The effects of biomass pretreatments on catalytic fast pyrolysis. (2018)
Type of ContentTheses / Dissertations
Thesis DisciplineChemical Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsXin, Xingshow all
Fast pyrolysis is a thermochemical process that converts biomass, the only carbon-based source of renewable energy, into bio-oil as the main product. Catalytic fast pyrolysis has been a promising pathway to improve the bio-oil quality. Biomass pretreatments have been employed to improve the yield and/or the quality of bio-oil in fast pyrolysis. The combination of biomass pretreatment with catalytic fast pyrolysis can be a potential process to achieve the optimum outcome. However, catalytic fast pyrolysis of pretreated biomass is not well understood yet. The objective of this study is to investigate the effects of biomass pretreatments on the product yields and composition in catalytic fast pyrolysis. Based on the results of this study, it is aimed to improve bio-oil yield and quality.
In this study, biomass pretreatments were first conducted including acid-leaching and torrefaction, and then the constitutive compositions and characteristics were analysed both for raw wood and treated wood. The acid-leaching was performed using 1 wt.% acetic acid solution and the torrefaction was done at the mild thermal conditions of 260 oC. A combined pretreatment of acid-leaching followed by torrefaction was also employed.
Following the above study, fast pyrolysis of these pretreated woods was conducted on two fluidised bed reactors, one at University of Canterbury and one at Scion, to understand the effects of the wood pretreatment on the outcomes in fast pyrolysis. However, fast pyrolysis of acid-leached pine wood suffered from bed material agglomeration in the fluidised bed reactor. The bed material adhered to the char and causing agglomeration. This issue had been frequently reported, but no solution had been demonstrated. Thus this issue was investigated in this study and the morphology of char samples was examined using scanning electron microscope. Approaches to overcome this issue were investigated and the mechanism of bed agglomeration was proposed.
A spray-dried HZSM-5 catalyst was applied for catalytic fast pyrolysis, and a hot filter was built as a modification to the reactor to capture dust caused by catalyst attrition. Methods for catalytic fast pyrolysis were developed to optimize the operating conditions. Following the developed methods, the catalyst remained active during the entire reaction time of 90 minutes and the total recovery of products (mass balance) was between 91-99 wt.%.
Catalytic fast pyrolysis of acid-leached wood was conducted at three temperatures (360 oC, 450 oC and 500 oC) and at three catalyst to biomass ratios (2.5, 4 and 6). Catalytic fast pyrolysis of torrefied wood and acid-leached-torrefied wood was conducted at three catalyst to biomass ratios (2.5, 4 and 6) at 500 oC. The effects of biomass pretreatments on the outcomes in catalytic fast pyrolysis were investigated in terms of the distribution of products and the quality of oil product. The bed agglomeration occurred in catalytic fast pyrolysis of acid-leached wood, hence this issue was further discussed and the solutions were proposed.
Finally, a pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) study complemented the understanding of the effects of biomass pretreatments in catalytic pyrolysis. The Py-GC/MS tests were conducted at four temperatures from 360 to 550 °C and four catalyst to biomass ratios from 0:1 to 6:1. The Py-GC/MS results were evaluated by two methods. Firstly, forty five identified products were grouped according to their chemical functionalities and the area percentages of the groups were compared. Secondly, principal components analysis (PCA) was employed to identify variances in the distribution of the products. These findings in the Py-GC/MS study were in agreement with those using a fluidised bed reactor. The chemical mechanism in catalytic pyrolysis of pretreated wood was discussed.
The key outcomes and findings of this study were:
(1) The issue of bed material agglomeration in pyrolysis of acid-leached pine wood on a fluidised bed reactor was overcome by the proposed approaches, thus catalytic fast pyrolysis of demineralised biomass on a fluidised bed reactor can be explored; (2) The acid-leaching pretreatment mildly impeded deoxygenation in catalytic fast pyrolysis, nevertheless it removed the ash from biomass which are deleterious to the zeolite catalyst; (3) The torrefaction pretreatment led to an increase in the yield of oil product in catalytic fast pyrolysis, and caused insignificant changes in the quality of the oil product; (4) The combined pretreatment, acid-leaching followed by torrefaction, was able to remove the ash from the biomass without introducing negative effects associated with the acid- leaching pretreatment on the catalytic fast pyrolysis, including bed agglomeration and impeded deoxygenation.