Biomass such as forest, agricultural and organic processing residues can be converted into commercial products via thermo-chemical processes [1
]. Biomass contributes ca.
14% of energy supply around the World, while in some developed countries, its contribution ranges from 40% to 50% [5
]. Because of its renewability, biomass is considered to be one of major potential energy sources in the future [6
]. Over the last two decades, special attention has been paid to the conversion of residual biomasses and renewable materials into fuels for power machines [9
]. Energy in biomass comes from solar energy. Biological conversion of low-value lignocelluloses biomass still faces challenges of low economy and efficiency [1
]. The characteristic of biomass is its huge amount and relative low energy content, which make any long distance transportation of biomasses to be uneconomical. Combustion, pyrolysis and gasification are three main thermo-chemical conversion methods used for biomass, which is traditionally combusted to supply heat and power for the process industry. The net efficiency for electricity generation from biomass combustion is usually low, ranging from 20% to 40% [2
]. The amount of biomass co-fired in existing combustors is usually limited to 5–10% of the total feedstock due to the concerns about plugging of the existing coal feed systems [3
]. Pyrolysis converts biomass to bio-oil in the absence of oxygen. The limited uses and difficulty in downstream processing of bio-oil have restricted the wide application of biomass pyrolysis technology [4
]. Besides the above mentioned thermo-chemical processes, gasification of biomass and its residues is a promising technology now and in the future. Gasification converts biomass through partial oxidation into a gaseous mixture of syngas (synthesis gas
) consisting of hydrogen (10–20%), carbon monoxide (15–30%), methane (2–4.5%), carbon dioxide (5–15%), nitrogen (45–60%) and water vapour (6–8%). In recent years biomass gasification has attracted huge interest by producing syngas which can be easily transported and utilized in different power facilities [12
]. Biomass gasification converts the intrinsic chemical energy in the biomass into combustible gas. The produced syngas can be standardized in its quality and it is easier and more versatile to use than the original biomass e.g., it can be used for inner combustion (IC) gas engines and gas turbines, or used as a chemical feedstock to produce liquid fuels and chemicals [14
The state of the art for gasification is the fixed bed gasifier with updraft and downdraft operation [15
]. Biomass snygas contains high concentrations of tarry compounds and particles, which must be purified before its utilization. The tars consist of a range of oxygenated hydrocarbons and hydro-carbons, typically containing aromatic, polyaromatic, and furanic backbone structures, with aliphatic and oxygenated functional groups (acids, aldehydes, ketones, and alcohols) attached to the backbone [16
]. These tars are notorious for condensing and subsequently polymerizing on downstream equipment such as compressors and IC gas engine surfaces if the gas is sufficiently cooled [8
]. The common facility for syngas utilization in Germany is the IC gas cogeneration engine [17
]. Its specification is an equivalent tar concentration lower than 50 mg/Nm3
. This value can only be achieved by enhanced gas purification, since the typical tar concentrations in syngas are always 0.1–6 g/Nm3
for downdraft gasifiers and 10–100 g/Nm3
for updraft gasifiers [17
Gas purification can be done by dry or wet processes. Cooling down below the dew point of the gaseous by products leads to condensates, mostly tar, which adheres to all inner surfaces of the system. A side effect of this process is an effective removal of dust particles. Wet systems are mainly absorption based processes where a solvent dissolves the tar products. The process is carried out in scrubber systems with parallel mist elimination [15
]. Both techniques have the same inevitable disadvantages: recovery of solvent and preparation boost cost. Tar condensates and eluant must be treated in order to avoid contamination.
This research work is an experimental feasibility study for a catalytic hydrocracking of tar by-products, including an increase of the calorific value of biomass syngas. Fuel feed is woody biomass. Palladium (Pa) is the catalyst for tar cracking and conversion. All processes are implemented in a reactor downstream of an updraft gasifier. Experiments and measurements are focused on the effects of catalyst temperatures and syngas flow rates. Sampling is done from raw gas and from gas after treatment. Samples are analyzed by flash-evaporation gas chromatography. The results show the efficiency of tar removal and conversion and an increase of high caloric value (HCV).