Beyond reducing the use of fossil fuels; Sweden has also made it its goal to replace nuclear energy with sustainable use of renewable resources. This goal is supposed to be implemented within an intermediate time period; mainly by the increased use of domestic biomass. Several estimates of present and future biomass fuel supplies have indicated that an expansion from the present 90 TWh/year to a maximum of about 150-210 TWh/year by the year 2025 could be feasible. However; as experience has already shown; switching from fossil fuels to using biomass and waste as fuels introduces some significant challenges that must be overcome. For example; biomass and waste can contain significant amounts of Na and K as well as trace elements such as Hg; P; Cu; Cr; As; Cd; Pb; Zn and Cl. The alkali metals together with P; Zn and Cl have a strong tendency to act as fluxing agents; decreasing the melting point of ashes to the point that; under combustion conditions; they form sticky; corrosive melts that are deposited on boiler surfaces. The deposits greatly reduce heat transfer in the furnace and corrode heat transfer surfaces. In addition; volatile metal chlorides are often formed; resulting in deposition on surfaces; with subsequent Cl-induced corrosion.
Simple measures to diminish these problems by using mixtures of fuels or inexpensive additives; thereby changing the melting and condensation temperatures and the specification of the ash system; have been proposed; but are primarily ad hoc and not guaranteed to work. At this point; solutions to these problems; which will allow biomass to be used sustainably; as desired by the politicians; can only come from increased knowledge of the underlying chemistry involving Na; K and the trace elements under combustion conditions. A survey of the literature quickly shows that there is very little available; especially of sufficient quality to allow models to be developed to allow the extent of fouling and corrosion problems to be identified and studied. The latter; of course; must be the outcome of any work in this area.
Work is currently underway at Ume√• and Link√∂ping Universities to determine and model the elementary; gas-phase; chemical reactions that occur when K; Na; Hg; Cd; etc.; are released into a combustion environment; as during the burning of biomass. Specially designed reactors have been built and coupled to a molecular-beam; mass spectrometer (MBMS). The latter has seen virtually no use in this field and yet allows the possibility of identifying and quantifying the presence of reactive intermediate species hereto immeasurable. Experience and results from this novel technique will be presented along with the models developed.
Proceedings from the Third International Disposal Conference; Karlskoga; Sweden; 10-11 November; 2003
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