B. R. Stanmore

The oxidation of soot: a review of experiments, mechanisms and models

Abstract Soot may be formed when carbonaceous fuels are burned under local reducing conditions. Its subsequent oxidation is of great significance for pollution control in industrial flames, auto engines etc. Oxidation (gasification) can be achieved with oxygen, carbon dioxide, water vapour or nitrogen dioxide. In this review, the experimental techniques which have been used to study the gasification of soot are described and the methods and results obtained by analysis of the data from them are considered. Firstly, the mechanism of soot formation and its structure are briefly discussed. The various scales of particulate which comprise it, i.e. spherule, particle and aggregate, influence its properties and behaviour. Next, the experimental equipment used in the study of its gasification is briefly described. Gasification kinetics at low temperatures are measured either in fixed beds or by thermogravimetry. The apparatus may be operated as a thermally programmed desorption system to identify the species involved. High temperature investigations have been carried out in entrainment burners and shock tubes. The chemistry of soot oxidation is discussed for both non-catalytic and catalytic conditions. The oxidation pathway involves interaction between adsorption and desorption processes, which determine the primary products, the order of reaction and the activation energy. The concensus is that two types of adsorbed surface species are present in uncatalysed combustion. The combustion mechanism of individual spherules is considered in terms of basic property changes. During thermogravimetry, the influence of the competition between reaction and oxygen diffusion in soot beds is analysed. The reaction of catalysed soot displays a different mechanism, as the primary products, the order of reaction and the activation energy all change. The lower activation energy and higher reactivity lead to lower ignition temperatures. Catalysts may be incorporated into the soot spherules by addition to the fuel, or may be added after formation. Two types of contact between the carbon and added catalyst have been identified, ‘loose’ and ‘tight’. Tight catalyst, which has been mechanically ground with the soot, produces more pronounced effects. Finally, the behaviour of soot during gasification by other oxidants, namely H2O, CO2 and NO2 is summarised.

The fate of heavy metals during combustion and gasification of contaminated biomass: a brief review

The literature on the presence of heavy metals in contaminated wastes is reviewed. Various categories of materials produced from domestic and industrial activities are included, but municipal solid waste, which is a more complex material, is excluded. This review considers among the most abundant the following materials - wood waste including demolition wood, phytoremediation scavengers and chromated copper arsenate (CCA) timber, sludges including de-inking sludge and sewage sludge, chicken litter and spent pot liner. The partitioning of the metals in the ashes after combustion or gasification follows conventional behaviour, with most metals retained, and higher concentrations in the finer sizes due to vaporisation and recondensation. The alkali metals have been shown to catalyse the biomass conversion, particularly lithium and potassium, although other metals are active to a lesser extent. The most prevalent in biomass is potassium, which is not only inherently active, but volatilises to become finely distributed throughout the char mass. Because the metals are predominantly found in the ash, the effectiveness of their removal depends on the efficiency of the collection of particulates. The potential for disposal into soil depends on the initial concentration in the feed material.

A review of NOx reduction on zeolitic catalysts under diesel exhaust conditions

The three-way catalysts currently used for controlling NOx gaseous pollutants from IC engines cannot be used under lean conditions such as diesel engine exhausts. Approaches to NOx reduction have therefore focused on direct decomposition of NO to its elements or selective catalytic reduction with hydrocarbons to N2. Metal-doped ZSM-5 catalysts are active in these reactions, with Cu-ZSM-5 receiving most attention. A survey of the literature regarding the performance, reaction mechanism and durability of these catalysts under engine conditions is presented. Although copper-based ZSM-5 has adequate catalytic properties, there remain doubts about its ability to withstand diesel exhaust conditions, since they deactivate at high temperature, due to dealumination or/and Cu migration.

Modeling NOx release from a single coal particle. I. Formation of NO from volatile nitrogen

Abstract For a single spherical coal particle, modeling has been undertaken of the processes of release of volatile fuel-nitrogen, and its subsequent conversion to nitric oxide during the devolatilization. The combustion conditions used were similar to those found in utility boilers. A finite volume numerical scheme was used to solve the equations of mass, species conservation, momentum and heat transfer about the devolatilizing particle. Particles in the size range 10–100 μm were modeled with gas temperatures from 1250 to 1900 K. Devolatilization was described by two competing reactions; results are compared with other single particle models and experimental results reported in the literature. The influence of coal rank was examined by using a lignite, a sub-bituminous and a high volatile bituminous coal. The conversion efficiencies of volatile fuel-nitrogen to nitric oxide are presented. It was found that there can be a significant conversion of the volatile fuel-nitrogen to nitric oxide before the liberated volatiles have reached the oxygen supply in the bulk gas. Conversion efficiencies of volatile nitrogen to nitric oxide as high as 54% are predicted for conditions of high gas temperature and high oxygen concentrations. For small particles (≈ 10 μm), most of the fuel-nitrogen is released as hydrogen cyanide (up to 70%) prior to mixing with the bulk gas.

The ignition and combustion of Cerium-doped diesel soot

Particulates (soot) were sampled from the regenerative trap of an automotive diesel engine run under three speed/load conditions. The fuel was doped with a cerium-based catalyst to promote oxidation of the soot bed. The soots were subjected to combustion testing in a DTG under both temperature ramping and isothermal conditions, and under temperature ramping in a small fixed bed. The combustion gas was 10% oxygen in nitrogen, which was supplemented in the case of the fixed bed with other gases found in diesel exhausts. The temperatures of initiation of combustion Tin were measured in the DTG and in the fixed bed. The kinetic rates of oxidation were calculated from the DTG results taking into account the influence of oxygen transport. The effect of gas composition on ignition and burnout in the fixed bed was determined. Ignition did not occur in the DTG, but rather burning progressed steadily. There was a large increase in the kinetic rate constants with cerium present, but the activation energy was unchanged. There was no difference in rates between the two cerium additives. No consistent differences were observed between the combustion kinetics of soots generated under different engine conditions. At temperatures above 600°C a decline in the catalytic effect of the doped samples appeared. In the fixed bed the catalysed samples tended to partly burn and then ignite and be totally consumed. The appearance of ignition was erratic with the addition of minor gases to the combustion gas. A simple combustion and heat transfer model was able to predict ignition temperatures. Loss of adsorbed hydrocarbons takes place before bed ignition.

A comparative study of carbon black and diesel soot reactivity in the temperature range 500–600°C—effect of additives

Abstract The oxidation reactivities of some soots generated in a small diesel engine were measured by thermogravimetry at 500°C and 600°C. The materials tested included carbon black as a reference carbon and some soots that contained iron and copper resulting from the use of diesel fuel additives. In all cases, mass transfer effects were present and the true reactivities had to be evaluated by their elimination. This was achieved by means of a one-dimensional model of oxygen transfer and by extrapolation of data from different bed sizes. Agreement between the two methods was good. Reliable values of the effective diffusion coefficients within particle beds are essential for successful modelling. The relative reaction rates between samples change markedly when mass transfer effects are eliminated. The presence of the metals increases the reactivity of soots, with copper producing an enhancement of 470% at 600°C.

Experimental and theoretical study of oxygen diffusion within packed beds of carbon black

Abstract Modelling the performance of regenerative soot traps for Diesel exhausts requires a knowledge of oxygen diffusivity within beds of Diesel soot. For convenience carbon black, a commercial product with similar properties to Diesel soot, is often used for combustion experiments. Both of these carbonaceous materials are composed of spherules of approximately 20 nm in diameter assembled within aggregates. The oxygen diffusivity within beds of carbon black with different densities was measured at 22°C. An increase in the porosity of the bed from 0.746 to 0.820 increased the diffusion coefficient of oxygen from 0.7×10 −6 to 2.0×10 −6 m 2 s −1 . Two simulations of molecular movement in aggregates of spheres or ‘cannonball’ solids were performed. The kinetic theory (KT) model assumes that the carbon spherules are regularly placed at the summits of cubes and a mean travel distance replaces the mean free path. The Monte Carlo (MC) model is based on a random walk among spherules placed randomly but homogeneously. The diffusivity values returned by the two models are strongly dependent on the tortuosity of the bed. The KT model returned oxygen diffusivities that were similar to those measured while the MC model gave values which were higher. When the MC values were modified to allow for non-homogeneous packing, i.e., uneven distribution of density, the results were improved. The concept of tortuosity, which is based on interconnected cylindrical pores, seems unrealistic in the case of beds of highly porous cannonball materials. Beds of Diesel soot are so open that Fickian diffusion of oxygen should occur, whereas combined Fickian and Knudsen diffusion seems to operate within the beds of carbon black. The mechanisms are uncertain since Knudsen diffusion within aggregates of carbon black or Diesel soot probably controls the overall diffusion process.

Experimental studies of ignition behaviour and combustion reactivity of pulverized fuel particles

A pulse ignition technique, whereby a small mass of fine coal or char particles is fed into a drop-tube furnace, was used to evaluate the ignition characteristics of pulverized fuel particles. Combustion rates were estimated from the ignition temperatures and were compared with rates determined experimentally using a fixed-bed reactor, DTG, and an entrained-flow reactor. Three Australian coals, two chars prepared from these coals and a petroleum coke were used. Coal rank and particle size were found to influence the ignition temperatures of the coal and char particles. However, these effects were reduced at high oxygen concentrations. In pure oxygen, the ignition temperatures of the chars and their parent coals were similar. Heterogeneous and homogeneous ignition theories indicated that at low O2 concentrations ignition was controlled by a homogeneous mechanism, whereas at high O2 concentrations heterogeneous ignition became dominant. The combustion rates of the chars determined by the four techniques were compared at an oxygen partial pressure of 10.1 kPa. The results of the fixed-bed, DTG and entrained flow experiments were consistent with each other and spanned the range of kinetic control from regime I at low temperature (fixed bed) to regime II at high temperature (entrained flow). The drop-tube experiments indicated consistently higher combustion rates than did the other techniques. The pulse ignition measurements therefore, while providing a valid means of characterizing coal and char ignition behaviour, result in overestimation of the char combustion rate.

The combustion characteristics of char from pulverized bagasse

A 90-125 μm sample of pulverized bagasse was charred in an ASTM volatile matter test, and the char combustion kinetics were evaluated by DTG. In the temperature range 450-550 °C, the combustion reaction occurred in regime II, with a Thiele modulus ranging from 470 to 3100. The activation energy was 180 kJ mol-1 and the reaction order with respect to oxygen was 0.65. These kinetic parameters are similar to those of low-rank coal.

Geometric effects on mass transfer during thermogravimetric analysis: Application to reactivity of diesel soot

The measurement of soot reactivity by thermogravimetric analysis is complicated by oxygen mass transfer limitations. The large surface area of the solid imposes local oxygen depletion, even with very small bed masses. An analytical solution has been developed to the equations describing flow above, and reaction within, a bed of soot contained within a rectangular crucible. Solutions are derived for a two-dimensional situation where the sides of the bed are either open to, or shielded from oxygen. The output of the model is modified by incorporating external mass transfer rates, evaluated for a number of bed configurations using the CFD package FLUENT. The influences of bed mass, the proximity of crucible walls and of heat release from the exothermic reaction are considered. It was found that the mass transfer coefficient is independent of sample mass and only slightly influenced by the walls. The depth of the soot bed is the most sensitive variable determining the derived reactivity. For satisfactory results, a small mass of sample carefully loaded into the crucible is recommended. New values for the reactivity of a Diesel soot previously examined are presented for the temperature range 600°-800°C.