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[EN]The adsorption isotherms of nitrogen at 77 K have been determined on two series of activated carbons and two samples of activated carbon fibres. The carbons were highly microporous and contained varying amounts of different forms of microporosity. Characterization of the carbons was undertaken using an alternative linear form of the BET equation and the results of the BET parameters obtained have been compared with those obtained from the usual classical linear form of the equation. The surface areas calculated from the two linear forms were similar while the C values were quite different. The C values obtained from the alternative linear form appeared to be more reasonable, being consistent with and related to the microporous character of these carbons. The alternative linear form gave BET plots which were more sensitive to deviations from linearity allowing the relative pressure range for the application of the BET equation to be determined without any ambiguity and more precisely. ; The authors thank DGICYT (Project PB90-0067) and FICYT for financial support. R. C. Bansal is grateful to the Ministerio de Educación y Ciencia, Government of Spain for a financial grant for his stay at the Instituto Nacional del Carb6n in Oviedo, Spain. J.C. de Sousa also thanks the Univ. Est. de Maringa and CNPq (Brazil) for a grant. ; Peer reviewed

This paper presents an experimental and numerical study on co-firing olive waste (0, 10%, 20% on mass basis) with two coals in an entrained flow reactor under three oxy-fuel conditions (21%O2/79%CO2, 30%O2/70%CO2 and 35%O2/65%CO2) and air–fuel condition. Co-firing biomass with coal was found to have favourable synergy effects in all the cases: it significantly improves the burnout and remarkably lowers NOx emissions. The reduced peak temperatures during co-firing can also help to mitigate deposition formation in real furnaces. Co-firing CO2-neutral biomass with coals under oxy-fuel conditions can achieve a below-zero CO2 emission if the released CO2 is captured and sequestered. The model-predicted burnout and gaseous emissions were compared against the experimental results. A very good agreement was observed, the differences in a range of ± 5–10% of the experimental values, which indicates the model can be used to aid in design and optimization of large-scale biomass co-firing under oxy-fuel conditions. ; L.A. acknowledges funding from the CSIC JAE program, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program). Financial support from the CSIC (Project PIE 201080E09) is gratefully acknowledged. ; Peer reviewed

The deployment of oxy-fuel combustion in utility boilers is one of the major options for CO2 capture. However, combustion under oxy-firing conditions differs from conventional air-firing combustion, e.g., in the aspect of radiative heat transfer, coal conversion and pollutants formation. In this work, a numerical study on pulverised coal combustion was conducted to verify the applicability and accuracy of several sub-models refined for oxy-fuel conditions, e.g., gaseous radiative property model, gas-phase combustion mechanism and heterogeneous char reaction model. The sub-models were implemented in CFD (Computational Fluid Dynamics) simulations of combustion of three coals under air-firing and various oxy-firing (21–35% vol O2 in O2/CO2 mixture) conditions in an EFR (entrained flow reactor). The predicted coal burnouts and gaseous emissions were compared against experimental results. A good agreement between the simulations and experiments was achieved, indicating a good applicability and reliability of the refined sub-models and suitability of use of the experimentally derived kinetic data in coal devolatilisation and char oxidation sub-models. The sub-models and the practices implemented in this work can be used in large-scale oxy-fuel combustion processes for reliable design and optimization. ; L.A. acknowledges funding from the CSIC JAE program, which was co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa Program). Financial support from the CSIC (Project PIE201080E09) is gratefully acknowledged ; Peer reviewed

The effect of co-firing coal and biomass on the ignition behaviour and NO emissions was evaluated under both air and O2/CO2 (21-35% O2) atmospheres. The results showed a worsening of the ignition properties in the 21%O2/79%CO2 atmosphere in comparison with air. Furthermore, in order to obtain similar or better ignition properties, the oxygen concentration in the O2/CO2 mixture must be 30% or higher. A decrease of the ignition temperature was observed with the addition of biomass in air and oxy-fuel conditions. The results also indicate that NO emissions in the 21%O2/79%CO2 atmosphere were lower than under air-firing conditions, although they increased in the 30%O2/70%CO2 and 35%O2/65%CO2 atmospheres. The addition of biomass resulted in lower NO emissions in all cases. ; This work was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund. L.A. and M.V.G. acknowledge funding from the CSIC JAE program, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program) ; Peer reviewed

The thermal reactivity and kinetics of four coal chars (HVN, UM, SAB and BA) in an oxy-fuel combustion atmosphere (30%O2–70%CO2) were studied using a thermobalance. The coal chars were obtained by devolatilization in an entrained flow reactor (EFR) at 1000 °C for 2.5 s under 100% N2 and CO2 atmospheres. The reactivity tests were carried out by isothermal thermogravimetric analysis at different temperatures in a kinetically controlled regime. Three nth-order representative gas–solid models – the volumetric model (VM), the grain model (GM) and the random pore model (RPM) – were employed in order to describe the reactive behaviour of the chars during oxy-fuel combustion. From these models, the kinetic parameters were determined. The RPM model was found to be the best for describing the reactivity of the HVN, UM and BA chars, while VM was the model that best described the reactivity of the SAB char. The reactivities of the chars obtained in N2 and CO2 in an oxy-fuel combustion atmosphere with 30% of oxygen were compared using the kinetic parameters, but no differences were observed between the two devolatilization atmospheres. The apparent volatile yield after the coal devolatilization under CO2 in the EFR was greater than under N2 for all the coals studied. According to the scanning electron microscopy (SEM) images of the chars, those obtained in the CO2 atmosphere experienced a greater degree of swelling, some particles showing partially reacted surfaces indicative of reaction between the char and CO2. ; This work was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund. M.V.G. and L.A. acknowledge funding from the CSIC JAE-Pre and CSIC JAE-Doc programs, respectively, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program). ; Peer reviewed

The ignition temperature, burnout and NO emissions of blends of a semi-anthracite and a high-volatile bituminous coal with 10 and 20 wt.% of olive waste were studied under oxy-fuel combustion conditions in an entrained flow reactor (EFR). The results obtained under several oxy-fuel atmospheres (21%O2–79%CO2, 30%O2–70%CO2 and 35%O2–65%CO2) were compared with those attained in air. The results indicated that replacing N2 by CO2 in the combustion atmosphere with 21% of O2 caused an increase in the temperature of ignition and a decrease in the burnout value. When the O2 concentration was increased to 30 and 35%, the temperature of ignition was lower and the burnout value was higher than in air conditions. A significant reduction in ignition temperature and a slight increase in the burnout value was observed after the addition of biomass, this trend becoming more noticeable as the biomass concentration was increased. The emissions of NO during oxy-fuel combustion were lower than under air-firing. However, they remained similar under all the oxy-fuel atmospheres with increasing O2 concentrations. Emissions of NO were significantly reduced by the addition of biomass to the bituminous coal, although this effect was less noticeable in the case of the semi-anthracite. ; This work was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund. M.V.G. and L.A. acknowledge funding from the CSIC JAE-Doc and CSIC JAE-Pre programs, respectively, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program). ; Peer reviewed

The thermal reactivity and kinetics of five coal chars, a biomass char, and two coal/biomass char blends in an oxy-fuel combustion atmosphere (30%O2–70%CO2) were studied using the non-isothermal thermogravimetric method at three heating rates. Fuel chars were obtained by devolatilization in an entrained flow reactor at 1273 K under N2 and CO2 atmospheres. Three nth-order representative gas–solid models – the volumetric model (VM), the grain model (GM) and the random pore model (RPM) – were employed to describe the reactive behaviour of the chars. The RPM model was found to be the best for describing the reactivity of the high rank coal chars, while VM was the model that best described the reactivity of the bituminous coal chars, the biomass char and the coal-biomass blend char. The kinetic parameters of the chars obtained in N2 and CO2 in an oxy-fuel combustion atmosphere with 30% of oxygen were compared, but no relevant differences were observed. The behaviour of the blend of the bituminous coal (90%wt.) and the biomass (10%wt.) chars resembled that of the individual coal concealing the effect of the biomass. Likewise, no interaction was detected between the high rank coal and the biomass chars during oxy-fuel combustion of the blend. ; This work was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund (ERDF). M.V.G. and L.A. acknowledge funding from the CSIC JAE-Pre and CSIC JAE-Doc programs, respectively, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program). ; Peer reviewed

The NO emissions of a semi-anthracite and a high-volatile bituminous coal were studied under oxy-fuel combustion conditions in an entrained flow reactor (EFR). The results obtained under oxy-fuel atmospheres (21%O2–79%CO2, 30%O2–70%CO2 and 35%O2–65%CO2), were compared with those produced in air. The replacement of CO2 by 5, 10, and 20% of steam in the oxy-fuel combustion atmospheres was also evaluated in order to study the wet recirculation of flue gas. The NO emissions during oxy-fuel combustion were lower than those in air-firing, and a slight increasing trend in NO emissions was observed as O2 concentrations in the oxy-fuel atmospheres increased. Similarly, the NO concentration was reduced by the addition of steam, both under air and under oxy-fuel atmospheres, but no relevant differences between the different steam concentrations were observed. ; Work carried out with financial support from the Spanish MICINN (Project PS- 120000-2005-2) co-financed by the European Regional Development Fund. L.A. and J.R. acknowledge funding from the CSIC JAE program, co-financed by the European Social Fund, and the Asturias Regional Government (PCTI program), respectively. ; Peer reviewed

The ignition temperature and burnout of a semi-anthracite and a high-volatile bituminous coal were studied under oxy-fuel combustion conditions in an entrained flow reactor (EFR). The results obtained under oxy-fuel atmospheres (21%O2–79%CO2, 30%O2–70% O2 and 35%O2–65%CO2) were compared with those attained in air. The replacement of CO2 by 5, 10 and 20% of steam in the oxy-fuel combustion atmospheres was also evaluated in order to study the wet recirculation of flue gas. For the 21%O2–79%CO2 atmosphere, the results indicated that the ignition temperature was higher and the coal burnout was lower than in air. However, when the O2 concentration was increased to 30 and 35% in the oxy-fuel combustion atmosphere, the ignition temperature was lower and coal burnout was improved in comparison with air conditions. On the other hand, an increase in ignition temperature and a worsening of the coal burnout was observed when steam was added to the oxy-fuel combustion atmospheres though no relevant differences between the different steam concentrations were detected. ; This work was carried out with financial support from the Spanish MICINN (Project PS- 120000-2005-2) co-financed by the European Regional Development Fund. L.A. and J.R. acknowledge funding from the CSIC JAE program, co-financed by the European Social Fund, and the Government of the Principado de Asturias (Severo Ochoa program), respectively. ; Peer reviewed

The thermal characteristics of pulverized coal have been studied under oxy-fuel combustion conditions using non-isothermal thermogravimetric analysis (TG). The atmospheres used were 21%O2/79%N2, 21%O2/79%CO2, 30%O2/70%O2, and 35%O2/65%CO2. Coal blends of coal with 10 and 20% of biomass were also studied under these atmospheres. The addition of 10 and 20% of steam was evaluated for the oxy-fuel combustion atmospheres with 21 and 30% of O2 in order to study the effect of the wet recirculation of flue gas. The results obtained were similar for all the different rank coals and indicated that replacing N2 by CO2 in the combustion atmosphere with 21% of O2 caused a slight decrease in the rate of mass loss and delayed the burning process of the coal, biomass and coal/biomass blend samples. When the O2 concentration was increased to 30 and 35% in the oxy-fuel combustion atmosphere, the rate of mass loss increased, the burning process occurred at lower temperatures and it was shorter in duration. An increase in the rate of mass loss and a reduction in burning time and temperature were observed after the addition of steam to the oxy-fuel combustion atmosphere. No relevant differences between the 10 and 20% steam concentrations were observed. ; The present study was carried out with financial support from the Spanish MICINN (Project PS- 120000-2005-2) co-financed by the European Regional Development Fund. L.A. and J.R. acknowledge funding from the CSIC JAE program, which was cofinanced by the European Social Fund, and the Asturias Regional Government (PCTI program), respectively. ; Peer reviewed

The devolatilisation step of coal is a vital stage in both air–coal and oxy-coal combustion and there is interest in whether methods of estimating the reaction parameters are similar for both cases. A network pyrolysis model, the FG-DVC (Functional Group-Depolymerisation Vaporisation Cross-linking) code was employed to evaluate the effect of temperature (1273–1773 K) and heating rate (104–106 K/s) on the devolatilisation parameters of two coals of different rank. The products distribution between char and volatiles, and volatiles and NH3/HCN release kinetics were also determined. In order to assess the accuracy of the FG-DVC predictions, the values for nitrogen distribution and devolatilisation kinetics obtained for a temperature of 1273 K and a heating rate of 105 K/s were included as inputs in a Computational Fluid Dynamics (CFD) model for oxy-coal combustion in an entrained flow reactor (EFR). CFD simulations with the programme default devolatilisation kinetics were performed. The oxygen content in oxy-firing conditions ranged between 21% and 35%, and air-firing conditions were also employed as a reference. The experimental coals burnouts and oxygen concentrations from the EFR experiments were employed to test the accuracy of the CFD model. The temperature profiles, burning rates, char burnout and NO emissions during coal combustion in both air and O2/CO2 atmospheres were predicted. The predictions obtained when using the CFD model with FG-DVC coal devolatilisation kinetics were much closer to the experimental values than the predictions obtained with the ANSYS Fluent (version 12) program default kinetics. The predicted NO emissions under oxy-firing conditions were in good agreement with the experimental values. ; The present study was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund. L.A. and J.R. acknowledge funding from the CSIC JAE program, which was cofinanced by the European Social Fund, and the Asturias Regional Government (PCTI program), respectively. MG acknowledges financial support from E.ON UK, and for an EPSRC Dorothy Hodgkin Postgraduate Award. We also thank Dr L Ma for helpful discussions. ; Peer reviewed

A fundamental investigation has been conducted on the combustion behavior of single particles (75–150 μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400 K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O2/79%N2) and four simulated dry oxy-fuel conditions: 21%O2/79%CO2, 30%O2/70%CO2, 35%O2/65%CO2 and 50%O2/50%CO2. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N2 gas of air with CO2 (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO2 to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO2. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas. ; The authors acknowledge financial assistance from the US-NSF award CBET-0755431. J. Riaza acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program). M.V. Gil and L. Álvarez acknowledge funding from the CSIC JAE program, co-financed by the European Social Fund. Support from the CSIC (PIE 201080E09) is gratefully acknowledged. ; Peer reviewed

The ignition behavior of coal and biomass blends was assessed in air and oxy-firing conditions in an entrained flow reactor. Four coals of different rank, an anthracite, a semi-anthracite, and two high-volatile bituminous coals, were tested in air and O2/CO2 (21–35% O2) environments. For all the coals, deterioration in ignition properties was observed in the 21%O2/79%CO2 atmosphere in comparison with air. However, the ignition properties were enhanced when the oxygen concentration in the O2/CO2 mixture was increased. Coal and biomass blends of a semi-anthracite and a high-volatile bituminous coal with 10 and 20 wt% of olive residue were also used in the ignition experiments under air and oxy-firing conditions. The ignition behavior of the coals improved as the additions of biomass increased both in air and oxy-firing conditions. In particular, the effect of biomass blending was more noticeable in the ignition of the high rank coal. Since industrial oxy-coal combustion with a wet recycle would result in higher concentrations of H2O(v), the effect of steam addition on ignition behavior was also studied. A worsening in ignition behavior was observed when steam was added to the oxy-fuel combustion atmospheres, although an increase in the steam concentration from 10 to 20% did not produce any significant difference in the ignition characteristics of the fuels. ; This work was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund. L.A. and M.V.G. acknowledge funding from the CSIC JAE programs, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program), respectively. Support from the CSIC (PIE 201080E09) is gratefully acknowledged. ; Peer reviewed