INTRODUCTION
In order to reduce CO2 emission, ELSAM (the electric utility company of the western part of Denmark) is looking into the possibilities for using biomass - mainly straw - for combustion in high-efficiency power plants. In this connection ELSAM has investigated 3 ultra supercritical boiler concepts for combustion of straw only or together with coal: 1) pulverized fuel boilers (PF-boilers); 2) circulating fluidized bed boilers (CFB-boilers); and 3) vibrating grate boilers with 100% straw. These investigations have mainly been full-scale tests with straw fed into existing boilers.
Corrosion tests have been performed in these boilers using temperature regulated probes and in-plant test tubes in existing superheaters. The corrosion has been determined by detailed measurements of wall thickness reduction and light optical microscopic measurements of the material degradation due to high temperature corrosion. Corrosion mechanisms have been evaluated using SEM/EDX together with thermodynamic considerations based on measurements of the chemical environment of the flue gas.
Great differences were found in the corrosion mechanisms for superheaters in PF-boilers and the CFB-boilers fired with almost the same share of straw and at the same metal temperatures.
The power generating capacity in Denmark is mainly based on coal combustion. Coal can be a troublesome fuel with respect to ash deposition and high temperature corrosion, but generally this can be solved by fuel selection or blending various types of coal.
In the Danish utility sector the fuel situation is constantly changing. One of the most recent changes is an increased use of biomass - especially strew - for electricity production. As straw is characterized by very high concentrations of C1 and K [1], a fuel with high corrosion and ash deposition propensities has been introduced.
The introduction of biomass for electricity production is one of the means to reach the goals set up in the Danish Energy Plan - ie. a 20% reduction of C02 emissions by year 2005 compared to the 1988 - level. In order to reduce C02 emissions by 20% and still stick to coal as the major fuel, it is important to maintain a highly efficient electricity generation and to include biomass, as biomass is considered to be a 100% renewable source of energy.
The newest and future power plants within the ELSAM area have steam parameters of 250 bar/580 ºC/560 ºC and 290 bar/580 °C/580 ºC/580 ºC giving efficiencies of 45 and 47% respectively based on LHV. The same efficiencies - and hence steam data - are aimed for with biomass combustion.
However, due to the high alkali and C1 content of straw, high-temperature corrosion of superheater tubes, and also ash deposition, could be limiting factors for efficient use of this fuel. Therefore, ELSAM has set up an R&D program aiming at co-firing straw and coal or straw alone in existing units and dedicated to the development of new technologies capable of firing large amounts of straw, with a view to ensuring high efficiency and maintaining high fuel flexibility.
