Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Chemical and Biochemical Engineering

Supervisor

Hugo de Lasa

Abstract

The recent discovery of an increase in greenhouse gas levels, specially CO2, raises concerns regarding the quality of life for future generations. Renewable energy sources such as biomass could contribute to solve this issue. Effective use of this available biofuel would improve the world’s ability to reduce toxic air transmissions, greenhouse gases, and its dependence on oil supply. Gasification arises as a potential alternative for converting biomass into a clean fuel. With the addition of steam as a gasifying agent, the problem on nitrogen in the syngas is neglected.

This research work investigated the effects of temperature, oxidizing agent and biomass composition in the quality of syngas from steam biomass gasification. Gasification of wood pellets, broza and bark was carried out with changes in temperature from 550°C to 600°C.

The atmosphere for gasification was also varied using He and CO2. Gasification of biomass materials are performed in a CREC Riser Simulator Reactor Unit under the expected conditions of a twin circulating fluidized bed gasifier.

Steam gasification performance was affected by changes in temperature, as well as endothermic reactions involved in the process. Thus, higher temperature favored steam reforming and carbon conversion with modest changes due the biomass type. The atmosphere in which gasification was carried out was found also to have an impact in the

produced syngas. Gasification of biomass under a water-CO2 atmosphere reached higher dry gas yields compared to steam gasification under water-helium. It is hypothesized that the decrease in the H2 yield was driven by the reverse water gas shift reaction.

This study also revealed the importance of the catalytic effect of the ash content in biomass.

Ash content affects the quality of syngas, with high-quality synthesis gas (H2/CO >2) suitable for direct alcohol synthesis, obtained under water-helium atmospheres.

Changes in the gasification product yields and their variation with operating parameters are found to be in general agreement with the chemical equilibrium predictions. Moreover, product yields approach chemical equilibrium as the ash content in biomass is larger. In this respect, syngas yields from water-helium and water CO2 gasification of broza, with an ash content of 12% were found to be close to the expected syngas yields at chemical equilibrium.

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