Ramprogram – Åtgärder för samtidig minimering av alkalirelaterade driftproblem, Etapp 2

Rapport, 2007

Combustion of an increasing amount of biofuel and waste woods has resulted in certain environmental advantages, including decreased emissions of fossil CO2, SO2 and metals. On the other hand, a number of chloride and alkali related operational problems have occurred which are related to combustion of these fuels. Alkali related operational problems have been studied in a project consisting of two parts. The overall scope has been to characterise the operational problems and to study measures to minimise them. The first part was reported in Värmeforsk report 997 where the results were summarised in a table of different measures. In part two, additional measures have been included in the test plan and initial corrosion has been studied linked to the different measures. The tests have also in part two been carried out at the 12 MW CFB boiler at Chalmers. The effect of the selected measures has been investigated concerning both deposit formation and bed agglomeration, and at the same time emissions and other operational conditions were characterised. The second part of the project has among other things focused on: • To investigate measures which decrease the content of alkali and chloride in the deposits, and consequently decrease the risk for corrosion (by investigating the initial corrosion). Focus was also on trying to explain favourable effects. • To investigate if it is possible to combine a rather low dosage of kaolin and injection of ammonium sulphate. This was done in order to reduce both bed agglomeration and problems from deposits during combustion of fuels rich in chlorine. • To investigate if co-combustion with sewage sludge, de-inking sludge or peat with high ash content, could give similar advantages as conventional additives. • Investigate if ash from PFBC is possible to use as an alternative bed material. By comparing the different measures in part two, it could be concluded that cocombustion of sewage sludge gave the best overall effect. The judgement was based on the effects concerning bed agglomeration, level of alkali chloride in the flue gas, deposits and initial corrosion. Simultaneous addition of kaolin and ammonia sulphate also had a favourable impact both in the bed and on the alkali chlorides in the gas phase. Dosage of kaolin did not reduce the effect of injected ammonium sulphate. Cocombustion of peat could also be a very attractive alternative, but it is critical to select a suitable peat type. It is of special concern to avoid peat with a high content of calcium, since it can increase the level of alkali chlorides by reacting with available sulphur. Change of bed material to ash from PFBC can decrease problems with bed agglomeration, but there is a risk of increased deposit formation and corrosion. A boiler operated with high steam data should consequently combine the change of bed material with injection of ammonium sulphate or another source of sulphur. Co-combustion of de-inking sludge from pulp and paper production can reduce problems with bed agglomeration. There is, however, an increased chance of corrosion due to the high content of calcium. Injection of sulphates may improve the situation concerning corrosion. The results also showed that low potassium chlorine content in the flue gas decreases the risk of corrosion. Increased amount of potassium chlorine content in the flue gas will not necessary give a high amount of chlorine content in the deposits if there is sulphur in the system. Key words: agglomeration, deposits, corrosion, alkali, kaolin, ammonium sulphate, sludge