Major bottleneck in biomass gasification: tar
Gasification technologies are expected to play a key role in expanding the
use of biomass as a major renewable energy source. Gasification processes,
converting the solid biomass feedstock into a gaseous fuel (or syngas), significantly
broaden the biomass utilisation spectrum. The syngas can be used for, e.g.,
co-firing in coal-fired power plants, electricity generation in stand-alone
conversion devices (gas engines, gas turbines, fuel cells), and production
of gaseous/liquid fuels or chemicals. These applications set different specifications
for (the contaminant levels in) the syngas.
Organic contaminants or 'tars' are considered as the major problem-causing
contaminants, which causes a large obstacle for the market introduction of
biomass-gasification based systems. Moreover, measurement methods as well as
definitions for tars are numerous and non-consistent. As a result, the comparison
of data and the definition of clear maximum allowable concentrations for tars
in the syngas are still problematic.
Need for tar measurement standardisation
In general, proper measurement of contaminant levels in biomass gasification-based
systems is crucial to their optimisation and implementation. For most contaminants,
well-developed measurement techniques exist which are similar to techniques
used for related technologies, such as coal combustion and coal gasification
(e.g., for dust, alkali metals, alkaline gases, etc). They allow, e.g., for
an undisputable determination of contaminant levels in the raw syngas, and
of required gas cleaning performance and clean syngas quality. For tars this
is not the case and the different sampling and analysis methods currently being
used cause a lot of confusion. This confusion is even increased by disputes
on the definition of tar.
The absence of a well-defined and widely accepted tar measurement method hinders
the market introduction of biomass gasification-based systems in the following
- It makes it difficult to come to a clear identification and definition
of tar-induced problems. As a consequence, also finding proper solutions
to these problems becomes more difficult.
- It is difficult to define clear specifications for individual system components
(e.g., syngas cooler, gas cleaning equipment, prime mover), which complicates/frustrates
the communication between different suppliers for these components. Tolerances
from which guarantees on performance, system lifetime etc. can be derived,
are difficult to define.
- It is difficult to design optimised integral biomass gasification systems
based on an optimal selection of system components.
The poor status concerning tar measurement in biomass gasification syngas
was clearly illustrated by a parallel measurement study, in which several institutes
conducted tar measurements at the same time and location .
Therefore, building on initial work started at a joint IEA/USDoE/EU-DGXVII
meeting in Brussels in 1998 and resulting in two draft protocols presented
at the 10th European Biomass Conference in Würzburg, the development
of a uniform Protocol (later called Guideline) was started in an EU project
with the title: "Development of a standard method (Protocol) for the measurement
of organic contaminants (tars) in biomass producer gasses" (contract ERK6-1999-20002).
Partners in this project were: ECN (co-ordinator), BTG, Danish Technical Institute – Denmark
(DTI), Kungl Tekniska Hogskolan – Sweden (KTI), Verenum – Switserland,
VTT – Finland, and CRE Group Ltd (renamed to EMC Environmental Engineering
Ltd) – United Kingdom. In addition, the project involved a group of (industrial)
This project resulted in a Tar measurement Guideline, which was largely based
on R&D activities in several parallel (mostly national) projects. For information
on the results of the project, click here...
"Tar Guideline" measurement method