Inorganic Species Behaviour in Thermochemical Processes for Energy Biomass Valorisation

Inorganic species from biomass (wood or agricultural waste) exhibit large variations in compositions and amounts, depending on the origin of the biomass (nature, growing conditions and location). Different thermal conversion processes (combustion, pyrolysis, gasification or other) and various technologies (grate furnace, fixed or fluidized bed, entrained flow reactor) using biomass, provide a wide variety of operating conditions with differences in atmosphere, pressure and temperature. During any thermal process and mainly depending on initial composition of the biomass, process temperature and atmosphere, some of the inorganic species react and may form liquid or gas compounds, alone or combined with other species: they may either be trapped at different locations during the process or released in the gas. The potential interactions of inorganic species with reactor walls, bed materials (in fluidized bed reactors), transfer lines and downstream process units are not well understood for most species. Both technical and economic issues about inorganic species behaviour are probably growing to become important in a near future: pressure on timber markets is growing and prices have already been rising : one solution is to replace this noble and clean resource (wood) by ash richer feedstock, like straw, dedicated energetic cultures, agricultural or even municipal solid waste. Biogas production from waste deposits is a good example to show how waste can be valorised. Going further (thermochemical conversion) with such ash rich feedstock will increase the potential of their re-use but also the technical difficulties in a dramatic extent; soil enrichment for the agriculture currently largely depends on the re-use of biomass, completed with artificial fertilisers. The question how to re-use the inorganic material in biomass after thermo-chemical conversion is an important subject. The objective of this paper is to present a global review of the technological difficulties ( corrosion, fouling, etc.): a detailed summary of the specific behaviour of the inorganic compounds in biomass thermal conversion facilities will be given, going from the description of inorganic species included in biomass up to the prevention and the solution, as far as possible. The following topics will therefore be developed: inventory of inorganic species in different biomasses; general review of the inorganic speciation during thermal conversion processes; technological barriers to overcome for economic and technological process improvements; specific aspects related to inorganic species behaviour investigated at CEA: inorganic species release and condensation; aerosol behaviour and deposits; hot particles filtration; slag formation and interactions with reactor wall.