A study of the importance of secondary reactions in char formation and pyrolysis : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Process Engineering at Massey University, Manawatū, New Zealand

Anthropogenic climate change, caused primarily by excessive emissions of carbon dioxide, has led to a renewed interest in char, the solid product of pyrolysis. When applied to soil as biochar it can both sequester carbon and improve soil function. To make its manufacture environmentally friendly and economically viable it is important to maximise char yield, which can be done by promoting secondary reactions. This research shows that secondary reactions, which are enhanced by prolonged vapour-phase residence time and concentration, not only increase the char yield but are the source of the majority of the char formed. All four biomass constituents (extractives, cellulose, hemicellulose and lignin) undergo secondary reactions concurrent with primary reactions over the entire pyrolysis range ≈ 140 to 500 °C, which makes it practically impossible to separate them. Secondary char formation was confirmed to be exothermic which affects the overall heat of pyrolysis. Impregnating the feedstock with the elements K, Mg and P, which are plant macro-nutrients naturally present in biomass, resulted in the catalysis of secondary char formation. The results reveal that a first order reaction model does not describe pyrolysis accurately when char formation is enhanced by catalysis and secondary reactions. Secondary char can be enhanced by increasing the particle size but there is a limit due to increased cracking and fracturing of the pyrolysing solid. This limitation is overcome by pyrolysis in an enclosed vessel, termed autogenous pressure pyrolysis, which was discovered to cause significant changes in the volatile pyrolysis products; indicating the co-production of a high quality liquid. This process, however, negatively affects the char properties relevant for biochar like the surface area, similar to self-charring and co-carbonisation of condensed volatile pyrolysis products. To increase research capabilities a unique high temperature/ high pressure reactor (600 °C at 20 MPa) was designed to allow the detailed characterisation of all three pyrolysis product classes under extreme pyrolysis conditions. This was demonstrated to be invaluable for understanding the underlying pyrolysis mechanism and physical processes at play

Measurement of the Effective Diffusion Coefficient of Water in Spray Dried Amorphous Lactose Particles

AbstractStickiness and caking phenomena in dairy powders have been attributed to the amorphous lactose component in dairy powders. The effect of water on the glass transition temperature of amorphous lactose is a key to understanding these phenomena. The speed at which the powder particles take up water is critical when modelling caking or sticking processes. There is little in the literature on the measurement of this. This paper presents a method that uses the absorption of water vapour into a monolayer of particles of mixed size to estimate the diffusion coefficient of water in amorphous lactose. The aim was to measure the diffusion coefficient of water in amorphous lactose. Amorphous lactose particles were produced by spray drying and freeze drying and residual free moisture removed by further drying in an oven at 105°C. A monolayer of the particles was spread over a Petri dish and the dish exposed to 30% RH air at 30°C. The change in weight with time was recorded. The particle size distribution was measured using a Malvern Mastersizer S. The size distribution was combined with a mathematical model for the absorption of water into a sphere, applied to each particle size simultaneously, to estimate the weight increase with time. The diffusion coefficient that minimised the sum of squares of the difference between the predicted and experimental values was taken as the diffusion coefficient of water in amorphous lactose. The diffusion coefficient of water in amorphous lactose was found to vary depending on how the particle was made. Values were (3.4±1.7) *10-14 and (6.6±0.7)*10-14 m2s-1 when made by spray drying from 30 wt% and 10 wt% solutions respectively, compared to (4.5±2.5)*10-11 m2s− 1 for freeze dried particles. This result indicates that the diffusion rate into amorphous lactose occurs faster than previously thought in freeze dried products

Studying carbonisation with raman spectroscopy

Raman spectroscopy can provide fast and non-destructive analysis of carbonaceous materials. As it is able to detect nanometre-sized structural features, Raman spectroscopy is widely used in the study of carbon nanotubes, fullerenes, graphenes, and many other carbon-rich materials. Raman analysis has previously shown potential for estimating the heat treatment temperatures (HTT) employed in the preparation of Japanese cedar charcoals which suggested future usefulness in quality control . In the current work, Raman spectroscopy was used to investigate the nanostructural development which had occurred within various chars prepared and carbonised at a range of heat treatment temperatures between ≈ 340°C and 1000°C. Chars were produced from sucrose sugar as standard precursor of high purity and two sources of biomass common in New Zealand (Radiata pine wood and Harakeke leaf fibres). In chars produced at lower HTTs, signals could be detected which were interpreted as representing hydrogen-rich amorphous carbon structures. In contrast, the Raman spectra of well-carbonised chars produced at higher HTTs featured signals consistent with graphene-like structures with coherent domains limited in size to below a few nanometres across. Measurement of such signals provides the ability to evaluate the extent of nanostructural development, identify char samples which are ‘undercooked’ when compared to other char samples, and estimate effective HTTs used in the production of a given char sample. More detailed Raman analysis of Radiata-derived chars was carried out, including analysis of chars produced from carbonising pyrolysis tars. Results of Raman analysis were correlated to H/C atom ratios obtained through elemental analysis for these chars produced from Radiata pine