Investigations into the Air Heater Ash Deposit Formation in Large Scale Pulverised Coal Fired Boiler
نویسندگان
چکیده
A mineralogical study was undertaken of air heater deposits in a 300 MWe pf boiler located in Western Australia to understand the deposit formation in air heater sections of boiler as an aid to implement possible remediation actions. Several air heater deposit samples were collected in the selected regions of the air heater along with samples of the feed coal, bottom ash and fly ash for comparison of ash chemistry and mineralogy. The deposit samples were examined using a combination of ash chemistry and quantitative X-ray diffraction analysis; the ash samples by bulk chemical analysis to determine the major element chemistry and mineralogy by quantitative X-ray diffraction. Samples of the deposits were also analysed using optical microscopy and QEMSCAN, an automated electron beam image analysis system. Chemical and mineralogical analysis showed that the deposits are unusual in containing high amounts of sulphate, particularly of aluminium and, to a lesser extent, iron. From the analyses it appears that the formation of the deposits is due to the high sulphate content which is acting as a cementing agent. There is an indication that temperature could be another factor in formation of the deposits, with a decrease in temperature leading to the formation of sulphurous acid which then reacts with the reactive glassy amorphous fly ash phase to form the aluminium and iron sulphates. Dew point calculations indicated that this is a possible deposit formation mechanism based upon air heater temperature data obtained from the utility. There was no evidence that unburnt carbon has played a significant role in deposit formation. Large temperature fluctuations resulting from the inherent nature of the operation of the air heater are a significant factor in deposit formation and a practical solution to consider would be the use of an SO2 absorbent placed prior to the air heater. Introduction It has been well established by several researchers over the last few decades that the ash deposition on boiler components and associated equipment within a power utility is a significant problem. The build-up deposit causes several operational problems leading to unnecessary outages requiring periodic maintenance [1-3]. The problems are much more pronounced when ash blockages occur on the heat recovery systems such as the air preheater. The purpose of an air preheater is to recover the heat associated with exit flue gases from the boiler mainly to enhance the thermal efficiency and control the heat likely to be lost in the flue gas. As a consequence of build-up deposits, on these heat recovery systems, the heat transfer rates will be altered significantly, thereby reducing the efficiency of thermal boiler systems. In this study, an investigation was made of the air preheater of a typical 300 MWe local power utility located in Western Australia. The power utility employs a typical rotary preheater (mainly used for large water tube boilers) wherein hot flue gas flows through one portion of the wheel while cool, clean combustion air passes through the remaining portion. As the wheel revolves, the cold combustion air passes through these hot surfaces and becomes heated which is then sent to the burners to mix with fuel in the firebox [4]. This preheater is often prone to ash build-up within short periods of operation, mainly due to to the wet steam which comes from the soot blower steam supply and the accumulation of hardened ash with flow marks and dribbles. With an effort to upgrade the steam quality supply from the soot blower, the steam temperature is increased via mixing with the main steam to arrive at a mixed steam temperature of 360°C. This enables prolonged operation and the interval between air heater washes increases further. Despite these measures, hardened ash on the bottom cold side of the heat exchange system elements at the cold end basket is reported to occur in this utility. (see Figure 1). The top end of the elements appears to suffer less from ash blockage due to higher flue gas temperatures. Given these build-up issues in air heater sections, an investigation was carried using detailed mineralogical analysis to understand the interaction between ash particles and the surface of the air heater elements (in terms of how it sticks initially and on the formation of subsequent ash layers) along with the deposit chemistry to establish the factors/mechanisms leading to the formation of hardened consolidated ash eventually causing blockages in the heat exchange elements. The study also examined other contributing factors and precursors for ash build-up as it is a well known fact within the power generation industry that soot from diesel burn, moisture and flue gas temperature below the SO2 dew point are contributing factors . Sampling and Analytical Procedures Five samples of the air heater deposits were provided for analysis. Three subsamples were taken of sample A as an initial visual examination indicated that differences existed between the fragments supplied for analysis (see Table 1 for details). Sub-samples were taken of the coarser fragments for optical and scanning electron microscopy and the reminder was ground and sub-sampled for chemical and X-ray diffraction analysis. The power utility also supplied representative samples of the feed coal, bottom ash and fly ash for chemical and X-ray diffraction analysis.. A mineral matter sample was prepared from the coal sample using low temperature radio-frequency plasma ashing, a technique which minimizes detrimental alteration of the coal mineralogy. Subsamples of the ashes and deposits selected for X-ray diffraction analysis were ground in an agate mortar and pestle then packed into an aluminium holder. Quantitative X-ray diffraction analysis was performed using SIROQUANTTM, a quantitative X-ray diffraction analysis software package that uses Rietveld procedures to generate a synthetic pattern which is then matched to the experimental data using a least squares minimisation fitting procedure. This approach has the advantage that the complete diffraction pattern is used to derive the quantitative results rather than relying upon one or two peaks for the determination. Although application of the Rietveld procedure usually requires that the phases be crystalline in order to calculate the synthetic XRD pattern, SIROQUANT makes use of experimentally derived structural data (observed hkl files) for amorphous or poorly crystalline phases and thus can be used to determine the amount of amorphous material present in the sample. Major element analysis of the ash samples was carried out using ICP-AES following fusion of the sample with lithium metaborate/tetraborate and dissolution of the fused sample in acid. The coal sample was ashed at 815C prior to borate fusion. Loss on fusion was determined at 600C and 1000C. Moisture and ash yield were determined using a LECO MAC analyser. Total carbon was determined using a LECO TruSpec CHNS analyser. Following determination of total carbon, carbonate carbon was removed by acid digest and carbon re-determined; the residual carbon assumed to be unburnt carbon and the difference, carbonate carbon. Sulphur was determined using a LECO TruSpec CHNS analyser.
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