Rafael Bilbao

Inhibition and sensitization of fuel oxidation by SO2

Abstract An experimental and theoretical study of the interaction of SO 2 with the radical pool under combustion conditions has been carried out. Experiments on moist CO oxidation were conducted in an isothermal quartz flow reactor at 1 atm; temperature ranged from 800 to 1,500 K and stoichiometries from fuel-rich to very lean. In addition, literature data on sulfur species concentration profiles and H atom decay in fuel-rich H 2 /O 2 flames doped with SO 2 were analyzed. The results show that under flow-reactor conditions SO 2 may inhibit or promote oxidation of fuel, depending on conditions. In a narrow range of operating conditions close to stoichiometric SO 2 promotes oxidation through the sequence: SO 2 + H ⇌ SO + OH, SO + O 2 ⇌ SO 2 + O. Inhibition of oxidation by removal of radicals can be explained in terms of the SO 2 +O+M reaction, even under fuel-rich conditions. From the shift in temperature for the onset of CO oxidation because of SO 2 addition under reducing conditions an upper limit of 3.0 × 10 14 cm 6 mol −2 s −1 at 1,060 K can be estimated for the rate constant of H + SO 2 + N 2 ⇌ HOSO + N 2 . This value is consistent with a significant barrier to reaction as proposed theoretically, but an order of magnitude lower than indicated by both ab initio calculations (Marshall and co-workers) and reaction rates derived from flames. However, we find that data on H atom decay in flames doped with SO 2 are not suitable for deriving rate constants because of uncertainty in important side reactions involving SO. Furthermore, we propose that the enhanced H atom decay observed in these flames may be attributed to recombination of H atoms with SO and S 2 species, rather than to a mechanism initiated by the H + SO 2 + M reaction.

Kinetic study for the thermal decomposition of cellulose and pine sawdust in an air atmosphere

Abstract Isothermal and dynamic experiments have been performed to obtain kinetic equations for the thermal decomposition of lignocellulosic materials in an air atmosphere. The experiments were performed with cellulose, which is considered to be the main component in these materials, and pine sawdust. Equations corresponding to the different ranges of temperature and solid conversions have been obtained. A comparison has been carried out between the kinetic constant values obtained with cellulose and pine sawdust under the same operating conditions.

CO2 as a gasifying agent for gas production from pine sawdust at low temperatures using a Ni/Al coprecipitated catalyst

Catalytic CO2 gasification of pine sawdust has been carried out at a relatively low temperature, 700°C, and at atmospheric pressure. The Ni/Al catalyst used was prepared by coprecipitation and calcined at 750°C for 3 h. The influence of the catalyst weight/biomass flow rate (W/mb) ratio on product distribution and gas composition was analyzed. Using a CO2/biomass ratio of around 1, the increase of the W/mb ratio increases H2 and CO yields while CH4 and C2 yields decrease. Deactivation of the catalyst was also observed under the experimental conditions employed. The influence of the W/mb ratio on the initial gas yields has been also analyzed. For W/mb ratios ≥0.3 h, no significant modifications are observed on the initial yields of different gases, and it is confirmed that under these conditions the initial gas composition is close to that for thermodynamic equilibrium. The influence of the reaction atmosphere on gas yields has also been carried out, analyzing the results obtained in pyrolysis, steam gasification and CO2 gasification.

Kinetics of the thermal decomposition of polyurethane foams in nitrogen and air atmospheres

Abstract The kinetic equations of polyurethane foam weight loss have been determined and the influence of the atmosphere, using nitrogen and air, has been analyzed. The results of the kinetic constants obtained from isothermal and dynamic experiments have been related. A comparison between the kinetic constant values corresponding to nitrogen and air atmospheres has also been performed. The significance of using the values of solid weight that can be lost at a given temperature is analyzed.

Experimental and theoretical study of the ignition and smoldering of wood including convective effects

Ignition, as one of the most important processes during the initiation and development of a fire, needs to be studied in different situations. In this work, an experimental and theoretical study of the ignition of wood, including convective effects, has been performed. The experimental study includes tests of both spontaneous and piloted ignition with air flows at different velocities over the sample. Depending on the conditions, smoldering was observed, followed either by ignition or extinction. In some cases, decomposition of the sample occurred without the appearance of a flame. A mathematical model has been used that includes the kinetics of thermal decomposition of wood, the latent heat of vaporization of water, and variable thermal properties. The model provided the temperature at each point in the solid, the local conversion of solid, the time to smoldering, and the time to ignition of the material. In general, reasonable agreement between experimental and theoretical results was obtained.

Modelling of the pyrolysis of wet wood

Abstract The influence of moisture content on the thermal decomposition of wood has been analyzed. Using wood slabs, experimental results of temperature profiles and solid conversion for different moisture contents and heat fluxes have been obtained. A mathematical model without adjustable parameters was used to calculate the temperature at different points in the sample and the average total solid conversion. Once the validity of different assumptions of the model had been analyzed, the experimental results were compared with those calculated by the model. Acceptable agreement was achieved.

Kinetics of thermal decomposition of cellulose: Part I. Influence of experimental conditions

The kinetics of weight loss in the thermal decomposition of cellulose have been determined by means of isothermal and dynamic experiments carried out under various conditions. Values for the pyrolyzable fraction, reaction order and kinetic constant have been obtained from isothermal experiments, while the important influence of the rate of heating of the system as well as the percentages of pyrolyzed solids at T ⩽ 150°C have been observed from the dynamic experiments.

The effect of lanthanum on Ni–Al catalyst for catalytic steam gasification of pine sawdust

The influence has been studied of the addition of lanthanum to coprecipitated Ni–Al catalysts used in the catalytic steam gasification of biomass. The experimental work was carried out at a relatively low temperature, 700 °C, and at atmospheric pressure. Three Ni–Al–La catalysts were prepared, the Ni content being the same in all cases but with varying amounts of La and Al. The evolution of gas yields over time was analysed and the results obtained with the modified catalysts compared with those corresponding to the Ni–Al catalyst. The addition of lanthanum to the Ni–Al catalyst resulted in an increase in CO, CO2, CH4, C2 and total gas yields, while the H2 yield remained almost unchanged. This demonstrates that when the Ni–Al catalyst includes lanthanum in its formulation, a higher transformation of the carbon contained in the biomass to valuable gases takes place and, consequently, less coke is formed over the catalyst. These effects were also analysed in relation with another reaction, CO2 reforming of CH4, where similar trends were noted.

The influence of the percentage of oxygen in the atmosphere on the thermal decomposition of lignocellulosic materials

Abstract Dynamic experiments with a heating rate of 12 °C min −1 have been carried out for different oxygen concentrations and several types of pine samples. The influence of the initial solid weight and the sample shape on the solid conversion for each oxygen concentration has been analysed. A simple method is proposed to predict the solid conversion for different oxygen concentrations. The kinetic equations corresponding to an air atmosphere are used in this method.

Evaluation of the use of different hydrocarbon fuels for gas reburning

Abstract Gas reburning is a NOx reduction technique that has been demonstrated to be efficient in different combustion systems. An experimental study of gas reburning performance in the low temperature range (at and under 1100°C) has been carried out. An evaluation of the use of different hydrocarbon fuels, such as natural gas, methane, ethane, ethylene and acetylene was performed and the influence of the temperature and stoichiometry is considered. The results show that the reburning process is effective under appropriate conditions at the low temperatures used in this work. However, as the temperature diminishes, the influence of the reburn fuel becomes more marked and the use of acetylene or ethane and ethylene leads to better performance than natural gas or methane, the classical reburn fuels for high temperature applications.