No abstract

A number of full-scale deposit probe measuring campaigns conducted in grate-fired and suspension-fired boilers, fired with biomass, have been reviewed and compared. The influence of operational parameters on the chemistry of ash and deposits, on deposit build-up rates, and on shedding behavior has been examined. The firing technology and the fuel utilized influence the fly ash and deposit chemical composition. In grate-firing, K, Cl, and S are enriched in the fly ash compared to the fuel ash, while the fly ash in suspension-firing is relatively similar to the fuel ash. The chemical composition of the deposits formed is determined by the fly ash composition and the flue gas temperature; increases in the local flue gas temperature lead to higher contents of Si and Ca and lower contents of Cl in the deposits. The net deposit build-up rates in grate-fired and suspension-fired boilers are at similar levels, 0–100 g/m2·h, while the ash deposit propensity is an order of magnitude larger in grate fired boilers than in suspension-fired boilers. Deposit build-up rates were found to increase at flue gas temperatures close to the melting temperatures of the fly ash. Furthermore, the rate of deposit build-up increased with the K-content of the fuel ash and fly ash for grate-fired boilers. For suspension-fired boilers, deposition rates are comparatively low for wood-firing and increase with increasing fuel straw shares. Shedding of deposits occurs by melting during straw-firing on a grate at high flue gas temperatures (>900 °C). At lower flue gas temperatures, the deposits can be removed by soot blowing. The required soot blower impact pressure is strongly influenced by the surface temperature, such that a high surface temperature makes the deposit more difficult to remove. During straw/wood-firing in suspension-fired boilers, shedding occurred by debonding with incomplete removal at flue gas temperatures of 600–1000 °C and by debonding with complete removal during wood-firing in suspension-fired boilers at high flue gas temperatures (1300 °C). Shedding events were not observed during wood suspension-firing at low flue gas temperatures (<900 °C). Here, a steady-state mass of deposits on the probe was observed. Increased exposure times and probe temperatures lead to deposits that are difficult to remove. This was observed for grate-firing of straw and for straw/wood firing in suspension-fired boilers.

In this work, a model for the nitrogen chemistry in the oxy-fuel combustion of pulverized coal has been developed. The model is a chemical reaction engineering type of model with a detailed reaction mechanism for the gas-phase chemistry, together with a simplified description of the mixing of flows, heating and devolatilization of particles, and gasÀsolid reactions. The model is validated by comparison with entrained flow reactor results from the present work and from the literature on pulverized coal combustion in O 2 /CO 2 and air, covering the effects of fuel, mixing conditions, temperature, stoichiometry, and inlet NO level. In general, the model provides a satisfactory description of NO formation in air and oxy-fuel combustion of coal, but under some conditions, it underestimates the impact on NO of replacing N 2 with CO 2 . According to the model, differences in the NO yield between the oxy-fuel combustion and the conventional combustion of pulverized coal can mostly be attributed to the recycling of NO (reburning effect) and to changes in the mixing patterns between fuel and oxygen. For pulverized-fuel combustion at high temperatures, we think that NO is mainly reduced by heterogeneous reactions involving both char and soot. Here, the tar yield of the volatiles is mainly converted to soot and H 2 , limiting the concentration of hydrocarbons and thereby the importance of gas-phase removal of NO. Our work emphasizes the need for accurate descriptions of mixing, volatile composition (fate of tar), and heterogeneous reactions. Furthermore, more work is desirable on the reduction of NO by CO on char at higher temperatures.

Two models for deposit formation in suspension firing of biomass have been developed. Both models describe deposit buildup by diffusion and subsequent condensation of vapors, thermophoresis of aerosols, convective diffusion of small particles, impaction of large particles, and reaction. The models differ in the description of the sticking probability of impacted particles: model #1 employs a reference viscosity in the description of the sticking probability, while model #2 combines impaction of viscoelastic particles on a solid surface with particle capture by a viscous surface. Both models were used to describe the deposit formation rates and deposit chemistry observed in a series of entrained flow reactor (EFR) experiments using straw and wood as fuels. It was found that model #1 was not able to describe the observed influence of temperature on the deposit buildup rates, predicting a much stronger influence of this parameter. Model #2 was able to provide a reasonable description of the influence of temperature on the deposit buildup rates observed in the EFR experiments. A parametric study was conducted to examine the influence of some physical parameters, including ash concentration, viscosity of ash and deposits, surface tension, Young’s modulus, and porosity. On the basis of this model evaluation, where a wide range of temperatures (700–1000 °C) and fuels (straw and wood) were applied, model #2 can be regarded as a promising tool for the description of deposit formation from biomass ashes.

In solid fuel flames, NO x is largely formed from the oxidation of volatile nitrogen compounds such as HCN and NH3. To be able to model the nitrogen chemistry in these flames, it is necessary to have an adequate model for volatile-N oxidation. Simple global models for oxidation of HCN and NH3 from the literature should be used cautiously, since their predictive capabilities are limited, particularly under reducing conditions. Models for HCN/NH3/NO conversion based on the systematic reduction of a detailed chemical kinetic model offer high accuracy but rely on input estimates of combustion intermediates, including free radicals. In the present work, simple, semiempirical expressions are presented for estimation of H, O, and OH radicals. Correlations are derived for volatile compositions representative of solid fuels ranging from bituminous coal to biomass, for temperatures of 1200−2000 K, and excess air ratios in the range 0.6 ≤ λ ≤ 2.0. The radical estimation tool is combined with the analytically reduced N-scheme of Pedersen et al. [Combust. Sci. Technol. 1998, 131, 193−223], and the combined model is tested against reference calculations with a comprehensive mechanism. For excess air ratios of λ ≥ 0.8 and temperatures of 1400 K and above, the prediction of NO formation from both HCN and NH3 is very good for volatile compositions representing all tested fuels. For lower values of λ, the predictions are good for biomass and lignite, while they become less accurate for the sub-bituminous and bituminous coals, especially at lower temperatures. The semiempirical correlations for estimating radical concentrations may also be useful in combination with models for other trace species, such as sulfur oxides, organic species, etc.