Phasing out peat from a co-fired 50 MWth circulating fluidized bed boiler : A theoretical sulfation potential study

Sammanfattning: The chemical company Perstorp AB has a 50 MWth circulating fluidized bed to supply the production plant with process steam. The fuel mixture used shows a large variation with about 20 different fuel fractions that can be divided into the categories, residue wood chips, sludge, forest fuel, animal waste, industrial residues, and peat. From an economic and environmental perspective, the use of peat should be minimized because it is classified as a fossil fuel. Peat has positive combustion characteristics due to a combination of sulfur and silica based minerals that can reduce alkali chloride related corrosion. Therefore it is not always possible phase out peat without negatively affecting the boilers availability. Besides reducing CO2 emissions, it is desirable to reduce the use of additives such as limestone and to use cheap waste fractions as fuels when operating a boiler. Similar to peat, reduced use of limestone and the introduction of a new waste fractions can affect the boiler availability negatively. If less limestone is used the risk of agglomeration can increase and often waste products can contain ash elements problematic in both a agglomeration and corrosion perspective.The aim of this master thesis is to investigate the possibility of reducing the usage of peat by minimizing the limestone content and to see if it would be possible to add a new Na-rich waste fraction to the fuel mixture. This was investigated by determining first what fuels that are used and in what amounts. Each fuel was either sent for new elemental analysis or existing analyses were used depending on if it was deemed to be still representative. With the help of experienced personnel working with the boiler, future possible cases for fuel mixtures could be determined: Case 1. Replacing peat with forest fuels. Case 2. Replacing peat with residue wood chips. Case 3. Replacing both peat and animal waste with residue wood chips. Case 4. Introducing Na-rich fuel. The theoretical available SO2 content could be determined for each case and three limestone levels. For each case the CO2 emissions and the economic savings could be estimated when the peat were fully removed and the limestone content halved. From an SO2 perspective, the results indicate that it could be possible to phase out peat for cases 1-2 by adjusting the limestone levels but this would not be enough for case 3. Adding the Na-rich fuel could also be problematic and more investigation has to be put into potential additives and fuels to compensate for the additional Na.When peat is fully phased out in case 1-3 the CO2 emission would decrease by 10 000 tonCO2/year. Due to the cost of CO2 emissions, this could result in considerable monetary savings. From the results it is estimated that case 1 could save 9.1 million SEK/year, case 2 10.3 million SEK/year, and case 3 6.5 million SEK/year when the peat is fully removed. This does not include changes in availability and maintenance costs.