The effect of oxygen and volatile combustibles on the sulphation of gaseous KCl

Sulphur/sulphate containing additives, such as elemental sulphur (S) and ammonium sulphate (NH4)(2)SO4), can be used for sulphation of KCl during biomass combustion. These additives convert KCl to an alkali sulphate and a more efficient sulphation is normally achieved for ammonium sulphate compared to sulphur. The presence of SO3 is thus of greater importance than that of SO2. Oxygen and volatile combustibles could also have an effect on the sulphation of gaseous KCl. This paper is based on results obtained during co-combustion of wood chips and straw pellets in a 12 MW circulating fluidised bed (CFB) boiler. Ammonium sulphate was injected at three positions in the boiler i.e. in the upper part of the combustion chamber, in the cyclone inlet, and in the cyclone. The sulphation of KCl was investigated at three air excess ratios (lambda = 1.1, 1.2 and 1.4). Several measurement tools were applied including IACM (on-line measurements of gaseous alkali chlorides), deposit probes (chemical composition in deposits collected) and gas analysis. The position for injection of ammonium sulphate had a great impact on the sulphation efficiency for gaseous KCl at the different air excess ratios. There was also an effect of oxygen on the sulphation efficiency when injecting ammonium sulphate in the cyclone. Less gaseous KCl was reduced during air excess ratio lambda = 1.1 compared to the higher air excess ratios. The optimal position and conditions for injection of ammonium sulphate were identified by measuring KCl with IACM. A correlation was observed between the sulphation of gaseous KCl and reduced chlorine content in the deposits. The experimental observations were evaluated using a detailed reaction mechanism. It was used to model the effect of volatile combustibles on the sulphation of gaseous MCI by SO3. The calculations supported the proposition that the presence of combustibles at the position of SO3 injection (i.e. AS) causes reduction of SO3 to SO2

The importance of SO2 and SO3 for sulphation of gaseous KCl - An experimental investigation in a biomass fired CFB boiler

This paper is based on results obtained during co-combustion of wood pellets and straw in a 12 MW circulating fluidised bed (CFB) boiler. Elemental sulphur (S) and ammonium sulphate ((NH4)(2)SO4) were used as additives to convert the alkali chlorides (mainly KCl) to less corrosive alkali sulphates. Their performance was then evaluated using several measurement tools including, IACM (on-line measurements of gaseous alkali chlorides), a low-pressure impactor (particle size distribution and chemical composition of extracted fly ash particles), and deposit probes (chemical composition in deposits collected). The importance of the presence of either SO2 or SO3 for gas phase sulphation of KCl is also discussed. Ammonium sulphate performed significantly better than elemental sulphur. A more efficient sulphation of gaseous KCl was achieved with (NH4)(2)SO4 even when the S/Cl molar ratio was less than half compared to sulphur. Thus the presence of gaseous SO3 is of greater importance than that of SO2 for the sulphation of gaseous KCl. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Two Strategies to Reduce Gaseous KCl and Chlorine in Deposits during Biomass Combustion - Injection of Ammonium Sulphate and Co-Combustion with Peat

Combustion of a biomass with an enhanced content of chlorine can result in operational problems including deposit formation and superheater corrosion. The strategies applied to reduce such problems include co-combustion and the use of additives. In this work a mixture of wood pellets and straw pellets was fired in a circulating fluidised bed boiler. Two strategies were applied to decrease the risk of superheater corrosion by reducing gaseous KCl and content of chlorine in deposits: sulphation of KCl by injection of ammonium sulphate and co-combustion with peat. During co-combustion of biomass with peat both sulphation of KCl and capture of released potassium in ash components can be of importance. The results were evaluated by means of IACM (on-line measurements of gaseous KCl), deposit probe measurements (chemical composition in collected deposits, initial corrosion) and ash analysis (chemical composition in fly ashes). The best overall performance was achieved with ammonium sulphate, which significantly reduced KCl. Meanwhile almost no chlorine was found in the deposits. Only a minor reduction of gaseous KCl was obtained during co-combustion although the chlorine content in the deposits was greatly reduced. The resistance to initial corrosion was improved during both injection of ammonium sulphate and co-combustion with peat