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

Influence of air-staging on the concentration profiles of NH3 and HCN in the combustion chamber of a CFB boiler burning coal

The characterisation of the concentration profiles of NH3 and HCN are of great importance for increasing the knowledge of the formation and destruction pathways of NO and N2O in a fluidized bed boiler. Further improvements of the sampling methods for the determination of both NH3 and HCN in the combustion chamber in full-scale CFB boilers are also needed. A gas-sampling probe connected to a Fourier Transform Infra Red (FTIR) instrument and a gas-quenching (GQ) probe in which the sample is quenched directly in the probe tip by a circulating trapper solution were used. The FTIR technique is based on analysis of hot combustion gases, whereas the trapper solutions from the GQ probe were analysed by means of wet chemistry. The tests were performed during coal combustion in a 12 MW CFB boiler, which was operated at three air-staging cases with the addition of limestone for sulphur capture. The concentration profiles of NH3 and HCN in the combustion chamber showed a different pattern concerning the influence of air-staging. The highest levels of NH3 were observed during reducing conditions (severe air-staging), and the lowest were found under oxidising conditions (no air-staging). The levels of HCN were much lower than those measured for NH3. The highest levels of HCN were observed for reversed air-staging and severe air-staging showed almost no HCN. The potential reactions involving NH3 and HCN in the combustion chamber as well as the potential measurement errors in each sampling technique are discussed for the three air-staging cases

Ammonia Addition for NOx Reduction in Fluidized Bed Boilers

Ammonia is added to the gases in the combustion chamber of fluidized bed briilers in order to investigate its effect ori the emission of nitrogen oxides. A stationary and a circulating fluidized bed boiler are used. In the sta­tionary fluidized bed boiler an addition corresponding to a (NH3/NOexit-molar ratio of 3 gives a 50% reduction of the NO emission. Larger doses lead to escape of ammonia and other disadvantages. In the circulating fluidized bed boiler no effect from injection of ammonia into the combustion chamber is noted. The ammonia is decomposed. If NH3 is to be used in circulating fluidized bed boilers it should be introduced in the particle-free gas after the particle separator

Oxidation of Volatile Nitrogen Compounds During Combustion in Circulating Fluidized Bed Boilers

In circulating fluidized bed boilers, the volatile nitrogen species of the fuel are mostly oxidized to NO. This is shown in a test programme where the effect of batch supply of fuel on the NO emission has been carried out in two different commercial-type fluidized bed boilers. Measurements of gas concentration profiles of CO, NO HCN, NHa and C1 to Ca hydrocarbons in the boilers support the conclusions from the batch supply tests

Emissions of nitrogen oxide from a circulating fluidized bed boiler--the influence of design parameters

In a circulating bed of silica sand, char is the principal agent for the reduc­tion of nitrogen oxide. The role of volatiles is not significant. Most features observed can be qualitatively explained as a consequence of the presence of char.In a circulating bed with limestone addition, the oxidation of nitrogen containing volatiles causes an increased emission of nitrogen oxide. This effect can be mitigated by avoiding excess addition of limestone in relation to the sulphur content of the fuel or by arranging the air supply in a suitable way, thus de­creasing the oxidation of ammonia

Sewage sludge as a deposit inhibitor when co-fired with high potassium fuels

The objective of this work is to survey the fate of potassium in the gas phase of a fluidised bed boiler and gain deeper understanding of the involved mechanisms during co-firing of municipal sewage sludge with biomass containing high amounts of potassium and chlorine. The results show that formation of alkali chlorides in the flue gas and corrosive deposits on heat transfer surfaces can be controlled by addition of municipal sewage sludge even though the fuel is highly contaminated with chlorine. The beneficial effects are partly due to the content of sulphur in the sludge, partly to the properties of the sludge ash. The sludge ash consists of both crystalline and amorphous phases. It contains silica, aluminium, calcium, iron and phosphorus which all are involved in the capture of potassium. (c) 2010 Elsevier Ltd. All rights reserved

Chemical fractionation for the characterisation of fly ashes from co-combustion of biofuels using different methods for alkali reduction

a b s t r a c t Chemical fractionation, SEM–EDX and XRD was used for characterisation of fly ashes from different cocombustion tests in a 12MW circulating fluidized bed boiler. The fuels combusted were wood pellets as base fuel and straw pellets as co-fuel in order to reach a fuel blend with high alkali and chlorine concentrations. This fuel blend causes severe problems with both agglomeration of bed material if silica sand is used and with deposits in the convection section of the boiler. Counter measures to handle this situation and avoiding expensive shut downs, tests with alternative bed materials and additives were performed. Three different bed materials were used; silica sand, Olivine sand and blast furnace slag (BFS) and different additives were introduced to the furnace of the boiler; Kaolin, Zeolites and Sulphur with silica sand as bed material. The results of the study are that BFS gives the lowest alkali load in the convection pass compared with Silica and Olivine sand. In addition less alkali and chlorine was found in the fly ashes in the BFS case. The Olivine sand however gave a higher alkali load in the convection section and the chemical fractionation showed that the main part of the alkali in the fly ashes was soluble, thus found as KCl which was confirmed by the SEM–EDX and XRD. The comparison of the different additives gave that addition of Kaolin and Zeolites containing aluminium- silicates captured 80% of the alkali in the fly ash as insoluble alkali–aluminium-silikates and reduced the KCl load on the convection section. Addition of sulphur reduced the KCl load in the flue gas even more but the K2SO4 concentration was increased and KCl was found in the fly ashes anyhow. The chemical fractionation showed that 65% of the alkali in the fly ashes of the Sulphur case was soluble

The Role of Fuel Volatiles for the Emission of Nitrogen Oxides from Fluidized Bed Boilers - A Comparison Between Designs

The nitrogen oxides formed during fluidized bed combustion of solid fuels originate from fuel nitrogen in both char and volatiles. In the course of combustion these fuel constituents not only contribute to the production but also take part in the reduction of the emission. In order to study the role of the nitrogen-containing volatiles for the production and re­duction of nitrogen oxides in fluidized bed boilers a series of tests has been carried out with fuels having different contents of volatiles in a range from anthracite to wood in two types of boiler, a circulating and a stationary fluidized bed boiler. It was found that the different designs of boilers result in different behaviours of the ni­trogen-containing volatiles. In stationary fluidized bed boilers the volatiles tend to reduce the emissions of NO comparable to what takes place in a pulverized coal flame, whereas in circulating fluidized bed boilers the volatiles do not have the same beneficial effect. In cir­culating fluidized bed boilers the emission of NO tends to increase with an increasing content of volatiles in the fuel, whereas the emission decreases from the stationarry fluidized bed type of boiler burning fuels with a larger amount of volatiles

Measurements of Gas Concentrations in a Fluidized Bed Combustor Using Laser-Induced Photoacoustic Spectroscopy and Zirconia Cell Probes

The dynamic combustion behavior of a circulating, fluidized bed boiler (CFB) was studied using two high-speed gas analysis systems during the combustion of coal, pear, and wood chips. Time-resolved concentrations of SO2 and NO were measured by laser-induced photoacoustic spectroscopy (LIPS). A zirconia-cell based probe (lambda-probe), synchronized with the LIPS probe, measured fluctuations between reducing and oxidizing conditions. The two probes were positioned in the same measurement volume on the centerline of the CFB combustion chamber. The purpose of the work was to investigate the behavior of the LIPS in a combustion chamber containing reacting gases in order to extend the previous h-probe measurements to other gas components. Correlations between oxidizing and reducing conditions and gas species concentrations in three locations in the combustion chamber are presented. The best correlations were found in the upper part of the CFB combustion chamber. In some cases the correlation between reducing conditions and the LIPS signal was caused by unburnt hydrocarbons. Average values of [NO] and [SO2] obtained by the LIPS system were compared with the results from a sampling probe connected to on-line analyzers. The measurements of [NO] and [SO2] were disturbed by interfering gases during reducing conditions. During a sufficiently long time of oxidizing conditions, however, reasonable agreement was obtained between LIPS measurements of [NO] and [SO2] and those of the on-line analyzers. On some occasions (low SO2 concentration) the concentration of the OH radical was also measured