Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 179

TABLE 6.1 Upper Limits of Biomass Gas Tar and Particulates

Particulate (g/Nm 3 ) Tar (g/Nm 3 ) Application

Direct combustion No limit specified No limit specified

Syngas production 0.02 0.1

Gas turbine 0.1 120 0.05 5

IC engine 30 50 100

Pipeline transport 50 500 for compressor

Fuel cells , 1.0

Source: Data compiled from Milne et al. (1998).

However, the flue gas produced after combustion must be clean enough
to meet local emission requirements.
Cofiring of gasified biomass in fossil-fuel-fired boilers is an example
of direct firing. Industrial units like ovens, furnaces, and kilns are also
good examples of direct firing. In such applications, it is not necessary to
cool the gas after production. The gas is fired directly in a burner while it
is still hot, in the temperature range of 600 900 C. Thus, there is little

chance of tar condensation. However, the pipeline between the gasifier
exit and the burner inlet should be such that the gas does not cool down
below the dewpoint of tar. If that happens, tar deposition might clog the
pipes, leading to hazardous conditions.
b. Internal-combustion engines, such as diesel or Otto engines, are favorite
applications of gasified biomass, especially for distributed power genera-
tion. In such applications, the gas must be cooled, and as such there is a
good chance of condensation of the tar in the engine or in fuel-injection
systems. Furthermore, the piston-cylinder system of an internal-
combustion engine is not designed to handle solids, which imposes tighter
limits on the tar as well as on the particulate level in the gas. Particulate
and tar concentrations in the product gas should therefore be below the
tolerable limits, which are 30 mg/Nm 3 for particulates and 100 mg/Nm 3
for tar (Milne et al., 1998, p. 41). The gas turbine, another user of bio-
mass gas, imposes even more stringent restrictions on the cleanliness of
the gas because its blades are more sensitive to deposits from the hot gas
passing through them after combustion. Here, the tar concentration should
be between 0.5 and 5.0 mg/Nm 3 (Milne et al., 1998, p. 39).
c. The limits for particulates and tar in syngas applications are even more strin-
gent, as tar poisons the catalyst. For these applications, Graham and Bain
(1993) suggested an upper limit as low as 0.02 mg/Nm 3 for particulates and