Extent of carbonate decomposition in CFB boiler firing oil shale with different properties

Transcription

1 th International Symposium Topical Problems in the Field of Electrical and Power Engineering, Doctoral School of Energy and Geotechnology Kuressaare, Estonia, January 9, 008 Extent of carbonate decomposition in CFB boiler firing oil shale with different properties Kristjan Plamus Tallinn University of Technology (TUT) Abstract In this paper the behavior of mineral matter firing different quality oil shale in CFB boiler is discussed. The firing tests at Balti power plant are prescribed and tests targets are given. Based on ash composition the extent of carbonate decomposition (ECD) for circulating fluidized bed (CFB) boiler is calculated. The calculation methodology of ECD developed at Tallinn University of Technology, Department of Thermal Engineering (TUT s DTE) is prescribed. The ECD values taken from each ash port samples are calculated. Short overview about arrangement of the tests and ash samples port locations are presented. The results of fuel and ash samples granular composition and ash samples chemical analyses are presented. Firing enriched oil shale the ECD value is higher then firing ordinary oil shale. The ECD value depends on fuel granular composition. The ash samples taken from several ports of CFB boiler have a different ECD values. Keywords Oil shale, CFB boiler, extent of carbonates decomposition, fuel granular composition, ash samples. Introduction Oil shale is the main energy resource in Estonia. Oil shale is known as fuel with high content of mineral matter. Burning that kind of fuel is associated with several problems concerning carbonate minerals dissociation and CO emission. The mineral matter of oil shale contains a significant amount of calcium carbonate (CaCO ), dolomite (CaMg(CO ) ) and siderite (FeCO ). The carbonaceous part of oil shale contains on average 99.% of total CaO and 9.7% of total MgO. The FeCO content in the carbonaceous part of oil shale is so small (0.% [] that it can be excluded from calculations in practice. During combustion the carbonaceous part of oil shale is dissociated to CaO, MgO and CO. The following reactions occur: CaCO CaO + CO () CaMg(CO ) CaO + MgO + CO () From carbonates dissociate formed CO amount will be added to the CO that is formed at burning of carbon. It means increased content of CO in flue gas. The amount of CO originated from decomposition of carbonate minerals added to the flue gas is up to 8-9% from total CO in case of full decomposition of carbonates. It is therefore very important to decrease the ECD in the boilers in view of the task of reducing the oil shale power plant CO emissions. Furthermore dissociation of carbonates is endothermic reaction and because of that some amount of the heat energy containing in fuel is not used in energy production [].. ECD calculation methodology based on ash composition Decomposition of carbonates in the process of fuel combustion is expressed as the extent of carbonate decomposition - k CO. This is the ratio of the amount of CO released from minerals to the total content of mineral CO in the initial material. The calculation methodology of ECD was developed at Tallinn University of Technology, Department of Thermal Engineering (TUT s DTE) []. As indicated in results of research into Estonian oil shale s mineral part, it is possible to determine the ECD in a relatively simple manner, on the basis of ash analyses alone. Thus from the viewpoint of determining the ECD, we are primarily interested in CaO and MgO content in ash samples. 9

2 The initial amount of CO bound with CaO and MgO before decomposition of carbonates can be established using the formulas below []: CO CaO = CaO ash /.08 () CO MgO = MgO ash /0. () CO sum = CO CaO + CO MgO () where CO CaO and CO MgO CO bound accordingly with CaO and MgO, calculated in percentages of the ash mass and CaO ash and MgO ash CaO and MgO content in the ash. If the carbonaceous CO content in the ash (CO ash ) is also determined by the chemical analysis, the ECD is calculated with the Eq. : k CO = CO ash / CO sum () Thermal decomposition of carbonate minerals depends on the furnace temperature, fuel particle size and partial pressure of CO in the surrounding medium [] and operation load of boiler []. During oil shale combustion in industrial pulverized firing (PF) boilers, where combustion temperature is high (00 C and even higher) and fuel particles extremely fine, carbonate minerals decompose to a large extent. The total extent of decomposition of 7 8 carbonates k CO = (the average value is 0.97). [] During oil shale firing in a circulating fluidized bed (CFB) furnace at atmospheric pressure (Fig. ), carbonate minerals decompose to a smaller extent because of the low combustion temperature ( C) and coarse particles. From the result obtained, one can assume that the extent of decomposition of carbonates k CO remains between Arrangement of the tests To reduce CO concentration in flue gas and minimize effect of dissociation of carbonates also it is required to reduce the amount of carbonate mineral in oil shale. Removing minerals (enrichment) makes the heating value of oil shale higher. At the moment the lower heating value of oil shale burned at Balti power plant (BPP) is approximately MJ/kg (ordinary oil shale). That fuel is appropriate for PF boilers. Because of different combustion temperatures and processes in CFB boiler furnace it is possible to burn oil shale with higher heating value (up to MJ/kg) more efficiently. 0 9 A E Se c o n d a r y a ir L B F F lu e g a s C D P r ima r y a ir G H I J K Fig.. Oil shale fired CFB boiler (sample ports) [] raw fuel silo, fuel feeder, grate, furnace chamber, separating chamber, fluidized bed internal heat exchanger (INTREX), 7 separator, 8 convective superheater and reheater, 9 economizer, 0 air preheater, electrostatic precipitator, fuel crusher. Fuel samples before (A) and after (B) crusher 0

3 Using the enriched oil shale some positive aspects can highlight:. The concentration of CO in flue gas reduces.. Consumption of fuel reduces per produced kwh electric energy.. Reduced fuel consumption means lower expenses for fuel handling and transportation.. As oil shale has high ash content the ash mass flow rate reduces (reduces expenses for ash transportation to ash field and its exploitation will be lower). To study the influence to the boiler thermal efficiency and CO emission burning enriched oil shale the firing tests need to be carried out. The tests were carried out on CFB boiler K- in BPP, power unit. To compare the boiler performance with different properties of fuel the tests divided into two test series based on fuel lower heating values (LHV): 8. and MJ/kg. The fuel with LHV 8. MJ/kg is supplied by Aidu open pit mine. Fuel with LHV MJ/kg was specially prepared. The tests run near the boiler nominal power level (80kg/s primary steam) three night days firing ordinary oil shale and five night days firing enriched oil shale. Testing the boiler on each test series several process data are collected. Concerned with calculation of ECD the ash samples are needed to estimate the chemical composition of ash. Ash sample collection ports are as follows (Fig. ): C bottom ash, D INTREX, E convective superheater and reheater, F economizer, G - air preheater. Also samples from electrostatic precipitator (ESP) are taken: H. field, I. field, J. field, K. field. Measured ash components are follow: CO, Al O, CaO, MgO, SiO, Fe O, K O, Na O, Cl and SO. The ash samples are analyzed by fuel chemistry laboratory of DTE (results in table and ). Ash samples were gathered manually following special procedure. The granular composition of ashes was also performed as follows: bottom ash, INTREX ash, superheater, ESP field and field. Granular composition of bottom and INTREX ashes were analyzed by sieving machine Fritsch at DTE. Others ash samples were analyzed with Malvern laser diffractometer ( µm) by ENAS OY (VTT, Finland). Table. Chemical composition and calculated ECD values of ash samples firing enriched oil shale Test day Chemical analysis results Extent of carbonate decomposition Ash port location CO CaO MgO CO CaO CO MgO CO sum k CO [%] [%] [%] [%] [%] [%] [-] Bottom ash Superheater Economaizer Air preheater ESP field ESP field ESP field ESP field Table. Chemical composition and calculated ECD values of ash samples firing ordinary oil shale Test day Chemical analysis results Extent of carbonate decomposition Ash port location CO CaO MgO CO CaO CO MgO CO sum k CO [%] [%] [%] [%] [%] [%] [-] Bottom ash Superheater Economaizer Air preheater ESP field ESP field ESP field ESP field

4 . Results of calculation ECD Results of the chemical analysis and calculated ECD values of the ash samples taken from several ports of CFB boiler burning enriched and ordinary oil shale are presented in tables and. ECD values were calculated for each test day. Here are the results presented for second test day firing enriched and for third test day firing ordinary oil shale. As we can see the bottom ash ECD values are generally lower than ECD values in ash samples after superheater (Inc.). The reason of that could be circumstance that bottom ash samples particles were coarser. The median sizes of ash samples are shown in table. There we can see the ash sample particles taken from superheater, ESP field and are much finer than bottom ash particles. The ECD value depends on granular composition of ash []. Pointing out in table the ECD value of ESP field is lower than even ECD value of bottom ash sample. The reason of that needs future investigation. Table. Ash samples median sizes R[0.] R[0.], µm Sample port Enriched Ordinary Bottom ash Superheater ESP field ESP field Taking into account ash mass flow rates (kg/s) from each port it is possible to calculate the average ECD value for the CFB boiler. Test series averages ash flow rates are illustrated in fig. The calculated ECD values for CFB boiler are shown in table. From table we can see that the test series average ECD value of CFB boiler burning enriched oil shale is higher than burning ordinary oil shale. It is noted that both test series the boiler operated near nominal power level and at the same furnace temperatures. 8 a) firing enriched oil shale 7, Total ash flow, kg/s Fuel feeding, kg/s Ash flow rate [kg/s],88,8 0 0,77 0,0 0,00 0,7 0,08 Bottom ash Superheater Economaizer Air preheater ESP field ESP field ESP field ESP field 8 7 7, Total ash flow, kg/s Fuel feeding 9, kg/s b) firing ordinary oil shale Ash flow rate [kg/s],090,7,0 0 0,7 0,0 0,07 0,00 Bottom ash Superheater Economaizer Air preheater ESP field ESP field ESP field ESP field Fig.. Ash flow rates of CFB boiler (steam production 80 kg/s)

5 Table. Calculated ECD values for CFB boiler Tests in 00 Enriched oil shale Ordinary oil shale Test day k CO Test series average k CO The reason of higher ECD value burning enriched oil shale could be explained with fuel granular composition. Precisely, the enriched fuel particles were finer. The results of fuels granular compositions are presented in fig. There we can see that the median size R[0.] for enriched oil shale was 0. mm, same time for ordinary oil shale the number was 0. mm. Cumulative Täisjääk, oversize, R(x), % R(x), % R[0.]=0, Enriche sept.0 Ordin nov.0 Enriched Ordinary 80 R[0.]=0, mm 90 0, 0 00 Sieve Sõela aperture ava, size, x, mm x, mm Fig.. Granular composition of ordinary (8. MJ/kg) and enriched ( MJ/kg) oil shale Conclusions The ECD is calculable for the CFB boiler on the basis of the CaO, MgO and carbonaceous CO content determined in the ash samples chemical analysis taken from several ports of CFB boiler. Taking into account ash flow rates from several ports the ECD value of CFB boiler burning enriched oil shale is 0.78 and burning ordinary oil shale is 0.9. The ECD value depends on fuel granular composition. The ECD value of CFB boiler calculated based on ash samples analyses taken from common ash silo burning ordinary oil shale was investigated by DTE []. Ash flows from several ports are directed to common ash silo. The ECD value of CFB boiler calculated based on ash samples taken from ash silo was 0.8. The ECD value calculated in this paper considering ash flow rates is 0.9. The ECD values calculated for ash samples taken from several ports showed that the ECD value is lower at bottom ash. The bottom ash particles are coarser. References. A. Ots, T. Pihu, A. Hlebnikov. The Influence of pressure on the behavior of oil shale carbonates. Oil Shale, Vol., No.. Special, 997, pp A. Ots. Oil Shale Fuel Combustion. 00, pp. 8.. H. Arro, A. Prikk, T. Pihu. Emission of CO from CFB boilers of oil shale power plants. Research report. DTE TUT, 00, pp. 8.. H. Arro, J. Loosaar, A. Ots, T. Pihu, A. Prikk. (DTE. TUT). P. Rusheljuk. (AS Narva Elektrijaamad). M. Hiltunen, A. Hotta, R. Parkkonen, K. Peltola. (Foster Wheeler Energia Oy). Firing Estonian oil shale in CFB boilers // 9 th FBC Conference from May -May 00. Proceedings Part II. Vienna. Austria.. K. Plamus. Determination of circulating fluidized bed boiler thermal efficiency burning oil shale with different properties // th International Symposium of doctoral school of energy and geotechnology. Kuressaare, 007, pp H. Arro, A. Prikk, T. Pihu. Calculation of CO emission from CFB boilers of oil shale power plants // Oil Shale, 00, vol., no., pp. -.