Page 201 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

178 Biomass Gasification, Pyrolysis and Torrefaction

IEA’s gasification task force appears most appropriate (Milne et al., 1998). It
is as follows:

The organics, produced under thermal or partial-oxidation regimes (gasification) of
any organic material, are called “tar” and are generally assumed to be largely
aromatic.

6.2.1 Acceptable Limits for Tar
Tar remains vaporized until the gas carrying it is cooled, when it either con-
denses on cool surfaces or remains in fine aerosol drops (,1 μm). This
makes the product gas unsuitable for use in gas engines, which have a low
tolerance for tar. Thus, there is a need for tar reduction in product gas when
the gas is to be used in an engine. This can be done through appropriate
design of the gasifier and the right choice of operating conditions, including
reactor temperature and heating rate. Even these adjustments may not reduce
tars in the gas to the required level, necessitating further downstream
cleanup.
Standard gas cleaning involves filtration and/or scrubbing, which not
only removes tar but also strips the gas of particulate matters and cools it to
room temperature. These practices clean the gas adequately, making it
acceptable to most gas engines. However, they result in a great reduction in
overall efficiency in the production of electricity or mechanical power using
a gas engine. Furthermore, gas cleaning greatly adds to the capital invest-
ment of the plant.
Biomass gasification is at times used for distributed power generation in
remote locations in small- to medium-capacity plants. For such plants, the
addition of a scrubber or a filtration system significantly increases the overall
plant costs. This limitation makes biomass-based distributed power-
generation projects highly sensitive to the cost of tar cleanup.
The presence of tar in the product gas from gasification can potentially
decide the usefulness of the gas. The following are the major applications of
the product gas:
a. Direct-combustion systems
b. Internal-combustion engines
c. Syngas production.
Table 6.1 presents data on the tolerance levels of tar and particulate con-
tents for several applications of gas.

a. In applications where the raw gas is burnt directly without cooling, there
is no need for cleaning. Such systems have little restriction on the amount
of tar and particulates as long as the gas travels freely to the burner and
as long as the burner design does not impose any restrictions of its own.