High contents of chlorine and alkalies restrict the use of biomass in energy production. Alkali chlorides vaporize during combustion. Chlorine tends to produce corrosive deposits and unacceptably high emissions of HCl and dioxins. Chlorine recovery and enrichment of Cl, Na, K, Ca, Al, and Si in coarse and fine fly ash were studied experimentally with two electrically stabilized bubbling fluidized bed (BFB) reactors capable of reproducing the particle residence times existing in full-scale BFB plants. Feedstocks were fir (mixture of heartwood and bark), paper sludge, and blends of fir with agricultural waste or plastic waste. Sulfur concentrations of feed components were low (<0.5 wt %), while chlorine and potassium concentrations ranged widely (0.02−3.2 wt % for Cl and 0.07−3.1 wt % for K). Aluminum-containing additives (kaolin, bauxite and fly ash from a pulverized coal plant) and limestone were added to the feedstocks at various dosages to evaluate their influence on Cl behavior and enrichment of the elements of interest. HCl was measured by FTIR and wet-absorption methods. Different ash samples (bed, cyclone and filter ash) were characterized for their Cl content and the major ash-forming constituents. Cl was completely volatilized from bed ash and recovered only in coarse (cyclone) and fine (filter) fly ash fractions. Al-containing additives increased HCl formation and decreased Cl concentration in the fly ash. In the case of Al−Si based additives, evidence was found of the formation of alkali aluminum silicates from alkali chlorides. The aluminum silicates were transferred mainly to the coarse fly ash fraction. Al-based additives also seemed to liberate Cl from alkali chlorides with reactions forming water-soluble alkali compounds. Limestone had the opposite effect to the Al-containing additives by binding Cl from gas phase to fly ash, but mainly to the coarse fly ash fraction. The results will be useful in optimizing the behavior of chlorine in bubbling bed combustion of Cl- and alkali-containing biomass.

Experiments on an electrically heated entrained flow combustion reactor were carried out in order to test the air-staging behavior of four bituminous coals of industrial interest. Through measurements of gaseous nitrogen-containing species profiles (NO, HCN) and sampling of char particles at different conversion levels, a study was elaborated about the impact of process parameters and coal type on NO formation and reduction, as well as on the nitrogen fate during the course of combustion. While the air-staging abatement efficiency was observed to be correlated with the volatile-nitrogen release from the coal, the presented analysis reveals that the contribution of char-nitrogen release cannot be neglected. This study shows that nitrogen release rates change significantly during the various phases of combustion, also revealing the effect of the operating conditions on the release rates. A simple computational modeling has been carried out in order to estimate the relative influence of the process parameters on char-nitrogen conversion into NO in the burnout zone. The results exhibit the influence of the NO concentration level in the gas phase as one possible explanation of the differences exhibited by the coals. The comparison of experimental data and the computational modeling also displays the necessity of a more detailed kinetic approach to describe char-nitrogen evolution by computer codes for the optimization of staged combustion processes.

The overall objective of the work described in this paper was to determine the behavior of wood ash under entrained-flow gasification conditions. Experimental work in atmospheric and pressurized entrained-flow gasification simulators, combined with thermodynamic equilibrium calculations, has shown that wood ash is not prone to form a molten slag at typical operating conditions of (pressurized, dry-feed, oxygen-blown) entrained-flow gasifiers, in spite of the presence of a relatively high amount of low-melting alkaline elements. This appears mostly due to the formation of mainly high-temperature-melting compounds (e.g., CaO) and only a small fraction of Ca silicates, which are characterized by a lower melting temperature. Phosphor and silicon may contribute to creating a higher melt amount, whereas low-melting alkali metal compounds are mostly partitioned into the vapor phase. Experiments, as well as modeling work performed for three types of wood, have shown consistent results. Addition of a fluxing agent is a promising option to improve the slagging behavior of wood-based systems by reducing the melting point of the slag. Moreover, thermodynamic calculations have shown that slag recycle may represent a feasible option in order to obtain sufficient slag coverage of the refractory wall despite the low ash content of woody fuels (typically 1 order of magnitude lower than in coal). In the present work, the determination of slag viscosity, a parameter critical for continuous operation of a slagging gasifier, has been addressed as well. The results of modeling work, showing the inapplicability of predictive formulas developed in the past for coal slags to wood-based slags, underline that further work is required to allow for a quantitative assessment of the slag viscosity as a function of slag composition and temperature.

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