This study examined the tar characteristic of wood pellet using a downdraft gasifier system. The wood pellet was used as a feedstock with a variation of moisture content between 2 to 6 wt. %. Tar sample was taken using the JP 2009-40885 method. Gas chromatography mass spectrometry and gravimetric analysis were used to identify and to analyze the tar sample, respectively. The results show that an increase in moisture content slightly decreased the total concentration of tar formation on the nine tar compounds of the biomass. Additionally, the lowest moisture content produced the highest amount of light tar on tar classification. The amount of light tar indicates the ease of ignition in the gasifier. Therefore, this condition will minimize the failure-of-equipment problem because of no clogs in the gasifier equipment. The lowest moisture content had the lowest tar density, and this minimized the deposit of tar in the gasifier. Therefore, the lowest moisture content had low tar production in the biomass gasification process using wood pellets.

To convert woody biomass into electricity with small capacity and high efficiency, it is reasonable to gasify the woody biomass and operate a reciprocating engine. On the other hand, bio-syngas produced from woody biomass have a low calorific value. Therefore, if the concepts of the thermal efficiency and the losses (the cooling loss, the combustion loss, the pumping loss, the exhaust loss, etc.) obtained by conventional fossil fuels are applied as these are, it is not possible to predict the thermal efficiency appropriately. In this paper, the bio-syngas as the reference is produced in a self-made gasification system, and the thermal efficiency and the losses of the reciprocating engine are experimentally examined to evaluate the significant effect of combustible components on these indicators. The combustible components are hydrogen, methane, and carbon monoxide. The nitrogen fraction in the bio-syngas is adjusted to control the low heating value. A commercially available small general-purpose spark-ignition reciprocating engine (GP-SIRE) was modified and used as the experimental engine. The results showed that (1) the operation with the referenced bio-syngas exhibits higher thermal efficiency (more than 10 points) than that with the conventional methane, and that (2) there is a trade-off between the combustion loss and the cooling loss.

In order to effectively utilize woody biomass, which has a low abundance density, it is necessary to develop a power generation system that can convert it with high efficiency even with a small capacity as less than 2 MW. For electricity generation, it is reasonable to use a small reciprocating engine. In the case of a naturally aspirated spark ignition reciprocating engine (SIRE), the amount of aspirated gas in one cycle is determined almost entirely by the displacement. The thermal efficiency of the SIRE generally increases with the power. Therefore, to improve the thermal efficiency, it is effective to make the low heating value (LHV) of the fuel higher to increase the power of the naturally aspirated SIRE. In this paper, three methods are used to increase the LHV of the bio-syngas: 1) reducing the nitrogen density of the bio-syngas (upgrade bio-syngas), 2) adding hydrogen to the bio-syngas, and 3) adding methane to the bio-syngas. Using these fuels, 1) the conditions for high power, and 2) the costs assumed for each condition, are evaluated through experiments and estimates. The results showed that the upgrade bio-syngas, obtained by gasification with oxygen-enriched air, had the highest power and the best cost-effectiveness.

In order to exchange the wood biomass energy for electric power with small capacity and high efficiency, it is most effective to use a reciprocating engine operated with a wood gasifier. On the other hand, such a small-capacity system is often installed in urban areas. Therefore, strict emission regulation should be observed. Normally, as the low heating value (LHV) of bio-syngas is small, the engine should be operated with a stoichiometric mixture to achieve a maximum power density. However, the emission with a stoichiometric mixture contains much unburned CO. This means that a stoichiometric mixture operation shows low efficiency and can’t observe the regulations. In this report, a mechanism of the unburned CO is considered, and a method to reduce the unburned CO ratio is shown with experimental results. In the experiment, a commercial reciprocating engine (4-stroke, modified single cylinder) is used. The bio-syngas, a producer gas from a fixed bed gasifier, is produced by a self-made wood pellet gasifier (fixed bed, auto thermal down-draft). The bio-syngas flow rate is calculated with the nitrogen ratio between input air and bio-syngas. The LHV is adjusted with the city gas (as an alternative to methane) and hydrogen. The CO volume ratio of the exhaust from the engine is more than 3 v% when the excess air ratio of bio-syngas/air mixture is 1.3, as the LHV of bio-syngas is less than 5.0 MJ/m³-LHV. On the other hand, the CO volume ratio of the exhaust under operation of the mixture, the bio-syngas, and methane with more than 7.0 MJ/m³-LHV was less than 0.2 v%. The CO in the exhaust with low LHV fuel means that the combustion is not finished in the chamber. The unburned ratio could be predicted in consideration of the gap/clearance as crevice, the temperature boundary layer, and the quenching distance.