The role of alkali metals in biomass thermochemical conversion

Environmental preservation concerns, coupled with those of energy supply security, are leading to a push for alternative fuels that are both green and sustainable. Therefore, biomass, which is a renewable low carbon energy source, is being increasingly utilized worldwide. The use of biomass in thermochemical conversion is not without problems, some of which are related to the inherent alkali metal content present in these types of fuels. The work conducted for this thesis mainly deals with topics related to thermal degradation kinetics of biomass and the influence of alkali metals on these kinetics, as well as their effects on the thermal behaviour of the fuel. Torrefaction is also studied with respect to kinetics studies, and the effects of mineral content. Thermal degradation was studied using thermogravimetric analysis (TGA) and kinetic models were evaluated to address two questions; first, what method of data analysis is appropriate for extracting reliable kinetic data from TGA experiments? Second, what kinetics are most suitable for high heating rate situations such as those present in pulverized fuel power stations? It was found that for low heating rate experiments (10 Klmin), the global first-order reaction kinetic models that tend to yield low activation energies (E), such as the reaction rate constant method, work well. High E kinetics can also work well at low heating rate, but only if the reaction is assumed to be due to the sum of a number of individual steps. For example, those derived when assuming the biochemical components degrade independently, or using the functional group approach. For higher heating rates (>103 Kls) high E kinetics predict conversion well, and this can be rationalized since primary cracking reactions will dominate under these conditions. However, at heating rates of 105 Kls and temperatures of 1500 QC (i.e., flame conditions), a compensation on the rates is seen and the choice of rate parameters is less critical. Two sets of kinetic data, E = 178.7 kJ/mol, A = 2.2 x 1013 S-1 and E = 48.7 kJ/mol, A = 6.84 x 103 S-I, both predict conversions in keeping With the available experimental data. The effects of alkali metals (K, Na, Cs) on thermal degradation kinetics of SRC willow in pyrolysis, combustion were studied using TGA, and single particle burning in a methane-air flame. The results revealed that all three metals had a strong and similar catalytic effect on pyrolysis and combustion. Combustion under flame conditions also showed a stark contrast between the strongly catalyzed degradation of samples in the presence of alkali metals, and the uncatalysed degradation of mineral-free samples. As with the low heating rate results, at flame conditions, the metal-impregnated samples behaved similarly to each other, implying that a similar thermal degradation. mechanism is followed when woody biomass contains any of the alkali metals. A mathematical expression directly linking the inherent potassium and sodium content of SRC willow to its thermal degradation kinetics was developed through modifying a Langmuir-Hinshelwood relation and applying it to pyrolysis data. This relation yields a maximum reaction rate and a metal saturation constant that can be used to predict a reaction rate of willow based on the pyrolysis temperature and the concentration of either of the metals in the biomass sample. I.e. the maximum reaction rate constant of 3.26 x 10-3 (S-I) and the potassium saturation constant of 0.56 wt% can be used to derive the pyrolysis reaction rate of any willow sample with a known potassium concentration. Similarly, a maximum reaction rate of 3.27 x 10-3 (S-I) and a sodium saturation constant of 0.36 wt% can be used to derive a reaction rate for any willow sample with a known sodium concentration. Ab initio (Density Functional Theory method) modelling was employed to explore the chemical mechanisms involved..Cellobiose was used as a model for cellulose. The cellobiose structure was first optimized at the HF level, and then at the B3L YP DFT level with a 6-31G(d) basis set and the structure frequency was checked to ensure the system was at ground state. The models showed that that both metal ions form multiple interactions with the hydroxyl and ether bonds in the cellulose structure. Structures with metal chelated at the C6 position in the ring have interactions with four oxygen atoms, while metals at the C2 position have interactions with only two oxygen atoms, although inter-molecular chelation between cellulose chains has not been considered. Structures are more stable when potassium or sodium can coordinate to more oxygen groups. Nevertheless, in all the structures investigated, chelation of potassium or sodium causes a change in the conformation of the rings (twisting) which may activate the structure towards cracking. The final area of investigation in this thesis is in torrefaction, a mild pyrolysis process. Torrefied biomass has many advantages over untreated biomass, but its ashcharacteristics remain similar to the parent biomass. In this thesis, inherent metals were removed prior to torrefaction. Impact on ash behaviour of this resultant fuel and the torrefaction process itself are reported. More specifically, the work examined the effects of altered mineral content (through a chemical fractionation procedure involving successive washings of the fuel in water, ammonium acetate, and hydrochloric acid), on the torrefaction, of four biomass fuels (SRC willow, Miscanthus, eucalyptus, and wheat straw), as well as on the pyrolysis and the ash behaviour of the torrefied material. Washing prior to torrefaction significantly reduced the ash content of the fuels, and ameliorated the ash fusion temperatures. The pyrolysis reaction rates of the HCI treated and torrefied fuels were found to be the highest, presumably due to the changes in the biomass structure caused by the acid. The results suggest that water washing is the most useful pre-treatment for the preparation of torrefied fuels. Washing with ammonium acetate or HCI would not be feasible because of the small advantage gained, the high costs induced, and the environmental implications. The significance of the impact of these changes in composition and reaction rates depends on the end application, power station or domestic heating. In the case of power station applications, washing with water reduces the ash content, and improves the ash melting behaviour of the remaining ash, which may be advantageous particularly in the case of straw; the slight increase in N would not be significant because of the normal NOx reduction methods used in power stations. In domestic applications the reduction of ash is not so important but the increase in N may be a significant disadvantage.