Introduction
HoSt acts as a turnkey supplier of (air blown) fluidized bed gasifier plants. By means of gasification biomass fuels can be converted to a combustible syngas, which can be cleaned and used in several kinds of power generation equipment.
As a turn key supplier of power plants, the main focus of HoSt is on applications where the syngas is burned to produce steam for use in a steam turbine. HoSt, in combination with partners, is developing and demonstrating a technology where the produced gas is used in gas engines.
The gasifier technology provides some serious advantages over conventional combustion technology, namely:
Clean technology, low emissions.
In a gasification plant an extremely low emission level can be achieved. In a combustion plant based on grate firing or a fluidized bed reactor, the process temperature is limited to approx. 800 – 900 oC to avoid melting of ash which is present in the fuel. In the HoSt gasification plants the fuel is converted to syngas at the same low temperature level, however the produced syngas is then burned at temperatures of 1400 – 1800 oC in a syngas burner , after removal of low temperature melting ashes. At this high temperature a 100% combustion can be achieved and all harmfull emissions (hydro carbons, CO, dioxines) are eliminated. Reports from independent measurement companies are available, which show that absolutely no dioxines are present in the produced flue gasses.
Technology suitable for ‘difficult’ fuels.
Several fuels like straw, sun flower husks, grasses are difficult to process in combustion systems due to the low melting temperatures of the ashes and the fouling of the downstream boiler components. In the fluidized bed gasifier temperatures can be controlled at levels as low as 750 – 800 oC. Since the syngas is combusted at high temperatures in the syngas burner, no problems with emissions will arise. Boiler fouling in combustion systems, is mainly caused by the presence of alkali metals (Na, K, P) in the flue gasses. In the HoSt gasifier concept the produced syngas first is cooled down to around 500 oC, after which the ash is removed and the syngas is burned. At this low temperatures, the alkali metals condense on the ash particles and are removed from the syngas with the ash. Due to this removal of alkali metals, no excessive fouling can take place in the boiler, increasing reliability and decreasing maintenance costs.
As an example, in a conventional system burning sun flower husk, the boiler has to be stopped every 2 -4 weeks in order to manually clean the heat exchanging surfaces. A boiler stop normally requires 2 -4 days to cool down, to clean and to start up again.
High electrical efficiency.
By firing the produced syngas in a gas engine, the electrical efficiency of a plant can be increased. At this moment the technology for this application is being developed and is demonstrated in a project in Portugal, together with some partners providing the technology for gas cleaning and the gas engine.