Bituminous Coal Char Research Articles

Liquid-Phase Adsorption of Phenol onto Activated Carbons Prepared with Different Activation Levels

The influence of the pore size distribution of activated carbon on the adsorption of phenol from aqueous solutions was explored. Activated carbons with different porous structures were prepared by gasifying a bituminous coal char to different extents of burn-off. The results of adsorption experiments show that the phenol capacity of these carbons does not proportionally increase with their BET surface area. This reflects the heterogeneity of the carbon surface for adsorption. The pore size distributions of these carbons, determined according to the Dubinin–Stoeckli equation, were found to vary with the burn-off level. By incorporating the distribution with the Dubinin–Radushkevich equation using an inverse proportionality between the micropore size and the adsorption energy, the isotherms for the adsorption of phenol onto these carbons can be well predicted. The present study has demonstrated that the heterogeneity of carbon surface for the phenol adsorption can be attributed to the different energies required for adsorption in different-size micropores.

Modelling of a pressurised entrained flow coal gasifier: the effect of reaction kinetics and char structure

A model for a pressurised entrained flow coal gasifier is presented, with the effect of pressure, reaction kinetics and char structure on the gasification reactions being outlined. A sensitivity analysis to reaction kinetics and char structure was performed, and model predictions are compared with published atmospheric and high-pressure gasification data. It was found that both reaction kinetics and char structure are important in predicting coal gasification. The initial surface area may be more significant than intrinsic reactivity for bituminous coal chars. Low-pressure gasification kinetics (i.e. pressure order) cannot be extrapolated to high-pressure conditions. A significant difference in predicted carbon conversion was observed between various char structural models. It is suggested that the random pore model gives a reasonable prediction. Volatile matter has significant effect on carbon conversion due to the formation of high-surface area char particles. Increasing vitrinite content in coal correlates with increased carbon conversion. The model predictions show reasonable agreement with published experimental data on both atmospheric and high-pressure gasifiers in terms of carbon conversion and gaseous product composition. Comparisons with previous models and sensitivity analyses suggest that it is necessary to include the effect of char structure, and more sophisticated reaction kinetics than single order rates are required when modelling coal gasification.

00/02278 The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically controlled conditions

00/02278 The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically controlled conditions

The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically controlled conditions

Bituminous chars with a strongly non-uniform porous structure have not been accounted for in existing pore models used to predict reactivity. A new approach has been developed to predict a particle structure parameter used in the random pore model. The reaction rate has been correlated with the carbon burn-off. Six bituminous coal chars were produced by pyrolysis in a pressurised drop tube furnace and their reaction rates were measured under various conditions using a pressurised thermogravimetric analyser. The char morphology and the porous structure of the char particles were obtained using image analysis. The porous structure of char particles was also characterised using mercury porosimetry. It is shown that after high-pressure pyrolysis, most of the char particles are of a high porosity, low surface area and a non-uniform porous structure with a large central void surrounded by a thin shell which will influence apparent char reactivity and ash formation. Image analysis also confirmed that the maceral composition of the coals and the pyrolysis pressure influence char morphology. A maximum reaction rate for each char was observed at a carbon conversion of 10–20 wt% during both char combustion and gasification. The particle structure parameters were predicted based on the measured porosity and surface area of macropores and micropores in the particle, and show reasonable agreement with those derived from experimental measurements. The predicted reaction rate agrees well with experimental measurements obtained under various conditions.

00/00663 The effect of the textural properties of bituminous coal chars on NO emissions

00/00663 The effect of the textural properties of bituminous coal chars on NO emissions

Effective diffusion coefficients in coal chars

Knowledge of effective diffusion coefficients in char particles is important when interpreting experimental reactivity measurements and modeling char combustion or NO and N 2 O reduction. In this work. NO and N 2 O reaction with a bituminous coal char was studied in a fixed-bed quartz glass reactor. Particle sizes in the range 0.05-5 mm were tested, and the effective diffusion coefficients were estimated from measured effectiveness factors using the Thiele modulus. At 1079 K the effective diffusion coefficients were 5.5×10 −6 m 2 /s and 6.8×10 −6 m 2 /s for N 2 O and NO, respectively. The experimental results were compared with theoretical values calculated from the mean pore radius and the cross-linked pore model. The method of mean pore radius underestimated the effective diffusion coefficient more than an order of magnitude. Using the cross-linked pore model, the bimodal pore size distribution, and a tortuosity factor of 5, a complete agreement between the experimental and the theoretical value was found. The conclusion was that for a char with a wide pore size distribution, the cross-linked pore model is a good choice for a theoretical calculation of the effective diffusion coefficient. In the case of strong pore diffusion limitations, the error in the interpretation of experimental results using the mean pore radius could be a factor of 5 on the intrinsic rate constant. For an average coal char reacting with oxygen at 1300 K, this would be the case for particle sizes larger than about 50 μm.

Interactions of gaseous no with char during the low-temperature oxidation of coal chars

The role of nitric oxide (NO) during the low-temperature oxidation (873 K, ca. 2% O2/He) of subbituminous and bituminous coal chars was studied experimentally in fixed-bed reactor systems. Analysis of the gas-phase products by gas chromatography (GC) and Fourier transform infrared spectroscopy (FTIR) identified nitrogenous products with typical abundance N2(54%), NO (22%), HNCO (12%), HCN (6%), and N2O (1%). Significant retention of char nitrogen (char N) occurred within the char during the early stages of burnout. This may be explained via the following mechanism: NO + 2C() → C(N) + C(O) (1) C(N) + NO → N2 + C(O) or CO (2) C(N) + O2 → NO + C(O) or CO (3) The observed extent of nitrogen enrichment of the char during burnout requires that up to 90% of the NO formed by oxidation of char N (3) is reincorporated into the char (1) in the burnout. Experiments also were conducted using a dispersed-bed reactor in which isotopically labeled 15NO (0–1000 ppm) was added to the reactant gases (2% O2 in He) during the oxidation of coal char. The concentrations of labeled HC15N and H15NCO that were observed by FTIR indicated a significant interaction between gaseous NO and the char surface. The ratios of isotopes present in the HCN and HNCO product distributions were similar, suggesting that these species may be formed by related processes.

Experimental investigation of no from pulverized char combustion

NO formation and reduction during pulverized char combustion in the temperature range 850–1150°C have been investigated in fixed-bed combustion experiments. Chars from a high-volatile bituminous coal and an anthracite have been used. Under single-particle conditions the selectivity for NO formation from combustion of char from both fuel types lies in the range 65–100%. The NO formation selectivity under single-particle conditions was observed to be lowest at 850 °C, to have values close to 100% at 1050 and 1150 °C, and to be independent of O 2 concentration. When conditions deviate from single-particle conditions, net NO formation is significantly lower due to NO reduction taking place simultaneously with NO formation. Rate expressions for NO reduction on char both in the presence and in the absence of O 2 have been determined. For bituminous coal char, these rates are 10–100 times more rapid than values previously reported in literature, but are consistent with reburn-type experiments employing char as fuel. This discrepancy is mainly attributed to rapid char deactivation prior to measuring of NO reduction rates in previous determinations. Shortly after pyrolysis, the effective NO-char reaction rate for pulverized bituminous coal char in the temperature range 850–1150 °C has been found to be given by r NO = 6 ⋅ 10 6 m 3 kg C ⋅s ⋅ e 14800 / T ( K ) ⋅ [ char C ] ⋅ [ NO ]

Expressing nth Order Char-Oxygen Reaction Initial Rate under Elevated Pressure.

Char combustion is investigated in the temperature range of 559–1273 K and pressure range of 0.1–1.6 MPa in a 0.5th-order reaction (a lignite coal char) and a first-order reaction (a bituminous coal char).The intrinsic combustion constant kv and the reaction order n do not vary with pressure, but the char combustion initial rate varies with the pressure in the chemical kinetics control and the internal diffusion influence regimes. The variation of initial rate with pressure was also sensitized to the reaction order n. In the chemical kinetics control regime, the char combustion initial rate was proportional to Pn with the exponent n, the reaction order. In the internal diffusion influence regime, the apparent dependency of char combustion initial rate on pressure changed with the exponent range from n to n/2. The reaction initial rate in the external diffusion control regime was invariant with pressure. A rate expression was developed for predicting the initial rate of nth-order char combustion under elevated pressure.

Porosity Development during CO2 and Steam Activation in a Fluidized Bed Reactor

Steam- and CO2-activated carbons from a Spanish HV bituminous coal char were obtained at 800−900 °C in a batch laboratory scale fluidized bed reactor. The N2 and CO2 adsorption isotherms and scanning electron microscopy allow for sample characterization. The development of porosity upon activation, using a fluidized bed reactor, is different depending on the activating agent. A much better activation development is attained with steam. Thus, total micropore volume linearly increases with the activation degree, attaining an apparent specific surface area of 1150 m2/g at 50% burnoff for steam-activated carbons. Although CO2 activation also produces a linear increase vs burnoff, much lower pore volume values are obtained, reaching an equivalent specific surface area of only 800 m2/g at 50% burnoff. SEM analysis indicates that CO2 produces external burning of the carbon particles. This different behavior of CO2 and steam activation was also observed for several runs carried out with almond shell char. The exp...

The effect of the textural properties of bituminous coal chars on NO emissions

NO is the primary product of the oxidation of char nitrogen, and in some combustion processes the NO can be reduced on the char surface to give N 2O and/or N 2. In this study a range of bituminous coals (low, medium and high volatile matter content) were pyrolysed in a fixed bed reactor at various heating rates. Textural characterisation was carried out by measuring true (He) and apparent (Hg) densities and N 2 (−196°C) and CO 2 (0°C) adsorption isotherms. Pore volume distributions and surface areas were obtained for the chars studied. A thermogravimetric analyser coupled to a quadrupole mass spectrometer (TG-MS) was used to study the combustion behaviour of the samples and the nitrogen compounds evolved during temperature-programmed combustion. Results are discussed in terms of the influence of both textural properties and reactivity on NO emissions and on the heterogeneous reduction of NO.

Assessment of Thermodeactivation during Gasification of a Bituminous Coal Char

The annealing-induced loss of gasification reactivity of a bituminous coal char with respect to carbon dioxide was studied by means of a combination of experimental techniques. Heat treatment of coal samples under inert atmosphere for different time intervals was accomplished at temperatures as high as 2000 °C by means of thermogravimetric analyzers and of a specially designed heated-strip reactor. Gasification reactivities of untreated and heat-treated coal samples were determined by isothermal thermogravimetric analysis in carbon dioxide atmospheres. The influence of the time−temperature history of the coal sample on the parameters of the apparent gasification kinetics (preexponential factor, activation energy) has been assessed. Results were further analyzed in the framework of a simple annealing kinetic model suitable for implementation in gasification modeling.

Reduction of NO with NH 3 over carbon catalysts – the influence of carbon surface structures and the global kinetics

The reduction of NO by NH 3 over carbon catalysts was studied at temperatures in the range 140–320°C, and at NO and NH 3 concentrations varying within 290–1160 ppm. The carbon catalysts of different surface areas were prepared by gasifying bituminous and sub-bituminous coal chars in CO 2 to different extents of burn-off. The NO reduction rate was found to increase with the carbon surface area, which is an increasing function of the burn-off level. However, the activity per unit surface area of the carbons decreases with the extent of burn-off. By adding oxygen in the reacting system, the NO conversion can be significantly enhanced. The apparent activation energy was found to be around 20 kJ/mol for the reactions with or without oxygen addition. The apparent reaction orders with respect to NO and NH 3 are functions of the reaction temperature and are affected by the presence of oxygen. It has been suggested on the basis of the kinetic study that the catalytic reduction of NO might be governed by two consecutive steps: the adsorption of NH 3 on active sites, followed by the addition of NO or NO 2 in the neighborhood of the C(NH 3) complexes.

Percolative fragmentation and spontaneous agglomeration

Captive particle imaging experiments were performed on over 200 coal and char particles in the pulverized size range from four coals of various rank at oxygen concentration from 3–19 mol-% and at gas temperatures of about 1250 K. Despite wide variations in single-particle behavior, the data set reveals two clear trends that provide new information on the nature of char combustion. First, the low-rank coal chars are observed to maintain their high reactivity through the late stages of combustion, thus avoiding the near-extinction events and long burnout tails observed for bituminous coal chars. Secondly, percolative fragmentation in the late stages of combustion is a rare event under these conditions. Some particles reach a percolation threshold late in combustion, but typically undergo spontaneous agglomeration rather than liberation of the incipient fragments. It is concluded that percolative fragmentation behavior in the pulverized size range is determined not only by solid-phase connectivity, but also by a real competition between disruptive and cohesive forces present at the time of formation of the colloidal-sized incipient fragments.

97/02795 Adsorption of SO 2 on bituminous coal char and activated carbon fiber

97/02795 Adsorption of SO 2 on bituminous coal char and activated carbon fiber

Adsorption of SO2 on Bituminous Coal Char and Activated Carbon Fiber

The SO2 adsorption behaviors of activated carbons produced from Illinois coal and of commercially prepared activated carbon fibers (ACFs) were compared. There was no relation between surface area of coal-based carbons and SO2 adsorption, whereas adsorption of SO2 on the series of ACFs was inversely proportional to N2 BET surface area. Higher surface area ACFs had wider pores and adsorbed less SO2; thus, pore size distribution is thought to play a significant role in SO2 adsorption for these materials. Oxidation with HNO3 and/or H2SO4, followed by heat treatment at 700−925 °C to remove carbon−oxygen complexes, resulted in increased SO2 adsorption for both coal chars and ACFs. This behavior was explained by an increase in the available number of free sites, previously occupied by oxygen and now available for SO2 adsorption. The use of nitrogen-containing functional groups on ACFs of proper pore size shows promise for further increasing SO2 adsorption capacities. Knowledge of the relationship among the number of free sites, pore size, and surface chemistry on corresponding SO2 adsorption should lead to the development of more efficient adsorbents prepared from either coal or ACFs.

97/01353 Chlorinated organic compounds evolved during the combustion of blends of refuse-derived fuels and coals

Chemical-looping combustion (CLC) is an innovative carbon-capture technology with potential to drastically reduce the cost of capture. By using a circulating bed material to transfer oxygen from the combustion air to the fuel, air and fuel are never mixed and the CO2 can be obtained as a separate flue gas stream, undiluted by N2. In other words, carbon capture is inherent to the CLC process. Here, we present findings from a 100 kW chemical-looping combustor for solid fuels. The oxygen carrier used in the present tests was an iron ore from Tierga, Spain. In total, 26 h of operation using bituminous coal and wood char as fuel was achieved. The highest gas conversion with bituminous coal was 87%, and the highest gas conversion using wood char as fuel was 93%. The carbon capture efficiency with bituminous coal was 94–98%, which is lower than what has been observed with ilmenite. For the wood char, the carbon capture was even lower. The fate of nitrogen and sulfur was investigated. It was found that 84 wt% of the S-containing gas was SO2, and only 16 wt% was H2S. The nitrogen analysis indicates that the fuel-N converted to gas was distributed as 10 wt% NO, 37 wt% NH3, and 53 wt% N2. It was found that the oxygen-carrier particles retained their high reactivity throughout the operational period. Furthermore, the expected lifetime of the iron ore was found to be approximately 300 h.

96/05316 Textural aspects of selected high volatile bituminous coal chars

96/05316 Textural aspects of selected high volatile bituminous coal chars

Effect of heterogeneous mechanisms during reburning of nitrogen oxide

AbstractNO reduction through heterogeneous mechanisms is a major concern when coal or lignite is used as reburning fuel. Reburning was simulated in an alumina flow reactor operated with a synthetic flue gas at a stoichiometric ratio of 1.1, at 1,100°C and with a residence time of approximately 0.2 s. Reburning fuels used include methane, one bituminous coal, two lignites, chars derived from the bituminous coal and two lignites, and the bituminous coal char impregnated with CaO. Reburning with chars indicates that heterogeneous mechanisms are more important than homogeneous mechanisms when lignites are used. This high reactivity renders lignite char an even more effective reburning fuel than the parent lignite, while char from the bituminous coal produces little activities at stoichiometric ratios above 0.75. Bituminous coal char impregnated with CaO demonstrates higher NO reduction efficiency than the original char. Effects of SR on NO reductions imply that oxygen atoms, required for the NO and HCN conversions in homogeneous mechanisms, are not required in heterogeneous mechanisms. In fact, they inhibit the surface NO reduction. The first‐level kinetic analysis indicates that the surface area is not the only contributor to a remarkable increase in NO reduction of lignite char, and chars of different origins seem to follow different mechanisms or to have different controlling steps in the overall reaction scheme.

The influence of pressure on char combustion kinetics

Combustion experiments in a fixed-bed reactor were performed at 0.2, 0.6, and 1.0 MPa for temperatures between 850 and 1200 K with Westerholt bituminous coal char sized in the range 90–106 μ m. The rate constants were first determined with the so-called “ n th-order” overall expression of the combustion rate. Then, a simplified heterogeneous reaction mechanism including a two-stage adsorption-desorption process was proposed to understand the influence of the total pressure on the adsorption and desorption reactions. Using the technique of sudden removal of oxygen, it was found that the total pressure has no effect on the desorption rate constants. Because the combustion is controlled by the adsorption of oxygen, the adsorption rate constant appeared very similar to the overall rate constant. Increasing total pressure up to 0.6 MPa was found to decrease the rate constants if the oxygen “concentration” is expressed as the oxygen partial pressure ( r=k P O2 n ). If the oxygen concentration is expressed as the oxygen mole fraction ( r=k y O 2 n ) there is only a weak effect of the total pressure on the rate constant. Another effect of a pressure increase is to decrease the transition temperature from a kinetics-controlled regime to a regime co-controlled by pore diffusion. To clarify the effect of pressure on combustion, a Langmuir adsorption isotherm was chosen and the adsorption constant was determined at 0.2, 0.6, and 1.0 MPa. It was then found that the Langmuir adsorption isotherm is particularly suitable when working at elevated pressure.