1.1 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 gasifier, clogging the gas passage and leading to system disruptions. Tar is highly unde­sirable, as it can create the following problems:

• 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.