Carlos A. Gurgel Veras

Biomass fire consumption and carbon release rates of rainforest-clearing experiments conducted in northern Mato Grosso, Brazil

Biomass consumption and carbon release rates during the process of forest clearing by fire in five test plots are presented and discussed. The experiments were conducted at the Caiabi Farm, near the town of Alta Floresta, state of Mato Grosso, Brazil, in five square plots of 1 ha each, designated A, B, C, D, and E, with different locations and timing of fire. Plot A was located in the interface with a pasture, with three edges bordering on the forest, and was cut and burned in 1997. Plots B, C, D, and E were located inside the forest. Plot B was cut and burned in 1997. Plot C was inside a deforested 9-ha area, which was cut and burned in 1998. Plot D was inside a deforested 4-ha area, which was cut in 1998 and burned in 1999. Plot E was inside a deforested 4-ha area, which was cut and burned in 1999. Biomass consumption was 22.7%, 19.5%, 47.5%, 61.5%, and 41.8%, for A, B, C, D, and E, respectively. The effects of an extended curing period and of increasing the deforested area surrounding the plots could be clearly observed. The consumption, for areas cut and burned during the same year, tended toward a value of nearly 50% when presented as a function of the total area burned. The aboveground biomass of the test site and the amount of carbon before the fire were 496 Mg ha−1 and 138 Mg ha−1, respectively. Considering that the biomass that remains unburned keeps about the same average carbon content of fresh biomass, which is supported by the fact that the unburned material consists mainly of large logs, and considering the value of 50% for consumption, the amount of carbon released to the atmosphere as gases was 69 Mg ha−1. The amounts of CO2 and CO released to the atmosphere by the burning process were then estimated as 228 Mg ha−1 and 15.9 Mg ha−1, respectively. Observations on fire propagation and general features of the slash burnings in the test areas complete the paper.

Experimental investigation of smouldering in biomass

Abstract The smouldering process of wood logs was studied experimentally in a laboratory facility and in prescribed forest burns. The main goal was to check the parameters that initiate and control the stability of the smouldering process. To do so, sample temperatures at five different locations and concentrations of CO, CO 2 and O 2 were measured and discussed. By varying the temperature and air supply of the flow tunnel apparatus, different rates of smoulder propagation were identified. In prescribed burns, the main characteristics of the self-sustained smouldering combustion front in logs of different sizes and species are reported. The average smouldering speed in the field is about one order of magnitude lower than that reported for different materials in laboratory experiments.

The Chemical Percolation Devolatilization Model Applied to the Devolatilization of Coal in High Intensity Acoustic Fields

The chemical percolation devolatilization model (CPD) was extended for the prediction of drying and devolatilization of coal particles in high intensity acoustic fields found in Rijke tube reactors. The acoustic oscillations enhance the heat and mass transfer processes in the fuel bed as well as in the freeboard, above the grate. The results from simulations in a Rijke tube combustor have shown an increase in the rate of water evaporation and thermal degradation of the particles. The devolatilization model, based on chemical percolation, applied in pulsating regime allowed the dynamic prediction on the yields of CO, CO2, CH4, H2O, other light gases as well as tar which are important on ignition and stabilization of flames. The model predicted the quantity and form of nitrogen containing species generated during devolatilization, for which knowledge is strategically indispensable for reducing pollutant emissions (NOx) in flames under acoustic excitation .

Probability of surface fire spread in Brazilian rainforest fuels from outdoor experimental measurements

This paper describes the development of a logistic model to predict the probability of surface fire spread in Brazilian rainforest fuels from outdoor experimental measurements. Surface fires spread over litter composed mostly of dead leaves and twigs. There were 72 individual outdoor experiments in eighteen sites. The fire propagated in 49% of the experiments. In each experiment, the litter height, litter temperature, unburned litter mass, wet and dry litter mass, soil temperature, wet and dry soil mass, ambient wind velocity, ambient air temperature, ambient air relative humidity and duration of fire spread were measured. Using these data, the rate of fire spread, litter bulk density, litter and soil moisture content, litter load and litter residue fraction were determined. For the sake of analysis, experimental results were classified into two groups: one for which the fire propagated and the other one for which the fire self-extinguished. Analyses of a logistic regression model showed that the relevant parameters for fire propagation are litter height and litter moisture content. Concerning the probability of successful fire propagation, the model showed a true positive rate of 71% and a true negative rate of 84%. The outdoor experiments also served to gather data to improve the understanding of surface fires and to provide input data for future computer simulations.

Effects of wet CO oxidation on the operation of engines and power generators

charcoal gasifiers to be utilized as alternative fuels in the operation of engines and gas turbines for power generation. Computational models of plug flow reactors and well stirred reactors are employed to simulate the reaction and post-flame zones, adopting different chemical mechanisms. In the simulations reactants enter the reactors at 1000 K, 1 atm and equivalence ratio 0.25. It was observed that mixtures about 3% to 4% in volume of water vapor allow to obtain optimal operation characteristics, including high blowout limit, low ignition delay, maximum reaction zone temperature, high CO2 prodution and low thermal NO formation. It was observed that increasing water contents reduce significantly ignition times up to 3% in volume, while blowout mass flow rates increase continuously up to 6 % in volume, the maximum value considered. Formation of NO decreases continuously with humidity after the flame zone, while there are peaks of NO formation within the flame zone below 1% in volume. Higher water vapor content decreases the final temperatures below 1700 K, leading to a lower thermal efficiency. The method can be used to estimate optimum operational conditions with other input parameters.

Effect of particle size and pressure on the conversion of fuel N to no in the boundary layer during devolatilization stage of combustion

The fate of fuel nitrogen evolved during pyrolysis in the boundary layer of a single particle is studied by modeling. The conservation equations for energy, species, and momentum are solved numerically in the boundary layer. The particles are treated as nonisothermal, because there can be great temperature gradients inside the particle, which affect the rates of devolatilization and nitrogen release. Global reaction mechanisms reported in literature are used for volatiles and nitrogen species in the gas phase. The fuel type (reactivity), particle size, gas temperature, oxygen concentration, and pressure affect the stoichiometric conditions in the boundary layer of a combusting particle and, consequently, the NO formation. The effect of particle size on the conversion of HCN and NH3 to NO in the boundary layer is studied. As examples, particles burning in air at two gas temperatures 1350 and 1900 K are considered. For small particles, the pyrolysis and char combustion stages become overlapping, and therefore, the conversion of nitrogen in char and volatiles takes place simultaneously. The NO formation was found to decrease with increasing particle size, when the size exceeded a critical size. The experimentally reported trend for the decrease of NO formation with increasing pressure is shown by model calculations for particle (diameter 80 μm) burning in oxidizing gas containing 10.0 wt % O2 at 1350 K.