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The calcium looping cycle is being developed as a method for capturing CO2 from both flue and fuel gases. It works by using CaO as a CO2 carrier and through repeated cycles of carbonation and calcination can extract CO2 from a gas with a lower partial pressure of CO2 (e.g., exhaust stream from a power station) and provide a pure stream of CO2 suitable for sequestration. A key problem in the development of calcium looping technology is the decrease in reactivity of the sorbent with an increasing number of cycles of carbonation and calcination. The hydration of calcined sorbent has been shown to be a promising way of periodically regenerating the sorbent, so that its reactivity can be recovered, reducing the requirement to purge material from the cycle. In previous work, the reactivity of sorbents after hydration has been mainly studied by thermogravimetric analysis or in a fluidized bed with an unrealistically low calcination temperature. For this work, a laboratory-scale reactor capable of operation under more realistic conditions has been designed, built, and commissioned. It consists of a computer-controlled, resistance-heated, fluidized-bed reactor capable of temperature cycling, allowing the sorbent to be exposed to repeated cycles of carbonation and calcination within the same vessel. The sorbent is “reactivated” by hydration after a number of cycles and then exposed to further cycles of CO2 capture and release. The reactivity of the sorbent is measured from the CO2 uptake and release during successive cycles of carbonation and calcination. Preliminary tests have been completed, and these show that, for limestone reacted under mild calcination conditions, the ultimate uptake of CO2 (the carrying capacity) of cycled Havelock limestone can be more than doubled upon hydration. As the calcination conditions before hydration become harsher (the temperature is increased), the regeneration technique becomes less effective. This is also observed, although to differing extents, with La Blanca and Purbeck limestones. This is shown to be due to mass loss from the fluidized bed because of the increased friability of the hydrated sorbent. A particle breakage model has been developed to describe this phenomenon.

Differences in the development of carbon structures between coal chars and metallurgical cokes during high-temperature reactions have been investigated using Raman spectroscopy. These differences are important for differentiation between different types of carbons present in the dust recovered from the top gas of the blast furnace. Coal chars have been prepared from a typical injectant coal under different heat-treatment conditions. These chars reflected the effect of peak temperature (from 900 to 2400 °C), residence time at peak temperature (from 2 s to 1 h), heating rate (from 1 to 6000 K/s), and pressure (from 3 to 40 bar a ) on the evolution of their carbon structures. The independent effect of gasification on the development of the carbon structure of a representative coal char has also been studied. A similar investigation has also been carried out to study the effect of heat-treatment temperature (from 1300 to 2000 °C) and gasification on the carbon structure of a typical metallurgical coke. Two Raman spectral parameters, the intensity ratio of the D band (1284-1600 cm -1 ) to the G band (ca. 1600 cm -1 ) (I D /I G ) and the intensity ratio of the valley between D and G bands to the G band (I V /I G ), have been found useful in assessing changes in carbon structure. An increase in I D /I G indicates the growth of basic graphene structural units (BSUs), across the temperature range studied. A decrease in I V /I G appears to suggest the elimination of amorphous carbonaceous materials and ordering of the overall carbon structure. The Raman spectral differences observed between coal chars and metallurgical cokes are considered to result from the difference in the time-temperature history between the raw injectant coal and the metallurgical coke. These observed differences may lay the basis for differentiation between metallurgical coke fines and coal char residues present in the dust carried over the top of the blast furnace.

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A suite of tuyere-level coke samples have been withdrawn from a working blast furnace during coal injection, using the core-drilling technique. The samples have been characterized by size exclusion chromatography (SEC), Fourier transform Raman spectroscopy (FT-RS), and X-ray powder diffraction (XRD) spectroscopy. The 1-methyl-2-pyrrolidinone (NMP) extracts of the cokes sampled from the “bosh”, the rear of the “birdʼs nest”, and the “dead man” zones were found by SEC to contain heavy soot-like materials (ca. 107–108 apparent mass units). In contrast, NMP extracts of cokes taken from the raceway and the front of the “birdʼs nest” only contained a small amount of material of relatively lower apparent molecular mass (up to ca. 105 u). Since the feed coke contained no materials extractable by the present method, the soot-like materials are thought to have formed during the reactions of volatile matter released from the injectant coal, probably via dehydrogenation and repolymerization of the tars. The Raman spectra of the NMP-extracted core-drilled coke samples showed variations reflecting their temperature histories. Area ratios of D-band to G-band decreased as the exposure temperature increased, while intensity ratios of D to G band and those of 2D to G bands increased with temperature. The graphitic (G), defect (D), and random (R) fractions of the carbon structure of the cokes were also derived from the Raman spectra. The R fractions decreased with increasing temperature, whereas G fractions increased, while the D fractions showed a more complex variation with temperature. These data appear to give clues regarding the graphitization mechanism of tuyere-level cokes in the blast furnace. These results from Raman spectroscopy were validated by XRD analyses of the demineralized and NMP-extracted cokes. The average lattice interlayer spacing d 002, stacking height L c, average crystallite diameter L a, and average number of lattice layers N c have been determined from the XRD patterns of the cokes. The raceway coke, which had been exposed to the highest temperature, was observed to possess the largest crystallite dimensions, possibly catalyzed by contact with iron. In contrast, the “dead man” coke had the largest interlayer spacing and smallest crystallite dimensions. The cokes were examined for the presence of alkalis. None were found in the raceway coke while the highest concentration was encountered in the “dead man” coke. These alkalis seemed to interact with carbon by diffusion/adsorption rather than through intercalation: the averaged number of lattice layers of “dead man” coke, with the highest alkali concentration, was still quite large. The TGA reactivity of the “dead man” coke was highest among the five samples, consistent with the degree of disordering of its structure and probably due also to the catalytic effect of the alkalis present. The combination of SEC, Raman spectrometry, and XRD was shown to provide considerable insight into coke structures present in a blast furnace and to give information on condit...

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HCN and NH 3 released during the gasification of sewage sludge have been measured during a program of tests with a laboratory-scale spouted-bed gasifier. The data have been compared with results from gasification tests with coal. The effect of altering the bed temperature has been investigated, and the results have been related to reactions involving gaseous N species known to occur in the gasifier. The effect of steam addition on the HCN release has been examined. It has been found that the HCN concentrations in the exit gas increase with the operating temperature, which is thought to indicate increased formation as a primary product of the decomposition of the fuel-N compounds. Increasing the height of the char bed caused a significant reduction in the HCN concentration at the exit, as this promoted the decomposition of HCN to NH 3 . Steam addition caused a rise in the HCN concentration during tests with sewage sludge and a similar effect had previously been reported on the NH 3 concentration during tests with coal. The NH 3 concentration decreased with increasing temperature, and this is thought to reflect the increased rate of the equilibration of NH 3 in the gas phase to form N 2 and H 2 .

Constraints within European Union (EU) countries on sewage sludge disposal routes are growing as former options meet with increasing environmental, legislative, and economic pressure. Gasification of sewage sludge for heat and power generation in combined heat and power (CHP) applications is an attractive concept that provides an environmentally acceptable, efficient, and economically viable means of generating energy from a waste disposal problem. The final solid residues are pathogen-free but may contain toxic elements such as barium, copper, mercury, lead, and zinc at levels that could make their disposal to landfills costly as well as environmentally unsound. Elements such as barium, copper, mercury, lead, and zinc are present in sewage sludges at levels significant to the disposal of the residual streams from a gasifier. The distribution of barium, copper, mercury, lead, and zinc to the ash residue streams has been studied in an air-blown laboratory-scale spouted-bed gasifier that is fueled by crushed, dried sewage sludge pellets. The gasifier was operated at temperatures of 770−960 °C, and samples of the solid residues were collected. In this study, measurements of trace element concentrations have been used to determine their overall retention in the solid streams, as well as their relative depletion from the coarser bed residue and enrichment in the fines carried to the gas-cleaning system. The effect of the gasifier bed temperature and the type of sewage sludge has been investigated. Under all of the conditions studied, no mercury retention in the solid residues was observed. Cobalt, copper, manganese, and vanadium were neither depleted from the bed residue nor enriched in the fines. The extent of barium, lead, and zinc depletion from the bed residue varies with sludge type, and the enrichment of lead in the fines seems to be enhanced by gasifier bed temperatures in excess of 900 °C. The observed behavior of these elements is discussed in relation to their speciation, as predicted by thermodynamic equilibrium modeling. The potential implications of these findings for process design, operating conditions, and residue disposal are discussed.