ON ASH DEPOSIT FORMATION DURING THE CO-COMBUSTION OF COAL WITH BIOMASS

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1 ON ASH DEPOSIT FORMATION DURING THE CO-COMBUSTION OF COAL WITH BIOMASS P. Abreu Mechanical Engineering Department, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal Supervisor: Mário Costa Abstract This article describes an experimental study on deposits formation during the co-combustion of coal with biomass in a large-scale laboratory furnace. Two types of biomass have been used (sawdust and olive stones). The main objective of this study was to relate the deposits deposition rates with the type of biomass burned and its thermal fraction in the blend. The thermal fraction of biomass in the blend was varied between 10% and 50%. For comparison purposes, tests have also been performed using only coal or only biomass. During the tests deposits were collected with the aid of an air-cooled deposition probe placed far from the flame region, where the mean gas temperature was around 640 ºC. A number of deposit samples were subsequently analyzed on a scanning electron microscope equipped with an energy dispersion of x-ray detector. Results indicate that blending sawdust with coal decreases the deposition rate as compared with the firing of unblended coal due to both the sawdust low ash content and its low alkalis content. The co-combustion of coal and sawdust yields deposits with high levels of silicon and aluminium which indicates the presence of ashes with high fusion temperature and, thus, with less capacity to adhere to the surfaces. On the contrary, in the co-combustion of coal with olive stones the deposition rate increases as compared with the firing of unblended coal. In this case, the co-combustion coal/olive stone produces deposits with high levels of potassium, which tend to increase their stickiness. Keywords Large-scale laboratory furnace, co-firing, coal and biomass, deposition rate INTRODUCTION The combustion of fossil fuels provides the major energy requirement worldwide. Due to its large reserves, coal will continue to be the dominant fuel for use in electricity production for the foreseeable future. The pollutant emissions that emerge from coal combustion are significant. Among these pollutants are oxides of sulphur (SO x ) and nitrogen (NO x ), which lead to acid rain and ozone depletion. In addition, greenhouse gas emissions (CO 2, CH 4, etc.) have become a global concern. A number of methods have been proposed for reducing such gaseous emissions from fossil fuel combustion. Some of these methods are expensive and, thus, increase the production costs. Among the less expensive alternatives, co-firing has gained popularity with the electric utilities producers. Recent studies have established that burning biomass with fossil fuels has a positive impact both on the environment and on the economics of power generation. The emissions of SO x and NO x were reduced in most co-firing tests, depending on the biomass fuel used, and the CO 2 net production was also inherent lower, because biomass is considered CO 2 neutral. In spite of co-firing using biomass as secondary fuel being reasonably well understood, many questions remain unanswered. In particular, that is the case of the ash related operational problems such as the effects of the co-firing on slagging and fouling in the system. Slagging and fouling reduces heat transfer and causes corrosion and erosion problems, which reduce the lifetime of the equipment and increase the probability of shutdown for maintenance and cleaning. The main contribution to fouling comes from the inorganic material in the fuel. Because biomass fuels generally contain a large variety of inorganic materials, as compared with coal, issues like fouling, corrosion and pollutant emissions need to be investigated. As compared with the deposits from coal combustion, the tenacity and the strength of the biomass combustion deposits are higher, with smooth deposit surfaces and little deposit porosity. This means that the deposits from biomass combustion may be hard to remove and may require additional cleaning effort.

2 The main aim of this study is to evaluate the ash deposition formation and its influences on heat transfer during the co-combustion of coal with biomass in a laboratory furnace. Related previous studies include those of Heinzel et al. [1], Jenkins et al. [2], Robinson et al. [3], Annamalai et al. [4], Turn et al. [5], Pronobis [6] and Kupka et al. [7, 8], among others. In an interesting study, Kupka et al. [7, 8] fired blends of pulverized coal and alternative fuels (municipal sewage sludge, sawdust and refuse derived fuel) in a laboratory furnace to examine the effect of the added fuel on the slagging propensity of the mixtures. The authors observed that firing coal alone yields light and easy removable deposits, whereas the co-combustion of coal with relatively small amounts of sewage sludge alters both the deposit structure and the deposition rate, with the latter increasing by a factor of two for the addition of 5% of refuse derived fuel. In contrast, the addition of sawdust to coal has a positive impact on the deposition behaviour, with the deposition rate decreasing. The authors have correlated the fuel slagging propensity with a slagging index based on the ratio of fluxing oxides to sintering oxides, with intensive slagging being observed when the index was between 0.75 and 2. Other interesting studies are those conducted by Jenkins et al. [2] and Turn et al. [5], who examined the effect of firing sugar cane bagasse and/or fiber cane with coal on the slagging propensity of the fuel mixtures. Overall, they observed that ash deposits from the fiber cane were two orders of magnitude greater than those from bagasse and ten times greater than those from coal alone. Along the same lines, Robinson et al. [3] investigated the deposit formation on the co-combustion of pulverized coal with biomass (wood, switchgrass and straw). They concluded that blending straw with coal reduces the high deposition rates observed in the firing of unblended straw, and that co-firing coal with wood yields lower deposition rates than those observed in the firing of unblended coal since wood presents lower ash and alkali metals contents. TEST FURNACE The tests were carried out in the Instituto Superior Técnico large-scale laboratory furnace, which is a vertical cylinder 0.6 m in diameter and 2.4 m in length, down-fired along its axis by a swirl burner. The furnace roof and the initial 1.2 m length of the cylindrical walls are refractory lined. The outer surfaces of the refractory walls are surrounded by cooling water jackets. The remaining 1.2 m length of the wall surfaces are water cooled only. The furnace is fully described elsewhere [9]. Figure 1 shows schematically the furnace roof and the burner arrangement. The burner geometry is typical of that used in power stations for wall-fired boilers and consists of a burner gun and a secondary air supply in a conventional double-concentric configuration, terminating in an interchangeable refractory quarl of half-angle 30. In this work, the burner gun used consisted of three concentric tubes: the central orifice was used for the introduction of transport air and pulverized coal, the intermediate orifice was used for the introduction of transport air and biomass and the outer orifice was used for the introduction of natural gas. The secondary air entered a plenum chamber situated above the burner, in which it encountered a moveable block swirl generator. The air then flowed through an interchangeable cylindrical duct and, subsequently, into the refractory quarl section. The secondary air was supplied by a fan of variable speed and its flow rate was measured with a calibrated orifice plate installed upstream of the fan. The natural gas was supplied by the Portuguese gas company via the IST grid, with the gas flow controlled with pressure regulators and valves and the flow rate measured using a digital rotameter. The coal and the biomass were transported to the burner with the aid of two identical loss-in-weight feeders, each associated with a compressed air ejector system. The solid fuels feeding system is fully described elsewhere [9]. The transport (primary) air for the solid fuels was supplied by an air compressor (10 bar), with the flow rates being measured using calibrated rotameters.

3 Ignitor burner Burner gun Quarl Water out Water out Cooling water Refractory cement Primary air + biomass Natural gas Primary air + coal Secondary air (swirl) 60 o Ø 56 Ø 120 ºC Termocouple type J Water out 300 Ø Dimensions in mm Figure 1. Schematic of the furnace roof and burner arrangement. EXPERIMENTAL METHODS Figure 2 shows a schematic of the air-cooled deposition probe and of the test section used to collect the deposits. As shown in the figure, the probe was placed inside the combustion chamber, through an existing port, 1.65 m downstream of the burner, in cross flow with the combustion gases. The probe diameter is 15 mm and its length 1 m. The test section, mounted in the central part of the probe, where the temperature is relatively uniform, has a length of 50 mm and a superficial area of about m 2. The test section is removable which permits the easy collection of the deposits for weighing and analysis after each trial. Note that a test section was used solely for one trial. In addition, the device is equipped with Cr/Ni thermocouples to measure the probe surface temperature (T s ), and the inlet (T in ) and outlet (T out ) air temperatures. The repeatability of the data concerning the mass of the deposits varied between 8 and 25% which is a good indicator of the uncertainty of the measurements. After weighing, the collected deposits were analyzed in a scanning electron microscope (SEM) JEOL, model JSM-7001F. The microscope allows obtaining 3-D images from a selected area through the sample irradiation by an electron beam. The microscope is also equipped with an energy dispersion of x- ray detector (EDS) that allows quantifying the ultimate composition of a deposits sample within a resolution of about 1 µm 2. The gases for the measurement of the flue-gas concentrations of O 2, CO, CO 2, unburnt hydrocarbons (HC), and NO x were withdrawn using a water-cooled stainless steel probe. The probe outlet was connected to a heated polytetrafluorethylene line through which the gases were conducted first to a cyclone separator immersed in an ice bath, to eliminate the moisture and the larger particulates from the gas sample, followed by a micro fiber filter, to remove the remaining particulates, and then on to the analyzers. The analytical instrumentation included a magnetic pressure analyzer for O 2 measurements, non-dispersive infrared gas analyzers for CO 2 and CO measurements, a flame ionization detector for HC measurements, and a chemiluminescent analyzer for NO x measurements. The analog outputs of the analyzers were transmitted via analogical/digital boards to a computer where the signals were processed and the mean values computed. Zero and span calibrations with standard mixtures were performed before and after each measurement session. The maximum drift in the calibration was within ± 2% of the full scale. At the furnace exit, where the gas composition was nearly uniform, probe effects were negligible and errors arose mainly from quenching of chemical reactions and sample handling. Samples were quenched near the probe tip to about 150 C and condensation of water within the probe was avoided by

4 controlling the inlet temperature of the cooling water (typically to around 60 C). Repeatability of the flue-gas data was, on average, within 7.5% Data logger Deposit layer T g Probe Test section Cooling air T s T out T in 50 Dimensions in mm Figure 2. Schematic of the air-cooled deposition probe and of the test section used to collect the deposits. Char sampling was performed with the aid of a stainless steel water-cooled, water-quenched probe. Upon leaving the probe, the char samples were collected in a glass container. Following decantation, the collected wet solid sample was placed in an oven at approximately 110 C to dehydrate. Complete dehydration was ascertained by repeated drying and weighing of the sample until the measured mass became constant. The solid samples were subsequently analyzed (ultimate analysis). The particle burnout data were obtained from the following equation: [ ( ω ω )] ( ω ) Ψ = 1 1 (1) k x where Ψ is the particle burnout, ω is the dry ash mass fraction, and the subscripts k and x refer to ash content in the input solid fuel and char sample, respectively. Uncertainties in particle burnout calculations based on the use of ash as a tracer are connected to ash volatility at high heating rates and temperatures and ash solubility in water. In order to determine the accuracy of the experimental procedure and measurements, carbon and oxygen elemental mass balances have been performed for all tested conditions. The overall mass balance discrepancies were below 12%. Radial traverses at the exit sampling location indicated no spatial variation in particle burnout and repeatability was, on average, within 10%. k

5 TEST CONDITIONS A United Kingdom bituminous coal, sawdust, olive stones and natural gas (CH 4 : 92.4%, C 2 H 6 : 4.95%, C 3 H 8 : 1.96%, and other components with minor concentrations) were used as fuels in the present work. The characteristics of the solid fuels are given in Table 1 and the particle size distributions are presented in Figure 3. Table 1. Properties of the solid fuels Parameter Coal Sawdust Olive stones Proximate analysis (% wt) Volatiles Fixed Carbon Moisture Ash Ultimate analysis (% wt) Carbon Hydrogen Nitrogen Sulphur Oxygen High heating value (MJ/kg) Low heating value (MJ/kg) % volume Coal Sawdust Olive stones Diameter ( m) Figure 3. Particle size distributions for the solid fuels. Table 2 summarizes the furnace operating conditions for the parametric trials. These tests have included the study of eleven flames that allowed us to quantify the effect of firing sawdust or olive stones with coal on the fouling/slagging propensity of the fuel mixtures. For comparison purposes pure flames of coal, sawdust and olive stones have also been studied. It should be stressed that in all flames a constant flow rate of natural gas was fed to the burner (see Figure 1) in order to help the flame stabilization process. Table 3 shows the flue-gas data, including the particle burnout for all flames studied. During the trials, the cooling air flow rate fed through the deposition probe was regulated so that a surface temperature of 500 C could be imposed on the surface of the test section used to collect the deposits (see Figure 2). Each trial lasted for about 2 hours, after which the deposition probe was removed from the combustion chamber, the test section removed from the probe, the mass of deposits weighted and, finally, the deposition rate calculated.

6 Table 2. Furnace operating conditions for the parametric tests. Parameter Coal Coal + Coal + olive Olive Sawdust sawdust stones stones Total thermal input (kw) 150 Thermal input: coal + biomass (kw) 40 Biomass (% thermal fraction) Coal mass flow rate (kg/h) Biomass mass flow rate (kg/h) Natural gas mass flow rate (kg/h) 7.18 Overall excess air (%) 1.15 Primary air mass flow rate (kg/h) Primary air temperature ( C) 30 Secondary air mass flow rate (kg/h) Secondary air temperature ( C) 30 Secondary air swirl number 0.9 Table 3. Gas composition and particle burnout measured in the exhaustion gas Parameter Coal 10% sawdust 20% sawdust 30 % sawdust 50 % sawdust 100% sawdust O 2 (%, dry volume) CO 2 (%, dry volume) CO (ppm, dry volume) NO x (ppm, dry volume) HC (ppm, dry volume) Particle burnout (%) Table 3. (continued) Parameter 10% 20% 30 % 50 % 100% olive stones olive stones olive stones olive stones olive stones O 2 (%, dry volume) CO 2 (%, dry volume) CO (ppm, dry volume) NO x (ppm, dry volume) HC (ppm, dry volume) Particle burnout (%) RESULTS AND DISCUSSION Influence of the Biomass Thermal Fraction in the Deposition Rate As mentioned above, the deposits were collected after an exposure time of about 2 hours. This exposure time guaranteed the collection of sufficient amounts of material for deposition rate calculations and subsequent analysis. However, it should be noted that with such test duration only the initial period of slagging can be analyzed [7]. Figure 4 shows the measured deposition rate, after two hours of exposure time, as a function of the biomass thermal fraction in the blend. In the case of the co-combustion of coal with olive stones it is seen that, with the exception of the case with 10% of biomass thermal fraction, the deposition rates are always higher than the reference case (i.e., coal firing). On the contrary, the addition of sawdust to coal has a positive effect on the deposition rate (Figure 4), which significantly decreases with the increase of biomass thermal fraction in the blend. Heinzel et al. [1], Robinson et al. [3], Kupka et al. [7, 8] and Tillman [10] have reported similar conclusions regarding the impact of wood residues on the deposition rate in co-combustion processes with coal in different combustion systems. Furthermore, they concluded that wood based fuels do not have propensity to cause any fouling problems and can even contribute to reduce its formation. Robinson et al.

7 [11] indicated that in the co-combustion of coal with biomass the deposition rates depend not only of the biomass characteristics but also of the interactions between the chemical elements present in their composition. Deposition rate (g / m 2 h) Sawdust Olive stones Coal Solid fuel in the blend (thermal %) Figure 4. Measured deposition rate, after two hours of exposure time, as a function of the biomass thermal fraction in the blend. Figure 5 shows the deposition rate as a function of the ash input rate into the furnace. As can be seen there is a linear relationship between the deposition rate and the ash content present in the blend used. The addition of sawdust, which presents an ash content of only 0.2% in its composition, reduces the deposition rate as compared with coal that has an ash content of 4.7% (see Table 1). In the co-combustion trials with 50% of sawdust, a reduction of about 40% in the deposition rate was achieved as compared with the pure coal firing. In contrast, the addition of olive stones to the blend increases the ash content in the mixture owing to the high ash content of the olive stones (13.1%) see Table 1. In this case, the co-combustion with 50% of olive stones led to an increase in the deposition rate of almost 70% as compared with the results obtained for pure coal firing (see Figure 4) % 100% Deposition rate (g/m 2 h) % 20% 10% 10% 20% 50% 30% 100% Sawdust Olive stones 100% coal Ash input rate (kg/m 2 h) Figure 5. Deposition rate as a function of the ash input rate into the furnace.

8 A comparison between the three solid fuels used suggests that the sawdust has little potential to cause fouling problems. In fact, its combustion alone originated a deposition rate of 56.5 g/m 2 h, while the pure coal firing and the pure olive stones firing presented formation rates of and g/m 2 h, respectively. Figures 6 to 8 show photographs of the deposits on the test section of the deposition probe, after two hours of exposure time, for most trials reported here. The photographs have been taking placing the camera perpendicularly to the leading edge of the probe. In the case of the co-combustion of coal with sawdust it is seen that the physical structure of the deposit layer created is similar for all test conditions. It is observed a relatively homogeneous layer that can easily removed and similar to the deposit layer obtained for the pure coal firing (Figure 6). In the case of the co-combustion of coal with olive stones, it was observed the existence of larger particles comparatively to the ashes that resulted from the cocombustion of coal with sawdust. A closer inspection of the photographs displayed in Figure 8 allows observing the agglomeration of significant amounts of deposits next to the leading edge of the test section of the deposition probe, which tends to increase with the increase of the olive stone thermal fraction in the blend. While scraping the test section surface to collect the deposits, it was observed that the deposits from the coal/olive stones blends had a stronger adhesion to the surface. In none of the trials was observed the sintering of the material, common at high temperatures and after long exposure times of the surfaces. Generally, deposits formed through this mechanism include reactions of alkaline metal with silica, forming alkaline silicates [12]. Figure 6. Photograph of the deposits on the test section of the deposition probe for coal firing, after two hours of exposure time. Influence of the Biomass Thermal Fraction in the Chemical Composition of the Deposits As mentioned earlier, after weighing, a number of deposit samples from various trials were analyzed with the aid of a scanning electron microscope equipped with an energy dispersive x-ray detector. Figure 9 presents the chemical analysis of representative deposits collected on the surface of the test section of the deposition probe for both the co-combustion of coal and sawdust and the co-combustion of coal and olive stones. For each deposit sample three measurements were made, being the averaged values reported here. Considering only the chemical elements present in amounts above 0.5% (the detection limit of the device), the maximum relative uncertainty obtained for the concentration of the chemical elements is 4.12%. The chemical analysis of the deposits is useful to help explaining the behavior of the solid fuels concerning the deposition formation rate. Figure 9 reveals that the elements Si and Al are present in relatively high amounts in the deposits. These elements are usually present in form of oxides, such as SiO 2 (silica) and Al 2 O 3 (alumina), being characterized by having a high fusion point. In the present deposits, the amount of Si and Al decreases with the increase of the biomass thermal fraction in the blend, being the reduction higher in the case of the co-combustion of coal and olive stones. It should be pointed out that ashes with lower concentrations of SiO 2 and Al 2 O 3 means particles with smaller softening points and, therefore, that can easier melt and adhere to the equipment surfaces.

9 Figure 7. Photographs of the deposits on the test section of the deposition probe for coal and sawdust firing, after two hours of exposure time. The co-combustion of coal with olive stones originates deposits with higher content of K, as compared with the co-combustion of coal with sawdust. Since the temperature of the surface of the test section of the deposition probe was maintained around 500 C, it is expected the K containing compounds exist in solid state, as K 2 SO 4 or K 2 CO 3, which can easily adhere to the surfaces [13]. In the trial firing the blend with 50% of biomass was observed that the K content of the resulting deposits was almost 5 times higher than that present in the deposits from the corresponding trial with sawdust. The existence of K in form of K 2 O also contributes to the reduction of the fusion temperature of the deposits [14]. Figure 8. Photographs of the deposits on the test section of the deposition probe for coal and olive stones firing, after two hours of exposure time.

10 Figure 9 reveals a higher increase in the K and S content in the deposits that resulted from the cocombustion of coal with olive stones. This means deposits with higher density which may difficult the heat transfer process should they have a reduced thermal conductivity. The presence of P only in the deposits that resulted from the co-combustion of coal with olive stones, for mixtures with upon 30% of biomass, could indicate the formation of P 2 O 5, which contributes to aggravate the fouling formation [6]. Figure 10 shows typical scanning electron microscope images of the deposits, in which some common chemical elements found in the deposits are indicated. One important advantage of using sawdust or olive stone in combustion processes is related to the fact that these fuels do not have Cl in its composition. The presence of Cl can cause corrosion on the metallic surfaces. Nonetheless, corrosion problems may still be a problem because the sulfates, particularly the alkaline tiosulfates, can be aggressive for the heat exchange surfaces [15]. This type of corrosion can be avoid by keeping the temperature of the tubes relatively low, which prevents the occurrence of reduction reactions, and by applying appropriate operations of sootblowing (cleanness). Available experimental results in the literature reveal that the co-combustion with wood residues poses hardly any problems as far as fouling is concerned due to its low ash and alkaline metals contents [2, 10]. In contrast, the use of herbaceous materials and agricultural residues can cause a number of problems because of the presence of alkaline metals and Cl. a) Concentration (% wt) Coal 10% sawdust 20% sawdust 30% sawdust 50% sawdust Si Al Fe K Cu Ca Mg Ti S Na Element b) Concentration (% wt) Element Coal 10% olive stones 20% olive stones 30% olive stones 50% olive stones Si Al Fe K Cu Ca Mg Ti S Na P Figure 9. Chemical analysis of representative deposits collected on the surface of the test section of the deposition probe. a) Cocombustion of coal and sawdust, b) co-combustion of coal and olive stones.

11 Coal x4 cenosphere, rich in C 90% coal + 10% sawdust x4 particle rich in Si 70% coal + 30% sawdust x2 particle rich in K 70% coal + 30% olive stones x3 particle rich in Al 80% coal + 20% olive stones x4 - particle rich in Si x5 - particle rich in Fe 50% coal + 50% olive stones x4 - particle rich in K x5 - particle rich in K and Ca Figure 10. Typical scanning electron microscope images of the deposits. CONCLUSIONS 1. The co-combustion of coal with sawdust does not pose operational problems related with the occurrence of slagging and fouling since the deposition rate decreases with the increase in the biomass thermal input in the blend. In fact, sawdust can be an excellent option for co-combustion processes given its reduced ash and alkali metals contents. Furthermore, the presence of high content of silicon and aluminium in the deposits, generally in form of oxides, also suggests the existence of ashes with higher fusion temperatures and, therefore, with less capacity to adhere to the surfaces.

12 2. On the contrary, in the co-combustion of coal with olive stones the deposition rate increases as compared with the firing of unblended coal. In this case, the co-combustion coal/olive stone produces deposits with high levels of potassium, which tend to increase their stickiness. REFERENCES 1. Heinzel, T., Siegle, V., Spliethoff, H., Hein K. R. G. Investigation of Slagging in Pulverized Fuel Co-combustion of Biomass and Coal at a Pilot-Scale Test Facility. Fuel Process Technology, 54, , Jenkins, B. M., Thy, P., Turn, S. Q., Blevins, L.G., Baxter, L. L, Jakeway, L. A., Williams, R. B., Blunk, S. L., Yore, M. W., Wu, B. C., C. E. Lesher, C. E. Composition and Microstructure of Ash Deposits from Co-firing Biomass and Coal. Bioenergy 2002, Proceedings of the 10 th Biennial Bioenergy Conference, Boise, Idaho, Robinson, A. L., Junker, H., Baxter, L. L. Pilot-scale Investigation of the Influence of Coal-biomass Cofiring on Ash Deposition. Energy and Fuels, 16, , Annamalai, K., Sweeten, J., Freeman, M., Mathur, M., O Dowd, W., Walbert, G., Jones, S. Co-firing of Coal and Cattle Feedlot Biomass (FB) Fuels, Part III: Fouling Results from a 500,000 BTU/h Pilot Plant Scale Boiler Burner. Fuel, 82: , Turn, S. Q., Jenkins, B. M., Jakeway, L. A., Blevins, L. G., Williams, R. B., Rubenstein, G., Kinoshita, C. M. Test Results from Sugar Cane Bagasse and High Fiber Cane Co-fired with Fossil Fuels. Biomass and Bioenergy, 30, , Pronobis, M. The Influence of Biomass Co-combustion on Boiler Fouling and Efficiency. Fuel, 86, , Kupka, T., Zajac, K., Weber, R. Influence of Fuel Type and Deposition Surface Temperature on the Growth and Chemical and Physical Structure of Ash Deposit Sampled during Co-firing of Coal with Sewage Sludge and Sawdust. Proceedings of the 8 th European Conference on Industrial Furnaces and Boilers, Algarve, Portugal, Kupka, T., Mancini, M., Irmer, M., Weber, R. Investigations of Ash Deposit Formation during Co-firing of Coal with Sewage Sludge, Saw-dust and Refuse Derived Fuel. Fuel, 87, , Casaca, C., Costa, M. Co-combustion of Biomass in a Natural Gas Fired Furnace. Combustion Science and Technology, 175, , Tillman, D. A. Biomass Cofiring: The Technology, the Experience, the Combustion Consequences. Biomass and Bioenergy, 19, , Robinson, A. L., Baxter, L., Junker, H., Shaddix, C. Fireside Issues Associated with Coal-Biomass Cofiring. Sandia National Laboratories, USA, Sandia National Laboratory. Alkali Deposits found in Biomass Boilers the Behavior of Inorganic Material in Biomass Fired Power Boilers. SAND , National Renewable Energy Laboratory, USA, Zhang, W., Leckner, B. Process Simulation of Circulating Fluidized Beds with Combustion/Gasification of Biomass. Contract JOR3CT980306, European Commission, JOULE 3, Final Report, Brussels, Belgium, Werther, J., Saenger, M., Hartge, E. U., Ogada, T., Siagi, Z. Combustion of Agricultural Residues. Progress in Energy and Combustion Science, 26, 1-27, Harb, J. N., Smith, E. E. Fireside Corrosion in PC-Fired Boilers. Progress in Energy and Combustion Science, 16, , Robinson, A. L., Buckley, S. G., Yang, N., Baxter, L. L. Experimental Measurements of the Thermal Conductivity of Ash Deposits: Part 1, Measurement Technique. Energy and Fuels, 15, 66-74, 2001.