Pilot scale steam-oxygen CFB gasification of commercial torrefied wood pellets. The effect of torrefaction on the gasification performance

Manuela Di Marcello,Georgios Archimidis Tsalidis,Giacomo Spinelli,Wiebren De Jong,Jaap H.A Kiel

doi:10.1016/j.biombioe.2017.08.005

Abstract

Torrefaction is a promising biomass upgrading technology as it makes biomass more coal alike and offers benefits in logistics and handling operations. Gasification is an attractive thermochemical conversion technology due to its flexibility in the product gas end-uses. Therefore, it is valuable to investigate whether additional benefits are foreseen when torrefaction is coupled with gasification. Therefore, two commercial torrefied wood fuels and their parent materials are gasified at 800–850 °C under atmospheric steam-oxygen circulating fluidized bed gasification conditions and magnesite as bed material. The torrefied feedstocks consisted of wood residues torrefied by Topell at 250 °C (Topell black), and mixed wood and wood residues torrefied by Torrcoal at 300 °C (Torrcoal black). The gasification results show that torrefaction resulted in an increased gas quality, as it yielded higher H2 and CO contents, a decrease of the CO2 content, increased gas yield and a significant decrease of the total tar content for both feedstocks. For the Torrcoal samples, torrefaction resulted in a decrease in the carbon conversion efficiency (CCE). In addition, the cold gas efficiency (CGE) remained approximately the same due to the increase in the H2 and CO contents. The Topell samples showed an increase in the CCE and CGE upon torrefaction, but this could be attributed to a significant grinding in the screw feeder. It is generally concluded that both torrefied fuels may offer benefits as a feedstock for steam-oxygen blown circulating fluidized bed gasification, in particular in terms of gas quality and yield.

Highlights

Biomass is considered as a potentially carbon neutral energy source
In this study, steam-oxygen blown circulating fluidized bed gasification experiments at 850 C have been performed with commercial torrefied woods and their parent materials in order to investigate the impact of torrefaction under our conditions
It is concluded that torrefaction affected the gasification performance of both woody feedstocks the same way with respect to the permanent gas composition, gas yield and total tar content, but in different ways regarding the conversion efficiency (CCE) and cold gas efficiency (CGE)

Summary

Introduction

Biomass is considered as a potentially carbon neutral energy source. Efforts are being made to develop upgrading processes which convert biomass into a fuel with superior properties in terms of logistics and end-use. During torrefaction the biomass becomes more coal alike; its energy density increases (on mass basis), it becomes more hydrophobic, more brittle and its O/C and H/C molar ratios decrease. If torrefaction is combined with a densification step, the energy density increases on a volumetric basis and its logistics and handling operations are improved [1]. Life cycle assessment studies have shown that torrefied wood offers environmental benefits in global warming impact when it is used for energy applications, such as co-firing with coal for electricity generation [2] and transportation fuels production [3]

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