Mika Järvinen Associate Professor at the Department of Energy Technology at Aalto University details how new technologies can further increase biomass potential
Reducing fossil CO2 emissions in our energy systems has driven research to new ways of biomass conversion to form methane and bio-oil. Up to now, the technologies applied, gasification 1 and fast pyrolysis 2 are close to being commercial technologies as applied in 3 recently commissioned plants.
As an example, Fortum Oy operates an integrated fast pyrolysis plant in Joensuu (Finland) producing 50000kt of bio-oil per year which is combusted in adjusted oil burners for power and heat production. In its GoBIGas plant Göteborg Energi (Gothenburg, Sweden) produces 160 GWh per year of bio-methane which is fed in the local gas distribution network. Lahti Energia (Finland) gasifies pre-sorted solid waste (solid recovered fuel, SRF) to yield a gas which is combusted to generate power and heat with an efficiency of 88%.
However, utilisation of solid waste and very wet biomass fuels for efficient energy conversion to high-grade fuels still bears challenges, but research in advanced technologies is ongoing.
The gasification of solid waste is very challenging as waste streams contain high amounts of ash, alkali, chlorine and heavy metals. Circulating fluidised bed (CFB) reactors are a promising conversion technology due to their high fuel-flexibility. Although in operation, gas cleaning before combustion in the waste gasification plant in Lahti still poses a challenge, resulting in considerable maintenance costs and plant availability issues. This situation could be alleviated by improving the conversion of solid waste into a cleaner gas product by optimizing operational parameters. Related research is conducted at Aalto University in a newly deployed test rig. Although still challenging, CFB gasification of waste has shown to be a highly efficient and clean pathway to recover energy from waste.
Another challenge imposed by biomass fuels is the often high moisture; 40-60% for vegetation biomass and up to 95% for liquid biomass such as sewage sludge and algae. Making them suitable for conventional gasification or combustion requires extensive drying which can account for up to 30% of the energy contained in the biomass which deteriorates the process efficiency.
Hydrothermal processes, in which the biomass’ water content is used as a reaction facilitator, are currently investigated. Among those, supercritical water gasification shows promising results for processes in which very moist biomass is converted into chemical fuels such as bio-methane or hydrogen. Water becomes supercritical at pressures and temperatures above 220 bars and 370˚C, respectively. At which, water’s physical properties combine an advantageous mix of vapour and liquid ones and allow more efficient gasification reactions. Experimental results obtained in cooperation between Aalto University and Åbo Akademy show that wet biomass can be gasified in supercritical water at high temperatures into high-grade syngas (mainly H2 and CM4) without significant contaminants.
1 Gasification is the partial combustion of a fuel at high temperatures in an oxygen-poor atmosphere. Organic compounds are broken down to form mainly hydrogen, methane, carbon monoxide and dioxide.
2 Fast pyrolysis is the decomposition in an oxygen-free environment at temperatures of approximately 500°C. The main product is a complex liquid mixture of higher hydrocarbons.
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