The effect of oxygen and volatile combustibles on the sulphation of gaseous KCl

Sulphur/sulphate containing additives, such as elemental sulphur (S) and ammonium sulphate (NH₄)₂SO₄), can be used for sulphation of KCl during biomass combustion. These additives convert KCl to an alkali sulphate and a more efficient sulphation is normally achieved for ammonium sulphate compared to sulphur. The presence of SO₃ is thus of greater importance than that of SO₂. Oxygen and volatile combustibles could also have an effect on the sulphation of gaseous KCl. This paper is based on results obtained during co-combustion of wood chips and straw pellets in a 12 MW circulating fluidised bed (CFB) boiler. Ammonium sulphate was injected at three positions in the boiler i.e. in the upper part of the combustion chamber, in the cyclone inlet, and in the cyclone. The sulphation of KCl was investigated at three air excess ratios (λ = 1.1, 1.2 and 1.4). Several measurement tools were applied including IACM (on-line measurements of gaseous alkali chlorides), deposit probes (chemical composition in deposits collected) and gas analysis. The position for injection of ammonium sulphate had a great impact on the sulphation efficiency for gaseous KCl at the different air excess ratios. There was also an effect of oxygen on the sulphation efficiency when injecting ammonium sulphate in the cyclone. Less gaseous KCl was reduced during air excess ratio λ = 1.1 compared to the higher air excess ratios. The optimal position and conditions for injection of ammonium sulphate were identified by measuring KCl with IACM. A correlation was observed between the sulphation of gaseous KCl and reduced chlorine content in the deposits. The experimental observations were evaluated using a detailed reaction mechanism. It was used to model the effect of volatile combustibles on the sulphation of gaseous KCl by SO₃. The calculations supported the proposition that the presence of combustibles at the position of SO₃ injection (i.e. AS) causes reduction of SO₃ to SO₂.     

The economics of reburning with cattle manure-based biomass in existing coal-fired power plants for NOx and CO2 emissions control

Coal plants that reburn with catttle biomass (CB) can reduce CO₂ emissions and save on coal purchasing costs while reducing NO_x emissions by 60–90% beyond levels achieved by primary NO_x controllers. Reductions from reburning coal with CB are comparable to those obtained by other secondary NO_x technologies such as selective catalytic reduction (SCR). The objective of this study is to model potential emission and economic savings from reburning coal with CB and compare those savings against competing technologies. A spreadsheet computer program was developed to model capital, operation, and maintenance costs for CB reburning, SCR, and selective non-catalytic reduction (SNCR). A base case run of the economics model, showed that a CB reburn system retrofitted on an existing 500 MW_e coal plant would have a net present worth of −$80.8 million. Comparatively, an SCR system under the same base case input parameters would have a net present worth of +$3.87 million. The greatest increase in overall cost for CB reburning was found to come from biomass drying and processing operations. The profitability of a CB reburning system retrofit on an existing coal-fired plant improved with higher coal prices and higher valued NO_x emission credits. Future CO₂ taxes of $25 tonne⁻¹ could make CB reburning as economically feasible as SCR. Biomass transport distances and the unavailability of suitable, low-ash CB may require future research to concentrate on smaller capacity coal-fired units between 50 and 300 MW_e.     

An in depth investigation of deactivation through carbon formation during the biogas dry reforming reaction for Ni supported on modified with CeO2 and La2O3 zirconia catalysts

The dry reforming of biogas on a Ni catalyst supported on three commercially available materials (ZrO₂, La₂O₃ZrO₂ and CeO₂ZrO₂), has been investigated, paying particular attention to carbon deposition. The DRM efficiency of the catalysts was studied in the temperature range of 500–800 °C at three distinct space velocities, and their time-on-stream stability at four temperatures (550, 650, 750 and 800 °C) was determined for 10 or 50 h operation. The morphological, textural and other physicochemical characteristics of fresh and spent catalysts together with the amount and type of carbon deposited were examined by a number of techniques including BET-BJH method, CO₂ and NH₃-TPD, XPS, SEM, TEM, STEM-HAADF, Raman spectroscopy, and TGA/DTG. The impact of the La₂O₃ and CeO₂ modifiers on the DRM performance and time-on-stream stability of the Ni/ZrO₂ catalyst was found to be very beneficial: up to 20 and 30% enhancement in CH₄ and CO₂ conversions respectively, accompanied with a CO-enriched syngas product, while the 50 h time-on-stream catalytic performance deterioration of ～30–35% on Ni/ZrO₂ was limited to less than ～15–20% on the La₂O₃ and CeO₂ modified samples. Their influence on the amount and type of carbon formed was substantial: it was revealed that faster oxidation of the deposited carbon at elevated temperatures occurs on the modified catalysts. Correlations between the La₂O₃ and CeO₂-induced modifications on the surface characteristics and physicochemical properties of the catalyst with their concomitant support-mediated effects on the overall DRM performance and carbon deposition were revealed.     

A comparative study of Pt/C cathodes in Sn0.9In0.1P2O7 and H3PO4 ionomers for high-temperature proton exchange membrane fuel cells

New Pt/C cathodes with many reaction sites for the oxygen reduction reaction as well as high tolerance to Pt corrosion have been designed for high-temperature proton exchange membrane fuel cells (PEMFCs), wherein a composite mixture of Sn_0.9In_0.1P₂O₇ (SIPO) and sulfonated polystyrene-b-poly(ethylene/butylene)-b-polystyrene (sSEBS) functioned as an ionomer. The microstructure of the Pt-SIPO-sSEBS/C cathode was characterized by homogeneous distribution of the ionomer over the catalyst layer and close contact between the ionomer and the Pt/C powder. As a result, the activation and concentration overpotentials of the Pt-SIPO-sSEBS/C cathode between 100 and 200 °C were lower than those of an H₃PO₄-impregnated Pt/C cathode, which suggests that the present ionomer can avoid poisoning of Pt by phosphate anions and the limitation of gas diffusion through the catalyst layer. Moreover, agglomeration of Pt in the Pt-SIPO-sSEBS/C cathode was not observed during a durability test at 150 °C for 6 days, although it was significant in the Pt-H₃PO₄/C cathode. Therefore, it is concluded that the Pt-SIPO-sSEBS/C electrode is a very promising cathode candidate for high-temperature PEMFCs.     

Optical analysis of the microstructure of a Mo back contact for Cu(In,Ga)Se2 solar cells and its effects on Mo film properties and Na diffusivity

The microstructures of molybdenum (Mo) thin films deposited at pressures from 3.3 to 10.3 mTorr were characterized, and the relationships between these microstructures and the properties of the films (residual stress and electrical resistivity) were investigated. In the low deposition pressure regime (region I, below 7 m Torr), the residual stress in the tensile direction increases with increasing pressure and the electrical resistivity increases gradually, but at high deposition pressures (region II, above 7 m Torr) the residual stress is reduced and the resistivity increases more steeply. These variations of the properties of the Mo films in the low pressure regime are due to the variation in grain size; the carrier mobility decreases due to increased grain boundary (GB) scattering and the tensile stress increases due to increased atomic attraction across the GBs. In contrast, the porosity of the Mo films increases significantly in the high pressure regime, as demonstrated with variable angle spectroscopic ellipsometry (VASE). Most of these pores are believed to be present along the grain boundaries of the Mo films, so their presence reduces the GB attraction and thus the tensile stress and enhances the carrier scattering. The high porosity of the Mo back contact was shown with secondary ion mass spectroscopy profiling to accelerate the Na diffusion from soda-lime glass into the Cu(In,Ga)Se₂ (CIGS) film.