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Tar & tar measurement
In general, producer gas from biomass gasifiers will contain tar. This tar forms a serious problem for the use of the gas in e.g. engines, turbines, fuel cells, syngas production, etc. Successful implementation of gasification technology depends on the effective and efficient removal and/or conversion of tar from the gas. As a result, there exists a strong need to quantify tar concentration for effective R&D work, successful demonstration and commercialization of biomass gasification plants
Tar sampling & analysis methods
Over the last decades several methods for the sampling and analysis of tars have been developed. Most methods are based on condensation in a liquid or adsorption on a solid material. Subsequently, the collected samples are analyzed gravimetrically or by means of a GC. Off-line methods used by BTG are the SPA method and Gravimetric analysis.
SPA Method The Solid Phase Absorption (SPA) technique developed in the 1990s at KTH, Sweden. A gas sample is sucked through an amino-sorbent collecting all tar compounds. Afterwards, the aromatic and phenolic compounds are collected separately and analysed by using different solvents and a GC. Although the sampling is quite easy and fast, the total analysis may take a few hours.
Gravimetric tar determination - The International CEN/BT/TF143 Standard “Biomass Gasification — Tar and Particles in Product Gases — Sampling and Analysis”
Development of an On-line Tar Analysis Method
By more advanced application of producergas in various applications, it is desired to have an on-line method to determine tar content for both research and implementation activities. A real continuous on-line measuring method for tar determination has the following advantages:
- methods with long response times are not very helpful for enhancing process performance; they give no momentary values, but an average over a certain sampling time
- a proper online method giving a continuous signal of the tar concentration allows monitoring and controlling the process and safeguarding equipment.
- it will enable to directly link the tar production to the gasifier performance and could therefore be helpful in research and development work.
- the method is cheap - both in investment and operation
- because there is no need for laborious sample treatment and analysis. It is expected that the analysers itself will cost around 10,000 euro
- it will contribute to and enhance the development of low tar gasification concepts.
The first semi-online method was developed by IVD-Stuttgart. This “online” analyzer is based on the comparison of the total hydrocarbon content of the hot gas and the gas with all tars removed. Sample loops with a defined volume and high temperature switching valves are used to guarantee high accuracy. Tar contents between 200 and 20 000 mg/m³ can be measured with a high reliably. The product gas sampling and tar analysis time is around 60-90 seconds for one run. Nowadays, the system is commercially available, but is further developed at IFK. The system is in particular suitable for relative high tar contents in the gas phase.
In 2005, BTG and KTH started the development of a real on-line tar measurement method based on Photo-Ionization (PID). This method is suitable for low concentration.
On line tar analysis based on Photo-ionisation (PID)
BTG and KTH started the development of a new on-line tar measurement technique based on the principle of photo-ionisation. Basically, the energy of a photon is used to release an electron from a molecule. The energy required to release an electron depends on the specific compound. Generally, it is easy (i.e. low energy) to remove an electron from large molecules, and in particular from aromatic structures (ring stabilization); a high energy input is required to remove an electron from small molecules like H2, N2 and CO.
Specific advantages and features of the PID
The online tar measuring method has the following advantages:
- The method can be used for process control; the response time of other off-line methods is too slow for this purpose
- An online method can be used as a safeguarding tool for expensive process equipment and/or prime movers
- It will enable to direct link the tar production with the gasifier performance and therefore assist research and development work on biomass gasification to determine the performance of the gasifier and/or gas cleaning device(s). Multiple PID analyzers can be used
- The method is relatively cheap – both in investment and operation – mainly because there is no need for laborious sample treatment and analyses like in the existing methods. The PID tar analyser is still under development and it is estimated that the analyser will cost about 10,000 - 15,000 Euro.