Biomass gasification– steam fluidized bed
The gasification process, first demonstrated at the Güssing power plant, is based on the steam gasification of biomass in the internally circulating fluidized bed. The heart of the plant - the fluidized bed - steam gasifier – onsists of two interconnected fluidized bed systems. In the gasification unit the dried biomass is being gasified at approximately 850°C under injection of steam (instead of air) as gasification which creates a nitrogen-free, low-tar producer gas with high calorific value. Before its further application it is cooled and cleaned. The flue gas produced in the oxidation zone is cooled down, dust is removed and finally it is purged by the stack. The heat is used to operate an ORC module in order to improve the electrical efficiency.
Feeding the fuel drying unit with the individual prepared biomass is done continuously via various supportive transport units from above.
Drying air, heated up by low-temperature heat (which otherwise is lost), is used in counter flow to the biomass. The power-regulated operation with low flow rates allows the water content of the wood material to reduce effectively without causing dust or noise.
Integration of such fuel drying systems increases the electrical efficiency significantly.
The gasification process is an endotherm reaction. To maintain the balance, heat must be applied. Therefore non-gasified carbon (charcoal) is transported by the rotating bed material (sand as heat transfer fluid) from the gasifier into the connected combustion chamber and burnt there. This exothermic reaction provides the energy for the gasification process by, in the combustion chamber, heated-up bed material being transported back into the gasifier where a thermal transfer from sand to biomass occurs. In addition to the product gas flow, a flue gas stream exists, which is similar to that of a conventional incinerator.
The result of the biomass gasification with steam is an virtually nitrogen-free product gas, which is, according to prevailing equilibriums, rate, and time spent, composed of mainly components H2, CO, CO2, CH4 and higher hydro-carbons.
Before going to the gas engine the gas needs to be cooled and cleaned.
Withdrawn from the gasifier at approximately 850°C it is cooled down to about 150°C in the subsequent product gas cooler. The heat obtained is used for district heating. Constructive design ensures high gas velocities and pipe wall temperatures and in this way reduces the tendency to pollution. The in-fabric filter-secluded dust is guided back into the combustion chamber due to its high carbon content. The following scrubber reduces the concentrations of tar, ammonia and acid gas components. As the producer gas is further cooled to provide an optimal inlet temperature for the gas engine, water and tars are condensed. An organic solvent is used as a washing medium. Condensate loaded with tars is continuously channelled out from the scrubber system and replaced by a fresh one. The discharged washing liquid is disposed in the combustion chamber, the resulting condensate is used for the production of steam needed in the gasifier.
Because of this special procedure, virtually all residues in the process can be recycled, which leads to a gas-cleaning process that is free of waste or waste water.
Power generation in the gas engine
The engine transforms the energy contained in the product gas into electricity and heat. Originally designed for natural gas, engines were adapted specifically to the needs of product gas usage. For this application, amongst other things, gas mixing units were redeveloped, thereby increasing efficiencies, which were previously unattainable at biomass power plants in the small range.
Synthetic products
Due to its good quality, constant composition and high availability, the product gas is ideal for additional applications / refinements:
To increase the power output of the plant and to become more independent of permanent heat sales, the plant is also equipped with an Organic Rankine Cycle (ORC) module. A thermal-oil circuit transports heat from the producer gas cooler, the flue gas cooler and the engine exhaust cooler to the ORC module at atmospheric pressure. The ORC module is similar to a conventional steam-turbine-cycle, where instead of water an organic fluid is used. This gives more flexibility in choosing pressure and temperature levels and eliminates the need for superheating the steam.