CO2 Gasification Reactivity of Char from High-Ash Biomass.

Biomass char produced from pyrolysis processes is of great interest to be utilized as renewable solid fuels or materials. Forest byproducts and agricultural wastes are low-cost and sustainable biomass feedstocks. These biomasses generally contain high amounts of ash-forming elements, generally leading to high char reactivity. This study elaborates in detail how chemical and physical properties affect CO2 gasification rates of high-ash biomass char, and it also targets the interactions between these properties. Char produced from pine bark, forest residue, and corncobs (particle size 4-30 mm) were included, and all contained different relative compositions of ash-forming elements. Acid leaching was applied to further investigate the influence of inorganic elements in these biomasses. The char properties relevant to the gasification rate were analyzed, that is, elemental composition, specific surface area, and carbon structure. Gasification rates were measured at an isothermal condition of 800 °C with 20% (vol.) of CO2 in N2. The results showed that the inorganic content, particularly K, had a stronger effect on gasification reactivity than specific surface area and aromatic cluster size of the char. At the gasification condition utilized in this study, K could volatilize and mobilize through the char surface, resulting in high gasification reactivity. Meanwhile, the mobilization of Ca did not occur at the low temperature applied, thus resulting in its low catalytic effect. This implies that the dispersion of these inorganic elements through char particles is an important reason behind their catalytic activity. Upon leaching by diluted acetic acid, the K content of these biomasses substantially decreased, while most of the Ca remained in the biomasses. With a low K content in leached biomass char, char reactivity was determined by the active carbon surface area.

Empirical Kinetic Models for the CO2 Gasification of Biomass Chars. Part 1. Gasification of Wood Chars and Forest Residue Chars.

The gasification kinetics of charcoals and biomass chars is complicated by several factors, including chemical and physical inhomogeneities, the presence of mineral matter, and the irregular geometry of the pore structure. Even the theoretically deduced gasification models can only provide empirical or semiempirical descriptions. In this study, an empirical kinetic model from the earlier works of the authors was adapted for the CO2 gasification of biomass chars. It is based on a versatile polynomial approximation that helps to describe the dependence of the reaction rate on the progress of the conversion. The applicability of the model was tested by the reevaluation of 24 thermogravimetric analysis (TGA) experiments from earlier publications. The adjustable parameters of the model were determined by the method of least squares by evaluating groups of experiments together. Two evaluation strategies were tested. In the regular evaluations, the same kinetic parameters were employed for all the experiments with a given sample. The use of experiments with modulated and constant reaction rate (CRR) temperature programs made it possible to employ another approach too, when the preexponential factor was allowed to vary from experiment to experiment. The latter approach allows a formal kinetic description of the differences in the thermal deactivation of the samples caused by different thermal histories as well as of some inevitable systematic errors of the TGA experiments. The evaluations were carried out by both approaches, and the results were compared. The evaluations were based on 12 experiments. As a test, each evaluation of the study was repeated with only 8 experiments. The results of the latter test calculations indicated that the information content of the employed experiments is sufficient for the evaluation approaches of this work.

Cooling aerosols and changes in albedo counteract warming from CO2 and black carbon from forest bioenergy in Norway.

Climate impacts of forest bioenergy result from a multitude of warming and cooling effects and vary by location and technology. While past bioenergy studies have analysed a limited number of climate-altering pollutants and activities, no studies have jointly addressed supply chain greenhouse gas emissions, biogenic CO2 fluxes, aerosols and albedo changes at high spatial and process detail. Here, we present a national-level climate impact analysis of stationary bioenergy systems in Norway based on wood-burning stoves and wood biomass-based district heating. We find that cooling aerosols and albedo offset 60-70% of total warming, leaving a net warming of 340 or 69 kg CO2e MWh-1 for stoves or district heating, respectively. Large variations are observed over locations for albedo, and over technology alternatives for aerosols. By demonstrating both notable magnitudes and complexities of different climate warming and cooling effects of forest bioenergy in Norway, our study emphasizes the need to consider multiple forcing agents in climate impact analysis of forest bioenergy.

Predictions of biochar yield and elemental composition during torrefaction of forest residues.

In this work, a direct prediction method coupled with a consecutive reaction model is developed to estimate the biochar yield and elemental composition in a biomass torrefaction process. Norway forest residues were chosen as feedstock and torrefied at different temperatures under nitrogen atmosphere in a thermogravimetric analyzer. Obtained data were modeled to predict the mass loss during torrefaction. Distributions of initial, intermediate and final solid products as well as torrefaction kinetic parameters are reported. Thereafter, a direct method to predict the elemental composition of biochar is introduced. The results show that the decomposition of initial biomass to form an intermediate solid has higher conversion rate than the degradation of the intermediate. Moreover, the predictions reproduce well the experimental thermogravimetric curves and show composition trends similar to the literature data. This method is useful for the design and optimization of industrial torrefaction processes with predictable biochar yield and elemental composition.

The effect of kaolin on the combustion of demolition wood under well-controlled conditions.

In an attempt to look at means for reduction of corrosion in boilers, combustion experiments are performed on demolition wood with kaolin as additive. The experiments were performed in a multi-fuel reactor with continuous feed of pellets and by applying staged air combustion. A total characterization of the elemental composition of the fuel, the bottom ash and some particle size stages of fly ash was performed. This was done in order to follow the fate of some of the problematic compounds in demolition wood as a function of kaolin addition and other combustion-related parameters. In particular chlorine and potassium distribution between the gas phase, the bottom ash and the fly ash is reported as a function of increased kaolin addition, reactor temperature and air staging. Kaolin addition of 5 and 10% were found to give the least aerosol load in the fly ash. In addition, the chlorine concentration in aerosol particles was at its lowest levels for the same addition of kaolin, although the difference between 5 and 10% addition was minimal. The reactor temperature was found to have a minimal effect on both the fly ash and bottom ash properties.