Bernard M. Gibbs

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.

Low-temperature oxidation of single and blended coals

The oxidation of different types of coal under self-heating conditions is considered using the crossing point method performed in a cubical wire-mesh basket. This study investigates the effect of particle size and the physical structure of coals (including pore size and surface area) on the self-heating character of high and low rank Indonesian coals and their blends. The results confirm that both particle size and surface area give considerably different effects on critical ambient temperature, the activation energy and the product of exothermicity and the pre-exponential factor of low and high rank coals. It was found that the self-heating characteristics of high rank coals were strongly dependent upon the particle size of the coals. For low rank coals, the dependence was weaker. A coal bed with mixed sizes is thus more vulnerable than one with segregated sizes. This means that one must carefully consider the particle size distribution to judge the spontaneous ignition behaviour of coal. It is suggested that the potential for spontaneous combustion of blended coal is greater following the increased percentage of more reactive coal.

Modeling NH3 and HCN emissions from biomass circulating fluidized bed gasifiers

Abstract A circulating fluidized bed biomass gasification model is developed in the present study. The model consists of sub-models for devolatilization, tar cracking and a chemical reaction network of main gasification reactions and nitrogen chemistry. A total of forty global chemical reactions are included in the model, of which twenty-eight reactions belong to fuel-nitrogen reaction network. Individual reaction rates are selected from the literature, wherever possible, based on studies of woody biomass fuels. Volatile nitrogen is assumed to consist of NH3, HCN and N2 with the distribution between three species as input parameters to the model. Modeling of the hydrodynamics of the riser is simplified by using solids concentration profile along the riser as an input to the model. Both gaseous phase and solids phase are assumed to be in plug flow. Modeling results are compared with the experimental results published in the literature. Predicted effects of bed temperature, equivalence ratio and fuel moisture content on main gaseous composition, tar and NH3 emissions generally agree with the literature data. A sensitivity analysis of some reaction rates included in the model on NH3 emissions has also been carried out. It has been revealed that the catalytic activity of bed materials towards the oxidation of NH3 has the greatest influence on the predicted NH3 emissions. In addition, the predicted NH3 emissions are also affected by the catalytic activity of bed materials towards the decomposition of NH3 and the homogenous reaction rates of NH3 decomposition and the reduction of NO by NH3 in the presence of oxygen.

Influence of operating parameters on NOx and N2O axial profiles in a circulating fluidized bed combustor

Abstract This paper reports an experimental investigation of the influence of operating parameters on the formation and reduction of NO x and N 2 O from coal combustion, carried out in a circulating fluidized bed combustor 6.2 m high and 0.161 m i.d. The main operating parameters studied were temperature, excess air factor, secondary/total air ratio, limestone addition and coal particle size. It was found that the NO x emission increased whereas the N 2 O emission decreased when the temperature increased, but the effect of the temperature depended on the excess air factor. Both the NO x and the N 2 O emissions increased with increasing excess air factor and decreased with increasing secondary/total air ratio. Moreover, the NO x emission increased and the N 2 O emission decreased with limestone addition and with increasing coal particle size. Unlike previous studies in which only the exhaust gases were the focus, here an attempt was made to analyse the NO x and N 2 O axial profiles to improve understanding of the formation and destruction of these pollutants. Analyses of the NO x and N 2 O concentration profiles along the riser height suggested that the NO x formed in the bottom was gradually reduced along the height of the combustion chamber and that the N 2 O concentration increased from the bottom to the top of the combustion chamber for all the operating conditions.

Evaluation of the optimal fuel characteristics for efficient NO reduction by coal reburning

Thirteen coals and a char, with volatile matter ranging from 7.9 to 50.4 wt% (daf) and nitrogen content from 1.2 to 2.1 wt% (daf), were evaluated as reburning fuels in an isothermal drop-tube reactor system. The NO reduction efficiency by coal reburning decreased with increasing primary-zone or secondary (reburning)-zone airfuel stoichiometry. The NO reduction efficiency was also nearly proportional to the extent of carbon burnout in the reburning zone and hence to coal reactivity, and generally increased with decreasing coal rank. With a high primary-zone NO concentration (∼650 ppmv) and reburning-zone fuel-rich conditions, the NO reduction efficiency could be improved by increasing the rate of input of reburning-coal volatile nitrogen into the reburning zone. A comparison of the reburning performance of a partly devolatilized char with that of the parent high-volatile bituminous coal showed that the parent coal always achieved a higher NO reduction than the char. A mechanistic model is proposal to determine the relative contributions of volatiles and char to the total NO reduction observed.

The influence of calcined limestone on NOx and N2O emissions from char combustion in fluidized bed combustors

Limestone addition to a coal-fired fluidized bed combustor usually results in an increase in NO/NOx emissions and a decrease in N2O emissions. This observation has been explained in the literature by the effect of limestone on: (i) the conversion of coal volatile-N to NO/NOx and N2O; (ii) the decomposition of N2O; and (iii) the pool of H, OH and/or O radicals due to sulphur capture. Since char-N is also an important source of NO/NOx and N2O under fluidized bed combustion conditions, any effect of calcined limestone on the conversion of char-N to NO/NOx and N2O can also contribute to the above observation. The effect of calcined limestone on the conversion of volatile-N species, i.e. NH3 and HCN has been well documented. However, so far few studies have studied the effect of limestone on the conversion of char-N under fluidized bed combustion conditions. In this study, a series of batch-type char combustion tests was carried out in a bench-scale bubbling fluidized bed combustor with different bed materials. These tests reveal that in addition to reducing N2O emissions from char combustion, calcined limestone also enhances the conversion of char-N to NO/NOx similar to the case of volatile-N. Possible reaction mechanisms to explain the experimental results of char combustion tests have been discussed.

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.

Using the crossing point method to assess the self-heating behavior of indonesian coals

Indonesian coal, although generally low in sulfur and ash content, is a lower rank coal with a relatively high moisture content (5–18%). Consequently, it has a greater propensity to self-heating behavior, and the aim of this paper is to assess the hazard for three typical coals that are used both domestically and for export. Experimental tests are reported using a new technique (the crossing point method) to determine the activation energy and reactivity of the low-temperature slow oxidation reaction. This uses the transient temperature profiles to deduce the activation energy and reactivity of the sample. The method relies on finding the center temperature at the point when a flat profile is observed (Called the crossing temperature) and has been used successfully to ascertain the kinetic parameters of three Indonesian coal samples. It is found that the method is valid as long as the coal experiment is not greatly supercritical when it is then difficult to determine an appropriate crossing temperature. The method has the distinct advantage over the traditional Frank-Kamanetskii approach, in that only one oven experiment is needed for any one point on the kinetic plot, and that the results are not heavily dependent on the fan setting of the oven, or on sample size. A comparable experiment using the Frank-Kamanetskii approach of finding the critical temperature for different sample sizes showed good agreement for estimating activation energy.

Modelling of circulating fluidized bed combustion of coal

Abstract In the proposed model, the hydrodynamics of a circulating fluidized bed (CFB) are described by an empirical correlation deduced from cold model experimental data. Volatile matter is assumed to be released instantaneously from coal on entry into the furnace, and is assumed to burn uniformly throughout the riser. The combustion of residual char particles is assumed to begin with oxygen diffusion to the burning char particle where the following reaction occurs: C + 1 2 O 2 → CO . The carbon monoxide produced is then oxidized by oxygen to produce carbon dioxide according to reaction: CO + 1 2 O 2 → CO 2 . A uniform gas temperature is assumed throughout the riser, whereas the temperature of a burning char particle is calculated from heat balances on the char particle. Agreement between experimental data and model prediction is encouraging.

On the prediction of thermal runaway of coal piles of differing dimension by using a correlation between heat release and activation energy

Greater use of international coal and low-sulfur/low-rank coals brings new challenges in monitoring the spontaneous combustion behavior of these coals during transport in stockpiles, and in coal milling systems Extensive experimental work has been carried out to measure the kinetic parameters of low-temperature oxidation of a number of coals for varying particle fractions. A substantial set of experimentally measured values of activation energy, E a , and the product of the exothermicity and the pre-exponential factor, QA , have indicated a strong relationship, which is shared by coals of different ranks and of different particle size. This correlation for E a and QA leads to a new approach for estimating the thermal runaway behavior of coal piles based on small-scale experiments. By use of Frank-Kamenetskii theory, the correlation curve fit of E a versus QA can be used to estimate hazardous conditions at varying scales by small-scale oven heating tests measuring the heat release rate at low temperatures. The results suggest that the increasing size of the pile alters the particle size effect. For large coal piles, large particle size can increase the potential for spontaneous combustion. This new approach provides a more cost-effective assessment of the susceptibility of various types of coal to self-ignition.