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The transformation of inorganic constituents in annual biomass was experimentally investigated at grate-combustion conditions. A laboratory fixed-bed reactor was applied to obtain quantitative information of the release of Cl, K, and S to the gas phase from six distinctively different annual biomass fuels. Samples of 4.0 g of biomass were combusted at well-controlled conditions at temperatures from 500 to 1150 °C. The elemental release was quantified by analysis of the residual ash and a mass balance on the system. The experimental results revealed that potassium was released to the gas phase in significant amounts at combustion above 700 °C. The potassium release increased with the applied combustion temperature for all biomass fuels; however, the quantity released was largely determined by the ash composition. At 1150 °C, between 50 and 90% of the total potassium was released to the gas phase. The biomass fuels with an appreciable content of silicate showed the lower release of potassium. Between 25 and 70% of the fuel chlorine was released below 500 °C; the residual chlorine was released by evaporation of KCl, mainly between 700 and 800 °C. Above 800 °C, the fuel chlorine was completely released to the gas phase for all of the samples. Between 30 and 55% of the fuel sulfur was released at 500 °C. The samples rich in K and Ca, but low in Si, displayed only a minor increase in the sulfur release as the combustion temperature was further increased. On the contrary, the sulfur release increased abruptly above 700-800 °C for the Si-rich samples. On the basis of the release quantification, the overall transformations of the ash-forming elements are discussed at grate-combustion conditions.

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When straw undergoes thermal treatment the initial process is a pyrolysis at which some K and Cl can be volatilized, and this may result in problems with deposit formation and corrosion of the reactor containment. A laboratory batch reactor was applied to study the release and transformation of K and Cl as a function of temperature, at an initial heating rate of approximately 50 °C/s. To facilitate the interpretation of the batch reactor experiments thermodynamic equilibrium calculations at reducing condition were conducted, and SEM (scanning electron microscopy) and leaching investigations were carried out on straw and char samples. The experiments showed that chlorine was released in two steps, about 60% was released when the temperature increased from 200 to 400 °C and most of the residual chlorine was released between 700 and 900 °C. Below 700 °C no significant potassium release was observe; above that temperature it increased progressively until about 25% potassium release at 1050 °C. During pyrolysis most K was released from the original binding sites, and the part that was not transformed to gas phase existed as redeposited discrete particles of KCl and K 2 CO 3 , as potassium silicates, or bound to the organic matrix. The initial release of potassium to the gas phase at approximately 700 °C was caused by evaporation of deposited KCl particles. The release of Cl to the gas phase was strongly affected by heating rate and sample size.

Combustion of wood for heat and power production may cause problems such as ash deposition, corrosion, and harmful emissions of gases and particulate matter. These problems are all directly related to the release of inorganic elements (in particular Cl, S, K, Na, Zn, and Pb) from the fuel to the gas phase. The aims of this study are to obtain quantitative data on the release of inorganic elements during wood combustion and to investigate the influence of fuel composition. Quantitative release data were obtained by pyrolyzing and subsequently combusting small samples of wood (∼30 g) at various temperatures in the range of 500–1150 °C in a laboratory-scale tube reactor and by performing mass balance calculations based on the weight measurements and chemical analyses of the wood fuels and the residual ash samples. Four wood fuels with different ash contents and inorganic compositions were investigated, including wood chips from spruce and beech, bark, and fiber board. The results showed a high release of Cl (∼85–100%) and S (∼50–70%) already at 500 °C, so that only small variations in the release trends of Cl and S were seen between the different fuels in the range of 500–1150 °C. The release of the alkali metals K and Na was, however, strongly dependent on both the temperature and the fuel composition under the investigated conditions. The release of the heavy metals Zn and Pb started around 500 °C and increased sharply to more than 85% at 850 °C in the case of spruce, beech, and bark, and was therefore mainly dependent on the temperature. By comparing the data to literature data, and by using tools such as scanning electron microscopy, chemical fractionation analysis, and equilibrium calculations, a better understanding of the release mechanisms was obtained. Mechanisms for the release of Cl, S, K, Na, Zn, and Pb during wood combustion are proposed.

Four groups of catalysts have been tested for hydrodeoxygenation (HDO) of phenol as a model compound of bio-oil, including oxide catalysts, methanol synthesis catalysts, reduced noble metal catalysts, and reduced non-noble metal catalysts. In total, 23 different catalysts were tested at 100 bar H2 and 275 °C in a batch reactor. The experiments showed that none of the tested oxides or methanol synthesis catalysts had any significant activity for phenol HDO under the given conditions, which were linked to their inability to hydrogenate the aromatic ring of phenol. HDO of phenol over reduced metal catalysts could effectively be described by a kinetic model involving a two-step reaction in which phenol initially was hydrogenated to cyclohexanol and then subsequently deoxygenated to cyclohexane. Among reduced noble metal catalysts, ruthenium, palladium, and platinum were all found to be active, with activity decreasing in that order. Nickel was the only active non-noble metal catalyst. For nickel, the effect of support was also investigated and ZrO2 was found to perform best. Pt/C, Ni/CeO2, and Ni/CeO2-ZrO2 were the most active catalysts for the initial hydrogenation of phenol to cyclohexanol but were not very active for the subsequent deoxygenation step. Overall, the order of activity of the best performing HDO catalysts was as follows: Ni/ZrO2 > Ni-V2O5/ZrO2 > Ni-V2O5/SiO2 > Ru/C > Ni/Al2O3 > Ni/SiO2 ≫ Pd/C > Pt/C. The choice of support influenced the activity significantly. Nickel was found to be practically inactive for HDO of phenol on a carbon support but more active than the carbon-supported noble metal catalysts when supported on ZrO2. This observation indicates that the nickel-based catalysts require a metal oxide as a carrier on which the activation of the phenol for the hydrogenation can take place through heterolytic dissociation of the O–H bond to facilitate the reaction.

In this work, the sulfur transformations during thermal conversion of two straw samples have been experimentally investigated. Sulfur was found to be associated partly as inorganic sulfate (40-50% of the total S) and partly as organic sulfur (50-40%) in typical Danish wheat straw samples. Batch pyrolysis and combustion experiments were conducted in a lab-scale tubular reactor in order to obtain quantitative information on the sulfur transformations during devolatilization and char burnout. The lab-scale experiments indicated that 35-50% of the total sulfur was released to the gas phase during the devolatilization. The release was predominantly caused by decomposition of organically associated sulfur. During char burnout at low temperature (<500 °C), no sulfur was released to the gas phase, but was instead completely retained in the straw ash. As the combustion temperature was increased, sulfur was gradually released to the gas phase; approximately 85% of the total S was released at 950 °C. Pyrolysis experiments with SO 2 addition indicated that additional sulfur may be fixed in char. Combustion of the sulfurenriched char samples resulted in significantly higher sulfur concentrations in the residual ashes. On the basis of the experimental results, the transformation and release to the gas phase of biomass sulfur is discussed.

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