Measuring Gasifier Tars: Draft Standard

Measuring Gasifier Tars: Draft Standard,
Paul de Wild, ECN September 2005


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 ways:

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 [1].

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) reviewers.

Description of the work

The work in this project is subdivided into three activities:

Standardisation of the existing Guideline into a Standard for measurement of tars in biomass producer gases;
Co-ordination of R&D activities
Dissemination of the results.

In the first activity, standardisation is performed in a Task Force (number CEN/TF/143) installed directly under the European Committee for Standardisation (Comité Européen de Normalisation, CEN). This Task Force has been applied for at CEN in the previous EU project and will be installed at the start of the current (new) project. The activities of this Task Force are twofold:

Communicate with CEN on progress of standardisation, following the CEN rules and procedures, document versions of the Standard (including formal enquiry stage and formal vote stage) and plan plus organise Meetings of the Task Force including the Technical Experts. Activities include holding the Secretary and the Chair of the CEN Task Force.
(b) Ensure technical input to the Task Force from Technical Experts. They bring in their expertise on tar measurement and use of the Guideline and define the specifications the Standard has to fulfil. This Group of Technical Experts discusses existing data on the draft Standard and takes action to ensure collection of data that are still missing. In particular, data on accuracy and reproducibility of the draft Standard are essential in the process of Standardisation.

Co-ordination of R&D activities
&D on the short – term is still required as data on accuracy and reproducibility of the Guideline method are necessary in order to achieve the Standard Status. The R&D will comprise mainly the following two routes:

Round Robin tests of gas chromatographic and gravimetric methods, where synthetic and real tar samples will be sent out for analysis in different laboratories.
Parallel testing, where different laboratories will carry out simultaneous tar measurements to find out whether the measurement method and measuring equipment result in comparable numbers.

In addition, also other interesting research activities (e.g. comparison of the Standard method with alternative methods, testing the Standard at low tar concentrations) will be stimulated and co-ordinated in the framework of the project. With this project covering only the co-ordination cost, the actual R&D will have to be covered by funding from other sources.

In the second activity, the results from this project will be disseminated to ensure widespread acquaintance with the Standard. Dissemination is aimed at the companies, institutes and universities working in the field of biomass gasification. Dissemination will be performed by means of an Internet site, by using Internet mailing lists/discussion groups and by means of papers and presentations at Biomass conferences.