W. Nimmo

Thermal analysis and devolatilization kinetics of cotton stalk, sugar cane bagasse and shea meal under nitrogen and air atmospheres.

Thermal degradation, reactivity and kinetics for biomass materials cotton stalk (CS), sugarcane bagasse 1 (SB1), sugarcane bagasse 2 (SB2) and shea meal (SM) have been evaluated under pyrolysis (N(2)) and oxidising (dry air) conditions, using a non-isothermal thermogravimetric method (TGA). In the cases of CS and SB1 the peak temperatures were 51 degrees C higher for pyrolysis compared with oxidative degradation, whereas for SB2 and SM the difference was approximately 38 degrees C. However, the differences in the rates of weight loss were significantly higher under oxidising conditions for all the materials studied. Maximum rate of weight loss (%s(-1)) under pyrolysis conditions ranged from 0.10 to 0.18 whereas these values accelerated to the range of 0.19-0.28 under oxidising conditions, corresponding to respective peak temperatures. Samples ranked in order of reactivity (R(M)x10(3)) (%s(-1) degrees C(-1)) are CS=1.31 approximately SM=1.30>SB2=1.14>SB1=0.94 for air and CS=0.54>SB2=0.49>SB1=0.45>SM=0.31 for nitrogen. Shea meal exhibited a complex char combustion behaviour indicating that there may be two distinct types of char derived from fibrous and woody components in the original material. Activation energy calculations were based on the Arrhenius correlation.

An investigation of important gas-phase reactions of nitrogenous species from the simulation of experimental measurements in combustion systems

Simulated results from a detailed elementary reaction mechanism for nitrogen-containing species in flames consisting of hydrogen, C1 or C2 fuels are presented, and compared with bulk experimental measurements of nitrogen-containing species in a variety of combustion systems including flow reactors, perfectly stirred reactors, and low pressure laminar flames. Sensitivity analysis has been employed to highlight the important reactions of nitrogenous species in each system. The rate coefficients for these reactions have been compared against the expressions used in three other recent reaction mechanisms: version 3.0 of the GRI mechanism, the mechanism of Glarborg, Miller and co-workers, and that of Dean and Bozzelli. Such comparisons indicate that there are still large discrepancies in the reaction mechanisms used to describe nitrogen chemistry in combustion systems. Reactions for which further measurements and evaluations are required are identified and the differences between the major mechanisms available are clearly demonstrated.

Shea meal and cotton stalk as potential fuels for co-combustion with coal

The efficient management of waste biomass is an important environmental problem in agricultural countries. Often land-fill is the main disposal route with ramifications including CH(4) release having 21 times greater global warming potential per molecule than CO(2). Biomasses are considered to be CO(2)-neutral fuels when combusted. Moreover, they are renewable and covered by the renewable obligation scheme and eligible for certificates in the UK. The overall objective of the investigation is to assess the performance of selected biomass and coal co-firing under two different modes of operation, air-staging and fuel-staging with the benefit of reduced-NO(x) and SO(2) emissions in power plant. The biomasses chosen for the study, shea meal (SM) and cotton stalk (CS) have very different cellulose/lignin compositions and different reported thermal behaviour. A series of experiments have been carried out in a 20 kW, down fired combustor using coal, shea meal-coal and cotton stalk-coal blends under un-staged, air-staged and fuel-staged co-combustion configurations. For air-staging, an optimum value of primary zone stoichiometry SR(1)=0.9 was found. Keeping it fixed, the shea meal and cotton stalk content in the coal-biomass blends was set to 5%, 10% and 15% on thermal basis. NO reductions of 51% and 60% were achieved using SM and CS, respectively, with an optimum thermal biomass blending ratio (BBR) of 10%. The results obtained were compared with un-staged and air-staged results for coal without the addition of biomass. Similarly for fuel-staging, keeping the length of the reburn and burnout zone fixed, SM and CS were evaluated as reductive fuel using different reburn fuel fractions (R(ff)) of 5%, 10%, 15% and 20%. NO reductions of 83% and 84% were obtained with an optimum R(ff) of 15% with an optimum reburn zone stoichiometry of SR(2)=0.8 for both SM and CS, respectively. SO(2) reduction and char burnout efficiency were also evaluated. It was found that addition of biomass coupled with air and fuel-staging techniques reduced-NO(x) and SO(2) simultaneously while at the same time improving the char burnout efficiency.

Effect of oxygenated liquid additives on the urea based SNCR process

An experimental investigation was performed to study the effect of oxygenated liquid additives, H(2)O(2), C(2)H(5)OH, C(2)H(4)(OH)(2) and C(3)H(5)(OH)(3) on NO(x) removal from flue gases by the selective non-catalytic reduction (SNCR) process using urea as a reducing agent. Experiments were performed with a 150kW pilot scale reactor in which a simulated flue gas was generated by the combustion of methane operating with 6% excess oxygen in flue gases. The desired levels of initial NO(x) (500ppm) were achieved by doping the fuel gas with ammonia. Experiments were performed throughout the temperature range of interest, i.e. from 800 to 1200 degrees C for the investigation of the effects of the process additives on the performance of aqueous urea DeNO(x). With H(2)O(2) addition a downward shift of 150 degrees C in the peak reduction temperature from 1130 to 980 degrees C was observed during the experimentation, however, the peak reduction efficiency was reduced from 81 to 63% when no additive was used. The gradual addition of C(2)H(5)OH up to a molar ratio of 2.0 further impairs the peak NO(x) reduction efficiency by reducing it to 50% but this is accompanied by a downward shift of 180 degrees C in the peak reduction temperature. Further exploration using C(2)H(4)(OH)(2) suggested that a 50% reduction could be attained for all the temperatures higher than 940 degrees C. The use of C(3)H(5)(OH)(3) as a secondary additive has a significant effect on the peak reduction efficiency that decreased to 40% the reductions were achievable at a much lower temperature of 800 degrees C showing a downward shift of 330 degrees C.

The production of ultrafine zirconium oxide powders by spray pyrolysis

Twin-fluid atomisation spray-pyrolysis has been investigated for the production of ZrO2 powders. The atomiser used in this study has a novel internal arrangement that can produce a spray with a mean diameter (SMD) of less than 5 μm. Spray pyrolysis tests with zirconium nitrate as a precursor salt were performed and the formation of ZrO2 powder was studied under substantially different heating rates and initial solution concentrations. A mean particle diameter, d(0.5), of 0.67 μm and 0.77 μm was achieved for 0.05 M and 0.5 M solutions, respectively. It was concluded that the new nozzle design performed well and was successful in producing ultra-fine ZrO2 powder with a principally tetragonal structure when the correct process conditions of heating rate and residence time were applied.

Evaluation of the chemical properties of coals and their maceral group constituents in relation to combustion reactivity using multi-variate analyses

Abstract The factors affecting the combustion rates of coals have been investigated using multivariate analysis. Coals from Australia, Colombia, Germany, UK and USA and their separated maceral group constituents were subjected to detailed optical and chemical characterisation followed by the measurement of combustion reactivity at low temperatures (up to 1323 K) using TGA, and at high temperatures (1300–1900 K) in an entrained flow reactor. The derived activation energies and pre-exponential factors were used in conjunction with eighty-six detailed chemical and petrographic parameters as the basis for statistical analysis. This demonstrated that the parameters which significantly influence the combustion rate could be used to divide the coals and maceral groups into two classes. One class containing predominantly the northern hemisphere coals with their corresponding maceral groups and the other containing the Australian and Colombian coals with their maceral groups. No significant relationships were found between the reactivity of the coals and the maceral groups within the coal, or between the same maceral group for all the coals. However the chemical composition of the organic matter content was found to be an important parameter in determining coal reactivity and such information could be used to supplement classical petrographical classification and proximate and ultimate analyses in order to predict the reactivity of a given coal. The effect of ash on the burning rate of the individual maceral components was also investigated and the implications for burnout in practical combustion systems discussed.

The Prediction of NOx Emissions from Spray Combustion

ABSTRACT The study of in-flame chemical processes involving nitrogenous species forms an important pan of the understanding of the design parameters which lead to lower NOx emissions from combustion systems. This paper presents data from experimental and modelling studies on the nitrogenous emissions from an oil fired furnace using staged combustion for the control of NOx emissions. Measurements of the in-flame NO concentration profiles are compared for the same burner operating in the unstaged and staged mode. The exit gas NO concentration was reduced by 30% during staged combustion with 35% secondary air. A post-processing NO model linked to the output from a commercial computational code was used to predict the rates of formation and concentrations of thermal, fuel and prompt-NO from the experimental system. The predictions showed that NO emission is in good qualitative agreement with the experimentally observed values when the effects of superequilibrium radical formation and turbulence/chemistry inte...

The control of spontaneous ignition under rapid compression

Evolution of the spontaneous ignition of n-butane is investigated under conditions of varying heat dissipation rates, controlled through the intensity of gas motion generated during mechanical compression of the reactants in a cylinder. As the heat dissipation rate is enhanced the minimum compressed gas temperature required for ignition is also raised, from 700 to 870 K in the present conditions. This increase is linked to the initial rate of cooling in the post compression period before significant chemical heat release begins to take place. At this threshold for reaction the common factor is the prevailing gas temperature, which at its minimum during a marginally supercritical ignition delay, is about 600 K. Heat release rates just balance dissipation rates here, permitting the evolution of “low temperature” degenerate chain branching. Moreover, the end-of-compression temperatures within which an inverse temperature dependence of the ignition delay is observed increase when the heat dissipation rate is raised. This feature is also rationalised in terms of the prevailing post-compression gas temperature, consistently in the range 660–725 K. The heat release rates derived from experimental results exhibit a negative temperature coefficient in this range and are the origin of the complex, overall time dependence. Some contrasts due to the much lower reactivity of isobutane are also presented.

Modelling the anaerobic digestion of solid organic waste – Substrate characterisation method for ADM1 using a combined biochemical and kinetic parameter estimation approach

This work proposes a novel and rigorous substrate characterisation methodology to be used with ADM1 to simulate the anaerobic digestion of solid organic waste. The proposed method uses data from both direct substrate analysis and the methane production from laboratory scale anaerobic digestion experiments and involves assessment of four substrate fractionation models. The models partition the organic matter into a mixture of particulate and soluble fractions with the decision on the most suitable model being made on quality of fit between experimental and simulated data and the uncertainty of the calibrated parameters. The method was tested using samples of domestic green and food waste and using experimental data from both short batch tests and longer semi-continuous trials. The results showed that in general an increased fractionation model complexity led to better fit but with increased uncertainty. When using batch test data the most suitable model for green waste included one particulate and one soluble fraction, whereas for food waste two particulate fractions were needed. With richer semi-continuous datasets, the parameter estimation resulted in less uncertainty therefore allowing the description of the substrate with a more complex model. The resulting substrate characterisations and fractionation models obtained from batch test data, for both waste samples, were used to validate the method using semi-continuous experimental data and showed good prediction of methane production, biogas composition, total and volatile solids, ammonia and alkalinity.

Experimental and kinetic studies on the effect of sulfur-nitrogen interactions on no formation in flames

The participation of fuel-sulfur (fuel-S) in the NO formation/destruction processes occurring in fossil fuel flames has been featured sparingly in the literature, although several studies were performed some years ago. This paper is concerned with the effects of fuel-S on NO formation and destruction processes in liquid-fuel, spray flames. Experimental in-flame measurements of NO, ammonia, and cyanide in a 150-kW test furnace are presented and discussed with the benefit of a kinetic study. Fuel-nitrogen (fuel-N) and fuel-S were simulated using quinoline and tetrahydrothiophene, respectively. The exit gas NO emissions were found to be affected by the addition of S to the fuel to an extent that was dependent on the fuel-S to fuel-N ratio and burner operating conditions. When the burner was operated in low-NO x mode by staging the combustion air, the interactions were found to be dependent on the primary-zone fuel/air equivalence ratios, with greater enhancement of fuel-NO formation observed for fuel-rich conditions. Thermal-NO emissions were reduced with S addition for fuel-lean operation of the primary stage but were relatively unaffected when operating fuel-rich. Comparative studies using SO 2 as the S additive produced similar results, but the magnitude of the changes in the NO emission were less than those found using tetrahydrothiophene. In-flame measurements of the concentration of NO x precursors, ammonia and cyanide, showed that where increases in fuel-NO were observed due to the effect of S, corresponding reductions in ammonia and cyanide were measured, particularly under fuel-rich conditions. Data obtained from the experimental furnace have been adapted for use with a detailed chemical mechanism with the CHEMKIN and the KINALC software packages. This has allowed the nature of the interactions between the sulfur chemistry and the hydrocarbon and nitrogen chemistry that influence NO formation to be investigated.