyrolysis is a process of thermal degradation of biomass under absence of any oxidising agents. The products are in solid (charcoal), liquid (pyrolysis oil) and gaseous form. The proportion of the fractions depends on process temperature, heating rate and residence time. At lower temperatures around 400 °C, the main product is charcoal, while at temperatures about 800 °C, mainly gas is yielded. Pyrolysis performed at high heating rates is known as fast or flash pyrolysis with residence time of seconds. In case of slow pyrolysis or carbonisation residence time of days is applied. Dry lignocellulosic biomasses such as perennial grass or wood can be converted to liquid fuels via a fast pyrolysis process in which biomass is rapidly heated to about 500 °C in the absence of oxygen and then quickly cooled in a reactor. The process converts the biomass into carbohydrate-based compounds that include condensable vapors; these are condensed into liquid bio-oil, the primary product of fast pyrolysis. This process can convert up to 75 % of biomass input into bio-oil, yielding about 135 gallons of bio-oil per ton of biomass. USDA-ARS have developed a FarmBio3 mobile pyrolysis system (see the figure below) that could be taken to on-site, simulating the real-world scenario of multiple on-farm units that convert feedstocks to bio-oil, which is delivered to a regional biorefinery (Brown and Harlow, 2015).
For conversion of woody biomass, the Netherlands based Biomass Technology Group (BTG) has designed a fast pyrolysis plant based on a rotating cone reactor developed by the University of Twente. In BTG’s technology, biomass particles at ambient temperature with an excess flow of hot sand particles, acting as carrier material, are introduced near the bottom of the cone reactor where the solids are mixed and transported upwards by the rotatory motion leading to rapid heating and a short gas phase residence time. After passing through several cyclones the vapours produced in the cone reactor enters a condenser where they are quenched by re-circulated oil. The pyrolysis reactor is integrated with a circulating sand system, a fluidized bed char combustor and a down-comer. The heat required for the pyrolysis process is provided by burning char in presence of air. The main product of this system is oil. The non-condensable pyrolysis gases are combusted and can be used e.g. to generate additional steam and excess heat can be used for drying the feedstock (BTG, 2016).
Dry woody biomass from LCMW of hedges and tree rows, olive pruning, riverbank maintenance e.g. reed could be utilized in pyrolysis systems.
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. Landscape maintenance residues can be converted into high quality syngas via thermo-chemical gasification to generate CHP electricity via usual combustion engine and generator. For example, gasification from woodchips obtained from the landscape maintenance in Germany has been successfully demonstrated by the LiPRO‐Energy GmbH & Co. KG in their small‐scale power plant (30 kWel and 60 kWth) that is able to convert solid biomass fuels ‐ like woodchips and other agricultural or landscape maintenance residues not suitable for biogas plants, into high quality syngas to generate CHP electricity via usual combustion engine and generator (see the figure below). The feedstock is heated up to ca. 700 °C by the process heat under a shortage of oxygen. The products are pyrolysis steam and charcoal, pyrolysis steam gets oxidized by about 1100 °C in the next step to crack long and ring shaped carbon-hydrogen molecules to avoid tar compounds in syngas. Third step is the reduction of CO2 by reacting with the hot charcoal to CO, due to several further reactions like water steam shift, syngas has following components: 3 % methane, 20 % hydrogen, 21 % carbon monoxide, 12 % carbon dioxide and 44 % nitrogen and a calorific value of 5.7 MJ/Nm³. After filtering the gas by a simple dry fabric filter the high quality gas is ready for the combustion engine. During the procedure, the charcoal‐ash mixture is produced and it is used in compost to improve ecological functions of soil. In 2015, during 7000 CHP operating hours, 210,000 kWhel were generated. Half of the annual feedstock amount comes from hedges of a farm and the other half from nearby forest residues (5 km radius). If the plant is operated for 8000 hours per year, it needs about 240 tDM wood or other lignocellulosic biomass. Biomass from roadside pruning, bush mulching, tree falling, wood processing residues, etc. can be used. Fuel needs to be dried to 15 % moisture content, which is done with help of the waste heat. The simplest and most cost‐efficient way to use roadside biomass or hedges in LiPRO plant is to manage these landscape biomass with a tractor equipped with a crane and a felling grapple. The trees are harvested from the thick end at about 20 cm diameter at breast height (DBH) e.g. every three years, and piled up at a place accessible to a truck. They are chipped with a self‐propelled chipper. The costs from harvest to storing place is about 9 € per loose cubic meter.
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)
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. Biochar is 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.