Utilization of char from biomass gasification for onsite energy production – Introduction and modelling of a secondary reactor

Gasification of biomass as a technological option for energy conversion is a concept that is widely discussed and taken into consideration. Due to financial incentives, i.e. increased feed – in tariffs, several small scale gasification units have been developed in Europe. In principle, the two main products of gasification are syngas, which is a gaseous fuel and char, which is a solid fraction which results from pyrolysis of carbon based materials. Char may have carbon content between 50% and 80% and has significant calorific value. It is the case that small gasification units tend to produce a considerable amount of char that can be as high as 10 % of the initial mass of the feedstock. Several potential pathways for the utilization of char have been identified, like manufacturing of filters or application on agricultural fields. Nonetheless, due to the scale restrictions of small scale units, organized strategies have not yet been developed. In addition, char contains heavy metals and PAHs in such concentrations that the utilization as material is not always possible. Thus, usually char is treated as a waste/ byproduct that needs proper disposal. Last but not least, gasification units demand electricity for the operation of the auxiliary units. This paper has the scope to investigate the possibility of onsite energy production of additional electricity by introducing a secondary reactor that would convert char to a gaseous fuel. The two streams entering the reactor would be the flue gases from the internal combustion engine and the char which is created as a by-product from the process of gasification. This secondary reactor may be applied in all types of small scale gasifiers, but the optimal condition is when it is applied on a downdraft gasifier. The conversion of the input streams to a gaseous fuel will propagate by means of char-gas reactions mainly the Boudouard reaction and the water-gas reaction. The produced gaseous fuel will be consisted of Hydrogen, Carbon Monoxide, Carbon Dioxide and Nitrogen. This fuel can be utilized for electricity production in a smaller internal combustion engine which can be located in the facility.