Page 200 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter 6

Tar Production and Destruction

6.1 INTRODUCTION

Tar is a major nuisance in both gasification and pyrolysis. It is a thick, black,
highly viscous liquid that condenses in the low-temperature zones of a gas-
ifier, clogging the gas passage and leading to system disruptions. Tar is
highly undesirable, as it can create many problems including:

Condensation and subsequent plugging of downstream equipment.
Formation of tar aerosols.
Polymerization into more complex structures.
Nevertheless, tar is an unavoidable by-product of the thermal conversion
process. This chapter discusses what tar is, how it is formed, and how to
influence its formation such that plants and equipment can live with this
“necessary evil” while minimizing its detrimental effects.

6.2 TAR

Tar is a complex mixture of condensable hydrocarbons, including, among
others, oxygen-containing, 1- to 5-ring aromatic, and complex polyaromatic
hydrocarbons (Devi et al., 2003). Neeft et al. (2003) defined tar as “all
organic contaminants with a molecular weight larger than 78, which is the
molecular weight of benzene.” The International Energy Agency (IEA)
Bioenergy Agreement, the US Department of Energy (DOE), and the
DGXVII of the European Commission agreed to identify as tar all compo-
nents of product gas having a molecular weight higher than that of benzene
(Knoef, 2005, p. 278).
A common perception about tar is that it is a product of gasification and
pyrolysis that can potentially condense in colder downstream sections of the
unit. While this is a fairly good description, a more specific and scientific
definition may be needed for technical, scientific, and legal work. Presently,
there is no universally accepted definition of tar. As many as 30 definitions
are available in the literature (Knoef, 2005, p. 279). Of these, one of the

Biomass Gasification, Pyrolysis and Torrefaction.
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