Ash and Deposit Formation in the Biomass Co-Fired Masnedø Combined Heat and Power Production Plant

Transcription

1 IFRF Combustion Journal Article Number , September 2003 ISSN X Ash and Deposit Formation in the Biomass Co-Fired Masnedø Combined Heat and Power Production Plant FRANDSEN Flemming 1, HANSEN Jørn 1, JENSEN Peter A. 1, DAM-JOHANSEN, Kim 1, HØRLYCK Steffen 2 and KARLSSON Asger 3 1 CHEC Research Centre, Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark 2 SK Power Company, Avedøre Power Station Hammerholmen 50, DK-2650 Hvidovre 3 Formerly with: EK Energi A.m.b.a., Lautruphøj 5, DK-2750 Ballerup, Denmark Corresponding Author: Flemming Frandsen CHEC Research Centre Department of Chemical Engineering Technical University of Denmark Building 229, DK-2800 Lyngby, Denmark ff@kt.dtu.dk

2 IFRF Combustion Journal Frandsen, Hansen, Jensen et al ABSTRACT A series of biomass co-combustion tests were conducted at the Masnedø combined heat and power (CHP) production grate-fired boiler, Denmark, in the period Nov March The fuel feedstock mixtures applied were: Danish wheat straw (100 % on a thermal base), wheat straw + wood chips ( % on a thermal base), wheat straw + olive stones ( %), and wheat straw + shea nuts ( %). During these combustion tests, air-cooled probes were exposed to the flue gas in the tertiary superheater. The metal surface temperature of the probes was kept constant at 500 ºC. Two probes were applied in each test, one was exposed for app. 1 h, the other was exposed for app. 18 hs. Probe deposits formed have been mounted in epoxy, cross sectioned, polished and studied in a Scanning Electron Microscope (SEM). The 1 h probes are in general covered with a thin oxide scale and a thin ash layer on top of this, while the 18 h probes in addition to a multilayer oxide scale also contain a thick ash layer. The ash layer consists of residual ash particles glued together by KCl (sometimes with a minor content of S also being present in this glue ). There seems to be a tendency to be more ash particles in the outer ash layers formed when co-firing straw and other biomasses, compared to pure straw-firing. This paper provides an outline of fuel characteristics, mapping of the partitioning of K, S and Cl in the boiler, and differences in ash and deposit formation when co-firing biomasses in grate-fired boilers.

3 IFRF Combustion Journal Frandsen, Hansen, Jensen et al INTRODUCTION The globally growing interest in the humanly caused climatic changes has led to a wish for reduction of the CO 2 -emissions from power production and put focus on substituting coal with biomass fuels. Biomass, i.e. in Denmark mainly wheat straw, is attractive as a fuel for two reasons: 1) biomass is CO 2 -neutral, meaning that during growth it accumulates the same amount of CO 2 as is released during combustion and 2) a fairly large surplus of straw occurs yearly in certain parts of Denmark, where on-site combustion has been forbidden. In Denmark, the government has committed the national power companies to reduce the CO 2 - emissions from Danish power stations by 20% based on the level from 1988, before the year This has led to a government demand to the power companies of burning a total of 1 million metric tonnes of straw, 0.2 million metric tonnes of wood chips and 0.2 million metric tonnes of straw or wood/willow chips per year from the year Biomass acts as fuel quite differently from coal, and in order to meet the goal of burning these large quantities of biomass, the most effective combustion concept has to be determined. Three boiler concepts have been proposed for this purpose: combustion of straw in grate-fired boilers, cocombustion of coal and straw in utility type boilers and co-combustion of straw, wood chips, and coal in circulating fluidised bed boilers. Each of the three concepts have advantages and drawbacks; however, one of the great advantages for the grate-fired boilers is that the straw is not mixed with any other fuel, meaning that the fly ash produced is not polluted, and can thus be applied by farmers as fertilizer. The use of straw for heat production in small scale furnaces, e.g. at individual farms, has been practiced for a number of years. However, power generation from biomass is a fairly new task, and grate-fired boilers utilizing biomasses have in many cases experienced serious problems with slagging, fouling, and corrosion of superheaters 1-5. These problems are primarily due to the large content of troublesome elements such as potassium and chlorine in the straw. Potassium and chlorine may either deposit directly on the heat transfer surfaces as KCl or the potassium may react with the silicate part of the ash, creating low-melting fly ash that has an increased tendency of sticking to heat transfer surfaces. The chlorine in the deposits are furthermore known for being able to attack the metal surfaces leading to serious corrosion problems in the boilers.

4 IFRF Combustion Journal Frandsen, Hansen, Jensen et al In Denmark, the deposition and corrosion problems experienced on the straw-fired boilers have been addressed by a number of people 1,3,6. Deposits collected on air- cooled probes have been shown to contain large quantities of K and Cl (40-80% (w/w)) in addition to varying amounts of fly ash particles 1,3. Corrosion rates has been measured by probes at the Rudkøbing CHP boiler and was found to be negligible at prevailing steam temperatures of 450 C. Increased but still acceptable rates were found for steam temperatures of C, whereas for temperatures above 520 C, the corrosion rates were significantly increased and found to be unacceptably high 6. Straw is often co-fired with wood chips, but experience on co-firing with other biomasses is minimal. It is important to gain more information about the possibilities and characteristics of co-firing straw with other biomasses, since this may become economically feasible in the future. Thus, in the period Nov Mar. 1999, the SK Power Company carried out a test programme for investigation of the potential of firing other biofuels than straw and wood chips in the grate-fired Masnedø CHP Plant. Wheat straw was burned pure and together with wood chips (25 % wood chips on a thermal base), olive stones (25 % olive stones on a thermal base) and shea nuts (30 % shea nuts on a thermal base). This paper reports the consequences of co-firing of these biomasses on formation of fly ash, and deposits formed on probes located in the convective pass of the Masnedø CHP Plant. THE MASNEDØ CHP BOILER The Masnedø CHP Plant was built by FLS Miljø/BWE, and commissioned in The plant is a 33 MW th unit producing 8.3 MW el,netto electricity and 20.8 MJ/s. The boiler produces heat for the town of Vordingborg, Denmark, and is operated by the SK Power Company. A sketch of the plant is shown in Figure 1. SH2 SH3 SH1 SH1 ECO Straw SH1 SH1 Air preheater Ash pit Figure 1: Sketch of the Masnedø CHP boiler.

5 IFRF Combustion Journal Frandsen, Hansen, Jensen et al The boiler is grate-fired with a water-cooled vibration grate. Straw is transported into the boiler by means of two screw feeders, which are supplied with straw from two separate straw shredders. The screw feeders deliver the straw on the (first) stationary grate, where the combustion process is initiated. The pyrolysis gases are ignited by means of air, added above the grate. The straw is moved over the first grate by means of the pressure from the continuously added straw (i.e. no separate transferring device is connected to the stationary grate). On the vibrating grate, the straw is moved step wise due to vibration of the grate. The grate vibrations do not occur at equal time intervals but are controlled by several operational parameters, e.g. the steam production. At the end of the vibrating grate, the ash hopper is located, in which the ash drops down into a water bath and is removed. The flue gas moves up through the furnace chamber to the secondary superheater (SH2) located at the top of the furnace, over the tertiary superheater (SH3) in the second pass, to the primary superheater (SH1) in the third pass. After passing the economizer, the air preheater and an electrostatic precipitator, the flue gas leaves the plant through the stack. Major plant data are given in Table 1. Capacity 33 MWth / 8.3 MWe Fuel Biofuels (mainly straw) No. of screw feeders 2 Steam data: 43.2 t/h (max. load), 522 ºC, 92 bar Geometry (h / l / w) 14 m / 5.3 m / 4.3 m Table 1: Masnedø CHP boiler data Air-cooled probes was inserted one probe at a time - in a measuring position in the middle of the tertiary superheater (SH3), see Fig. 1. Exposure times of approximately 1 and 18 hours were intended during each of the four test-firings (straw, straw + wood chips, straw + olive stones and straw + shea nuts). The surface temperature of the probes was maintained at 500ºC by cooling with air. In parallel with the deposition measurements, gas analyses and temperature measurements were carried out at the plant 7. FUEL AND ASH ANALYSES Based on the fuel analyses provided in Table 2, the inlet concentrations of K, S, and Cl to the furnace are shown in Figure 2.

6 IFRF Combustion Journal Frandsen, Hansen, Jensen et al Fuel Straw Wood chips Olive stones Shea nuts Share 100 % 0 % 25 % 0 % 25 % 0 % 25 % Ash Moist HHV C H N S O Cl K Table 2: Fuel analyses, %(w/w) dry base. O is calculated by difference. The higher heating value (HHV) is provided in MJ/kg. The share of alternative biofuels is on a thermal base. 0 % refers to the composition of straw applied in the co-firing experiments, while 25 % refer to the actual biomass blend moles / 100 MJ Ktotal Kwsol Cl+2S 0.00 None 25 % wood chips 25 % olive stones Straw Co-Firing Share 35 % shea nuts Figure 2: Fuel inlet (moles/100 MJ thermal input) of K, Cl and S to the Masnedø CHP boiler. Ktotal is the total amount of K, while Kwsol is the water soluble fraction of the K. Data provided by the SK Power Company. Notice in Figure 2, that there is an increased amount of K, S, and Cl when co-firing biofuels, compared to the firing of straw alone. For potassium, this increase is in the case of co-firing of wood and shea nuts, partly due to a higher concentration of potassium in the straw (Table 2, column 2 + 6). In the case of co-firing of straw with olive stones respectively shea nuts, the increased inlet of potassium to the furnace is also (partly) due to a very high content of potassium in olive stones and shea nuts (Table 2, column 5 + 7). Another characteristic feature in Figure 2 is that in the case of pure straw-firing and co-firing of straw and shea nuts, the amount of water soluble K and the sum of Cl + 2S (ie. the theoretical amount of K being present as KCl and K 2 SO 4 ) are almost equal, while there is an excess of the anions in the case of co-firing of straw with wood chips and olive stones.

7 IFRF Combustion Journal Frandsen, Hansen, Jensen et al In Figure 3, the same data are plotted, but now showing the amounts of K and Cl + 2S in the bottom ash. Notice, that less than 15 % of the potassium in the bottom ash is water soluble, a fact which has also reported previously 1. In all the cases, there is a minor excess of Cl + 2S compared to the amount of water soluble potassium. The excess of total K, compared to Cl + 2S is most likely being present as K-Ca silicates in the bottom ash. moles / 100 g bottom ash None 25 % wood chips 25 % olive stones 35 % shea nuts Ktotal Kwsol Cl+2S Straw Co-Firing Share Figure 3: Bottom ash content (moles/100 g bottom ash) of K, Cl and S at the Masnedø CHP boiler. Ktotal is the total amount of K, while Kwsol is the water soluble fraction of the K. Data provided by the SK Power The content of K (total and water soluble) vs. the content of Cl + 2S in the fly ash generated during biomass co-firing is shown in Figure 4. Notice, that almost all the potassium in the fly ash is water soluble, confirming the fact that the fly ash originates from inorganic metal moles/100 g fly ash None 25 % wood chips 25 % olive stones 35 % shea nuts Ktotal Kwsol Cl+2S Straw Co-Firing Share Figure 4: Fly ash content (moles/100 g fly ash) of K, Cl and S at the Masnedø CHP boiler. Ktotal is the total amount of K, while Kwsol is the water soluble fraction of the K. Data provided by the SK Power Company.

8 IFRF Combustion Journal Frandsen, Hansen, Jensen et al species which are volatilized in the flame and subsequently recondensed heterogeneously or homogeneously. Furthermore, with the exception of co-firing of straw and shea nuts, in the fly ash, there is a balance between K and Cl + 2S. In the shea nut derived fly ash, the excess potassium, compared to Cl + 2S, occur most likely as K silicates. The fly ash were investigated for its structure and chemical composition in a Scanning Electron Microscope (SEM), see Figure 5. A small sample of fly ash is distributed on a thin cylindrical plate, which is placed in the microscope and analysed. In Figure 5, it is seen that the fly ash which in this case originates from the experiment with straw-firing in reality is an agglomerate of a large number of small spherically, primary particles - rich in K and Cl. Fly ashes sampled from the electrostatic precipitator during the co-firing experiments, had a similar appearance when analysed in the microscope. Figure 5: Scanning Electron Micrograph of residual fly ash formed during biomass co-firing at the Masnedø CHP. Fuel: Straw Straw + 25 % wood Straw + 25 % olive Straw + 30 % shea chips stones nuts Na Mg Al Si P S Cl K Ca Table 3: Chemical composition of Residual fly ash from biomass co-firing tests at the Masnedø CHP boiler. Analyses are performed by Energy Dispersive X-Ray analysis. The results are provided as %(w/w) on a C- and O-free base.

9 IFRF Combustion Journal Frandsen, Hansen, Jensen et al In Table 3, the composition of residual fly ash sampled from the electrostatic precipitator during the biomass co-firing tests are provided. The analyses are performed by Energy Dispersive X-Ray analysis (EDX), on 5 areas of each fly ash sample in the SEM. The results shown are the mean value of the five analyses two times the estimated standard deviation, as determined from standard statistics. It is seen in Table 3, that all four residual fly ashes analysed, are dominated by K and Cl. Minor differences in the Si, P and S content of the fly ashes are observed. Notice, in particular, that the residual fly ash generated during co-firing of straw and shea nuts, have about twice as high a content of S, as the other fly ashes. The results shown in Table 3, were confirmed by performing SEM-EDX mappings of fly ash samples. These mappings show that the dominating elements in the four residual fly ashes were K and Cl. DEPOSIT INVESTIGATION As mentioned above an air-cooled probe was inserted in the middle of the tertiary superheater (SH3, see Figure 1). The surface temperature of the probe was maintained at 500 ºC. Two measurements were performed during each test-firing: one having a short exposure time, app. 1 hour, the other a somewhat longer exposure time, app. 18 hours. Anyhow, in the case of straw shea nut co-firing, for operational reasons, the long exposure time only reached app. 12 hours. The probes were mounted in epoxy, cross sectioned, and polished, and thereafter analysed in a SEM. All the long-term exposure probes were covered with a grey, fin-shaped deposit on the upstream side of the probes, while a thin layer of light grey dust covered the downstream side of the probes. Thus, the main focus in the SEM analysis of the probes have been on the chemistry and structure of the upstream deposits. In Figure 6, photographs of the 1 hour respectively the 12 hours probe from the straw-shea nut co-firing experiment, are shown. It is seen that the 1 hour probe does only contain a thin oxide scale, and virtually no deposit, while a nice fin-shaped deposit has formed on the

10 IFRF Combustion Journal Frandsen, Hansen, Jensen et al upstream side of the 12 hours probe. This has been confirmed during the subsequent SEM analysis of the probes. Figure 6: Photographs of air-cooled probe after exposure to straw shea nuts derived flue gas for app. 1 hour (left) and app. 12 hours (right). Notice that the 1 hour probe has virtually no deposit on it, while a nice fin-shaped deposit has formed on the upstream side of the 12 hour probe. The main difference between the short- and the long-term probes is, not surprisingly, that a thick outer deposit is formed on the upstream side of the long-term probes. Furthermore, the deposits formed during long-term exposure have a similar structure: an inner oxide scale (in most cases multi-layered) followed by an inner and an outer deposit layer. Anyhow, one important exception need to be emphasized: More residual fly ash particles and thereby a thicker deposit layer are observed in the outer deposit when co-firing biofuels compared to straw-firing. Thus, below, the discussion will focus on a comparison between the long-term probes from the straw-firing and the straw-shea nut co-firing experiment. In Figure 7, an overall SEM micrograph of the upstream deposit formed during straw-firing is shown. The deposit consists of an oxide scale, an inner and an outer deposit layer. The oxide scale is multi-layered, each layer consisting of Fe X O Y, with varying concentrations of S and K in the layers. On the top of the oxide scale is a thin, dense layer rich in K, S, and P, probably a mixture of K 2 SO 4 and K 3 PO 4. A close-up of the oxide scale and the thin K-S-P - rich layer are shown in Figure 8. For comparison, in Figure 9, a SEM micrograph of the overall upstream deposit structure formed when co-firing straw and shea nuts, is shown. This deposit was formed during an exposure time of 11 hours and 38 min. As it was the case with the straw deposit, this deposit

11 IFRF Combustion Journal Frandsen, Hansen, Jensen et al consists of a thin oxide scale closest to the tube, followed by a number of inner deposit layers rich in K and Cl, and on the top of this, a thick porous outer deposit layer rich in Ca- and Sirich particles, glued together by KCl. A close-up of the outer deposit layer, are shown in Figure 10. Figure 7: SEM micrograph of overall structure of upstream deposit formed after app. 18 hours exposure to a straw-derived flue gas at the Masnedø CHP. Notice that on top of the multilayer oxide scale is located the outer deposit layer, which consist of a dendritic structure of primary KCl particles. Compare with the structure of the residual fly ash shown in Figure 5. Only very few other residual fly ash particles are located in the outer deposit layer. Figure 8: Close-up of multi-layered oxide scale formed on air-cooled probe exposed to strawderived flue gas at the Masnedø CHP plant.

12 IFRF Combustion Journal Frandsen, Hansen, Jensen et al Figure 9: SEM micrograph of overall structure of upstream deposit formed after app. 12 hours exposure to a straw-shea nut derived flue gas at the Masnedø CHP. Notice that on top of the Fe X O Y oxide scale is located first a porous layer rich in KCl, followed by an outer deposit layer, rich in Ca- and Si-rich particles, glued together by KCl. A close-up of the outer deposit layer is shown in Figure 10. Figure 10: Close-up of the outer deposit layer of a deposit formed during straw-shea nut cofiring at the Masnedø CHP. The dark gray phases on the micrograph is Ca- and Si-rich particles, while the light gray phases are KCl acting as glue, binding the residual particles together. Several residual particles are observed in this outer layer. A close-up of the oxide scale and the inner deposit layers seen on Figure 9, is shown in Figure 11.

13 IFRF Combustion Journal Frandsen, Hansen, Jensen et al Figure 11: Close-up on oxide scale and inner deposit layers of upstream deposit formed during co-firing of straw and shea nuts at the Masnedø CHP plant. The light gray/white phases in the middle horisontal band of the mocrograph (see eg. EDX-area analysis point 4) is KCl acting as glue, binding residual ash particles rich in Ca and Si, together. The observation that the upstream deposits formed when co-firing biofuels contain much more residual ash particles than is the case during straw-firing, is supported by the fact that the fly ash mass loading in the flue gas channel is increased when co-firing biofuels, see Figure 12. g/nm3[dry flue gas] None 25 % wood chips 25 % olive stones Straw Co-Firing Share 35 % shea nuts Figure 12: Fly ash loading when firing straw alone or co-firing straw with alternative biofuels at the Masnedø CHP plant. The ash loading (mass of ash per volume flue gas) increases when co-firing biofuels.

14 IFRF Combustion Journal Frandsen, Hansen, Jensen et al The ash loadings shown in Figure 12, are estimated based on a measured ash split in the furnace, ie. measured flows of bottom and fly ash. These mass flows are then divided by a calculated amount of flue gas, estimated from the fuel composition and an air excess number, adjusted to fit the measured oxygen concentration in the stack of the plant. SEM-EDX mappings were made on all short- and long-term co-firing experiments, ie. experiments where straw was co-fired with wood chips, olive stones or shea nuts. No mapping was made on straw deposits, since EDX spot analyses on these have shown that their outer layers consisted primarily of K and Cl. In Figure 13, a mapping of the upstream deposit formed on an air-cooled probe after app. 18 hours exposure to a straw-wood chips derived flue gas is shown. Several residual fly ash particles are seen in the outer layer of the deposit, glued together by KCl. Fe is only present in the thin oxide scale and in the tube material. Figure 13: SEM-EDX mapping of upstream deposit formed on an air-cooled probes exposed to a flue gas from co-firing of straw and 25 % (thermal base) wood chips at the Masnedø CHP boiler. The image analysed is show in the upper left corner of the figure. Notice that several Ca- and Si-rich particles are glued together by KCl in the outer deposit. Fe is only present in the thin oxide scale and in the tube material.

15 IFRF Combustion Journal Frandsen, Hansen, Jensen et al The consequence of an increased fly ash mass loading in the flue gas channel, may be increased deposit build-up rates. Assuming that the main mechanism for deposit build-up is inertial impaction, a simple model for quantifying the deposit build-up rate, N, is: N = ηimp ηstick U C ash where η imp is the impaction efficiency, ie. the weight fraction of particles that reaches the tube surface, η stick, is the sticking efficiency, ie. the weight fraction of those particles reaching the surface, that also stick to the surface, U is the bulk linear flow velocity (m/s) and C ash is the ash loading (g/m 3 ) in the flue gas. As a first approximation, the deposit build-up may be quantified by the above simple equation, stating a linear dependence between the flux of sticking particles and the fly ash mass loading in the flue gas. Thus, an increased build-up rate is to be expected when the fly ash mass loading is increased eg. during co-firing of biomasses. SUMMARY AND CONCLUSIONS A fly ash and deposit investigation was carried out as part of the SK Power Company test programme on co-firing of biomasses in a grate-fired boiler. The results can be summarized as follows following: Two of the alternative biomasses (olive stones and shea nuts) contain more K, S, and Cl, than wheat straw. The amount of Cl+2S does in general equal the amount of K in the fuel feedstock, bottom ash (BA) and fly ash (FA). Fraction of water-soluble K: o Fuel: 50 %(w/w) o Bottom ash: < 15 %(w/w) o Fly ash: 100 %(w/w) No significant change in deposit structure was observed when co-firing alkali-rich biomass: KCl glues residual ash particles together, independent of the feedstock mixture.

16 IFRF Combustion Journal Frandsen, Hansen, Jensen et al Higher fly ash mass loading (mass of fly ash/volume of flue gas) to be expected when co-firing the alternative biomasses applied in this study, with wheat straw. More residual ash particles occur in the outer deposit layers when co-firing alternative biomasses with wheat straw. The low fraction of water-soluble K in the fuels is surprising, but may be related to the specific procedure applied at the test laboratories. No further details on this are available. Thus, based on the experience gained within this study, it can be concluded that co-firing of the actual biomasses in boilers designed for straw-firing at the present shares is not problematic, from an ash formation and/or deposit build-up point-of-view. Anyhow, as indicated above, the increase in ash mass loading in the flue gas, may cause increased build-up of particulate deposits in the convective pass of the boiler. Thus, co-firing of biomasses over long time periods may cause a necessary change in the soot-blower operation (increased frequency of soot-blowing may be required). ACKNOWLEDGMENT This work was carried out within the Combustion and Harmful Emission Control (CHEC) Research Centre, Technical University of Denmark. The CHEC Research Centre is co-funded by Elkraft, Elsam, the Danish and Nordic Energy Research Programmes, and the European Union. Ellen ter Haar Hansen is greatly acknowledged for mounting, cutting and polishing the samples for SEM analyses. Center for Microstructure and Surface Analysis, at the Teknological Institute, Taastrup, Denmark, and in particular Dorte Larsen is greatly acknowledged for operating the SEM. REFERENCES 1. Michelsen, H.P., Larsen, O.H., Frandsen, F.J., and Dam-Johansen, K., (1998), Deposits and High Temperature Corrosion in a 10 MW Straw Fired Boiler, Fuel Processing Technology, 54, 95 (1998) 2. Stenholm, M., Jensen, P. A., Hald, P. The Fuel and Firing Characteristics of Biomass - Combustion Trials, Final Report, EFP Project No. 1323/ (In Danish) (1996).

17 IFRF Combustion Journal Frandsen, Hansen, Jensen et al 3. Jensen, P. A., Stenholm, M., Hald, P. Deposition Investigation in Straw-Fired Boilers, Energy and Fuels, 11:5, 1048 (1997). 4. Frandsen, F. J., Nielsen, H. P., Jensen, P. A., Hansen, L. A., Livbjerg, H., Dam-Johansen, K., Hansen, P. F. B., Andersen, K. H., Sørensen, H. S., Larsen, O. H., Sander, B., Henriksen, N., Simonsen, P. "Deposition and Corrosion in Straw and Coal-Straw Co- Fired Utility Boilers - Danish Experiences", Proc. Eng. Found. Conf. "Impact of Mineral Impurities in Solid Fuel Combustion", Kona, Hawaii, USA, November 2 7 (1997). 5. Frandsen, F. J., Nielsen, H. P., Hansen, L. A., Hansen, P. F. B., Andersen, K. H., Sørensen, H. S. Ash Chemistry Aspects of Straw and Coal-Straw Co-Firing in Utility Boilers, Proc. 15 th Annual Int. Pittsburgh Coal Conf., GreenTree Marriott Hotel, Pittsburgh, PA, USA, September (1998). 6. Larsen, O. H., Henriksen, N., Inselman, S., Blum, R. The Influence of Boiler Design and Process Conditions on Fouling and Corrosion is Straw and Coal/Straw-Fired Ultra Supercritical Power Plants, 9 th European Bioenergy Conference, Copenhagen, Denmark, June (1996). 7. Lans, R. P. Gas Concentrations and Temperature Measurements during Straw and Biomass Co-Firing in the Grate Furnace of Masnedø CHP Plant, CHEC Report No. 9914, Department of Chemical Engineering, Technical University of Denmark (1999).