Tar is neither a chemical name for certain molecular species, nor a clearly defined terminology in materials. Tar has been operationally defined in gas­ification work as the material in the product stream that is condensable in the gasifier or in downstream processing steps or subsequent conversion devices and parts [45]. This physical definition is inevitably dependent upon the types of processes, the nature of the feedstock, and specifically appli­cable treatment conditions. Producer gases from both biomass gasification and coal gasification contain tars. The generalized composition of tars is mostly aromatic and the average molecular weight is fairly high. Even with the very same biomass feed, the amount and nature of tars formed are dif­ferent depending upon the gasifiers used and process conditions employed. Similarly, the same gasifier would generate different amounts and types of tars depending upon the feedstock properties and compositions. Therefore, successful implementation of efficient gasification technology depends on the effective control of tar formation reactions as well as the efficient removal or conversion of tar from the produced gas.

A number of investigators studied various aspects of tar in terms of its for­mation, maturation scheme, properties, molecular species, and relationship

between the tar yield and the reaction temperature. Elliott extensively reviewed the composition of biomass pyrolysis and gasifier tars from vari­ous gasification processes and proposed a tar maturation scheme [46], as shown in Figure 5.2.

Nickel-based catalysts are known to be effective in biomass gasification for tar reduction to produce synthesis gases, because of their relatively lower cost and good catalytic effects. Several different types of nickel-based cata­lysts for biomass gasification were reviewed with respect to tar reduction efficiency by Wu and Williams [47].

Several methods for the sampling and analysis of tar have been devel­oped. Most of these methods are based on the condensation of tar in a liquid phase or adsorption of tar on a solid material. The collected sam­ples are subsequently analyzed gravimetrically or by using a gas chro­matograph (GC). The SPA (solid-phase absorption) method was originally developed by KTH, Sweden, and according to the SPA method a gas sam­ple is passed through an amino-sorbent which collects all tar compounds [48]. The ensuing step is to use different solvents to collect aromatic and phenolic compounds separately. These compounds are then analyzed on a gas chromatograph and positive identification of the condensed mate­rial is achieved by a gas chromatograph-mass spectrometer (GC-MS). The tar amount determined by the GC analysis is called GC-detectable tar. Tar can also be analyzed gravimetrically and the gravimetrically determined tar is called gravimetric tar. Gravimetric tar is evaporation/distillation residue from particle-free solution(s) determined by gravimetric analy­sis. Both GC-detectable and gravimetric tar are reported in mg/m3. Both chromatographic and gravimetric determination of tar is based on the European Technical Specification, TC BT/TF 143 WI CSC 03002.4: 2004 (E), developed by Technical Committee CEN/BT/TF 143 "Measurement of Organic Contaminants (Tar) in Biomass Producer Gas" [49]. This technical specification is applicable to sampling and analysis of tars and particles in the concentration range between 1 mg/m3 to 300 g/m3 at all relevant sampling port conditions (0-900°C and 0.6-60 bars). The application of the

technical specification allows determination of four different analytical values [49]:

• The concentration of gravimetric tars in mg/m3

• The sum of the concentrations GC-detectable tars in mg/m3

• The concentration of individual organic compounds in mg/m3

• The concentration of particles in mg/m3