Explosion de vapor

STEAM EXPLOSION OF BIOMASS TO PRODUCE DURABLE WOOD PELLETS

by

PAK SUI LAM

M. Sc., Hong Kong University of Science and Technology, Hong Kong, 2005 B. Eng., Hong Kong University of Science and Technology, Hong Kong, 2004

A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (CHEMICAL AND BIOLOGICALENGINEERING)

THE UNIVERSITY OF BRITISH COLUMBIA (VANCOUVER) May 2011 © Pak Sui Lam, 2011

ABSTRACT
Wood pellet is regarded as a clean fuel for combustion with low ash content (less than 1% by weight) and a high heating value around 21500 MJ/m3 compared to a heating value of 5400 MJ/m3 for dry wood chips. However, pellet is easily disintegrated into fines due to impact or moisture sorption duringhandling and storage. Fines may promote dust explosion during handling or self-heating of pellets in storage. The present study investigates the use of steam explosion pretreatment to improve the pellet durability in terms of mechanical strength and moisture sorption resistance. In this research, a batch steam explosion unit consisting of a steam generator, a steam treatment reactor, and controldevices was developed. Steam explosion experiments were carried out on Douglas Fir at 2 temperatures (200oC and 220oC), 2 treatment durations (5 min, 10 min), and 2 particle sizes (0.4 mm and 0.9 mm). It was found that the bulk density and tapped density of steam treated wood increased with the treatment severity. The pellets made with biomass treated at different combinations of temperature-timewere 1.4 to 3.3 times stronger than untreated pellets. The steam treated biomass required 12% to 81% more energy to form durable pellets than the untreated biomass. Energy input to produce 45000 metric ton regular pellets and steam exploded pellets was estimated. The input energy ranged from 2.80 to 3.52 MJ/kg. Producing pellets from untreated biomass consumed the least energy while pellets made frombiomass treated with saturated steam at 220oC for 10 minutes consumed the highest. A kinetic model for pseudolignin formation during steam explosion was developed. Based on the experimental data in this research and published literature, it was postulated that the creation of pseudolignin is responsible for improved durability of steam exploded pellets. A reaction model was developed to predictthe formation of pseudolignin and evaluate the optimized treatment condition for making durable and water repellent wood pellets.

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PREFACE
The literature review and the design of the research plan in chapter 1 were prepared by the major author (Pak Sui Lam) for the PhD comprehensive exam. The design and construction of steam explosion unit (Chapter 2) and pellet die system (Chapter 3) andthe control system installation (Appendix I) were done by the major author. The major supervisors, Dr. Sokhansanj, Dr. Bi and Dr. Lim provided guidelines and advice on the work from Chapter 1 to Chapter 6. Parts of the chapter 2 of the lab work (TGA, moisture content, sieving and moisture sorption measurement) were carried out by Dr. Sylvia Larsson, a postdoctoral researcher at UBC in 2009. Thedesign of experiment, sample preparation of steam explosion at different conditions, other physical and chemical characterizations, statistical analysis, and regression analysis were done by the major author. Parts of the chapter 2 were presented in the ASABE Annual International Meeting held in Pittsburgh from June 20th to June 23rd, 2010 under the title “Effect of Temperature, Time, Particle sizeand Moisture content on Physical and Chemical Properties of Steam Exploded Woody Biomass”. Parts of Chapter 2 were also submitted for publication to the journal of “Fuel” on April 25th, 2010 and under revision. Other than Sylvia Larsson, other co-authors include Shahab Sokhansanj, Xiaotao Bi, Jim Lim and Staffan Melin. Parts of Chapter 3 were presented in the AICHE annual meeting held in Salt...