William F. DeGroot

The influence of exchangeable cations on the carbonization of biomass

Abstract Cations of calcium and potassium added to wood through ion exchange have unique effects on the thermal decomposition of the wood. Addition of calcium ions increases the decomposition temperature of the wood and affects the char yield only slightly, while addition of potassium ions reduces the decomposition temperature and significantly increases the char yield. These effects are distinct from the effects of salts of the same elements added to the same or similar materials through absorption of a solution of the salt. For instance, potassium carbonate absorbed on cellulose significantly increases its decomposition temperature, although it has the opposite effect when added to wood through ion exchange. The primary sites for ion exchange are believed to be glucuronic acids in the hemicellulose fraction, and it is therefore likely that the effects of exchanged cations are due primarily to their influence on the decomposition of this component. When naturally occuring inorganic species are removed from wood by acid washing, the char yield is reduced and the cellular structure of the wood can be lost during carbonization, particularly at high heating rates.

First chemical events in pyrolysis of wood

Wood has been heated at 250°C on a thermal balance and the evolved gases have been analyzed by Fourier transform infrared spectroscopy (FTIR). The heated wood has been analyzed for glycoses, uronic acids and by nitrobenzene oxidation to vanillin and syringaldehyde. About 60% of the weight loss is accounted for by five compounds, which are the only products detected in the gases by FTIR. These products are water, carbon dioxide, methanol, acetic acid and formic acid. By relating the rates of formation of these compounds to weight loss, the following major conclusions are reached regarding the first chemical events in pyrolysis of wood. Uronic acids in the hemicelluloses and pectic substances decompose very readily to yield carbon dioxide, water, char (or char precursors) and perhaps some methanol. This decomposition may lead to further pyrolysis of the xylose units to which the uronic acids are attached in the hemicelluloses. Acetyl ester groups in the hemicelluloses are much more resistant to pyrolysis, but are released slowly as acetic acid. A small proportion of the potential methanol product is released very readily and at least part of this product is derived from lignin. Formic acid is released at a slow and continuing rate at 250°C by unknown mechanisms and is probably derived from degradation of hemicelluloses.

Kinetics of gasification of Douglas Fir and Cottonwood chars by carbon dioxide

Abstract Arrhenius kinetic parameters have been determined for the CO 2 gasification of chars (heat treatment at 1000 °C) prepared from well-characterized samples of a hardwood, a softwood and a Montana lignite. The effects of pre-pyrolysis addition of inorganic salts of the alkali, alkaline earth and transition metal groups to the wood samples have also been determined. The reactivities of the chars of the cottonwood and lignite samples exceeded that of Douglas fir char by a factor of four to seven between 700 and 900 °C. The reactivity of the wood char was related to the inorganic content of the sample. There was very little difference in the reactivity of chars prepared from the hardwood and the softwood after treatment with similar quantities of inorganic salts. The inorganic content of the lignite char was more than five times greater than that of cottonwood char, but its reactivity was similar. The carbonates of sodium and potassium were equally effective gasification catalysts. The transition metal salts were the most effective catalysts initially, but they lost their activity well before the gasification was complete. The data indicate that treatment of wood with aqueous salts results in replacement of some of the natural minerals by ion exchange, and that these exchangeable ions play a major role in controlling reactivity of the chars.

Relative rates of carbon gasification in oxygen, steam and carbon dioxide

Rates of gasification of a cellulose char have been determined in 0.1 atm of O2, H2O, and CO2. Gasification was carried out in a reactor designed to provide precise control of reaction conditions and to reduce diffusion limitations of the reaction rate. Apparent activation energies were determined for each reaction to allow extrapolation of rates determined in different temperature ranges to a common temperature. The apparent activation energies for reaction in O2, H2O and CO2 were found to be 28.0, 38.1 and 66.8 kcal/mole, respectively. The relative rates of reaction of the same gases at 800°C and a reactant gas pressure of 0.1 atm were found to be 1.6 × 104, 18 and 1.0, respectively.

Influence of pyrolysis conditions and ion-exchanged catalysts on the gasification of cottonwood chars by carbon dioxide

Abstract Wood chars of heat treatment temperature 800–1000 °C were gasified in carbon dioxide at 800 °C. Chars were prepared from untreated black cottonwood and from wood treated with gasification catalysts, primarily by ion exchange on cell wall carboxylic acid groups. Catalyst treatments were carried out on wood from which the indigenous mineral matter was removed by acid washing (ash-free wood) to avoid complications of interfering catalytic effects. Catalysts were selected to represent the significant inorganic constituents of wood as well as more active gasification catalysts, such as the transition metals. The results of chemical analysis and gasification rate determinations indicate that calcium is the dominant catalytic species controlling the gasification of the untreated wood. Ion-exchanged calcium and cobalt were the most active catalysts under the conditions employed in this study, and ion-exchanged potassium was surprisingly ineffective as a catalyst. CP/MAS 13 C-n.m.r. spectra of chars prepared from untreated and catalyst-treated wood indicate that their chemical structures are very similar, therefore catalytic effects are not associated with the catalysts influence on the formation of the pyrolytic char.

Gasification of cellulosic chars in oxygen and in nitrogen oxides

Abstract Rates of gasification of celluose char (HTT 1,000°C) have been determined in the temperature range 400–600°C in 0.1 atm of oxygen, nitrous oxide (N2O), and nitric oxide (NO) respectively. The results show that the overall reactivity in gasification reactions decreases in the order O2 >NO >N2O.

The effects of ion-exchanged cobalt catalysts on the gasification of wood chars in carbon dioxide

Abstract The high catalytic activity of cobalt in gasification of wood chars by carbon dioxide has been studied by determining the effect of heat treatment temperature (HTT) on reactivity of the char and the chemical state of the catalyst. The pyrolytic transformations occurring as low-temperature chars (HTT 400 °C) are heated to higher gasification temperatures have also been studied in order to determined first, the nature of the interaction of the catalyst with the pyrolytic carbon; and second, the type of gases produced by pyrolytic reactions prior to or during gasification by carbon dioxide. Maximum catalytic activity was not observed until the HTT exceeded 600 °C. The maximum rate of gasification was not strongly dependent on HTT, but as the HTT was increased the duration of the maximum level of reactivity was reduced, and an increasingly smaller fraction of the carbon maintained contact with the catalyst. The most reactive chars were those of HTT 600 °C; these chars gasified completely in approximately 5 min at 600 °C. Elemental cobalt was detected in chars of HTT 1000 °C by X-ray diffractions, and this is believed to be the active catalytic species at lower temperatures as well. The activity of the catalyst is believed to depend upon reduction to the elemental state, and is further related to the rate of agglomeration of the catalytic species and the efficiency of the reduced catalyst in dissociating carbon dioxide.

CHEMISORPTION OF OXYGEN AND OF NITRIC OXIDE ON CELLULOSIC CHARS

Abstract Cellulose chars have been subjected to chemisorption reactions in the temperature range 100–200°C in oxygen and nitric oxide (nitrous oxide did not chemisorb). The products of chemisorption reactions were analyzed by temperature-programmed desorption (TPD) using combined thermogravimetry and mass spectrometry. The results show that nitric oxide is much more reactive than oxygen in chemisorption reactions, although separately we have found that cellulose chars gasify at a higher rate in oxygen. The results of this study are discussed in terms of possible mechanisms of surface oxidation and desorption.

Influence of inorganic additives on oxygen chemisorption on cellulosic chars

Abstract Chemisorption of oxygen on cellulosic chars is the initial step leading to gasification and is a significant factor in controlling chemical reactivity and heat release in smoldering and glowing combustion of cellulose. Oxygen chemisorption kinetics have been determined for chars (HTT 550°C) prepared from cellulose and cellulose treated with inorganic additives. Elovich kinetic analysis indicates that combustion behavior can be correlated with chemisorption kinetics. Addition of the same inorganic additives by grinding with pure cellulose chars had little or no effect on chemisorption kinetics. These data indicate that the mode of action on inorganic additives in enhancing or inhibiting the solid phase combustion of cellulose chars involves their influence on char functionality developed during pyrolysis. Chemisorption of oxygen on chars results in a decrease in free radical concentration, and heat treatment at 400°C in flowing nitrogen restores the original concentration. However, free radical concentrations do not differ significantly between additive treatments over most of the temperature range studied. Therefore, combustion behavior cannot be explained strictly in terms of changes in free radical concentration and other functional groups must also play a significant role.

Kinetics of Wood Gasification by Carbon Dioxide and Steam

Gasification of wood and related biomass fuels provides a means of converting these renewable resources to an energy form that can be stored, transported and burned more cleanly and efficiently. The availability of biomass materials, particularly in industries such as the food processing and forest products industries, and their lower nitrogen, sulfur and ash content compared to conventional fuels such as coal, offer distinct advantages for the use of biomass as gasification substrates. Furthermore, gasification can be used in combination with other thermal processes such as pyrolytic saccharification of wood1 to recover the fuel value of the energy-rich char which is a byproduct of this process.