Extending the operational limit for fuel water content in a stratified downdraft gasifier from 15 to 22 m% by increasing the reactor height

Small-scale gasification of biomass combined with a gas engine and a generator (combined heat and power CHP) is an important technology, as it is one of the best ways to replace fossil fuels in electricity and heat generation. Small-scale gasification plants generally have strict fuel property requirements, which makes them economically unattractive.

In particular, the water content of the fuel is crucially important, as commercial plants usually only allow a water content of up to 15 m%. In this study, a simple approach was taken to extend the operational limit of a stratified downdraft gasifier by extending the reactor height. In turn, this extends the permissible range of fuel water content. In the experimental part of this study, the reactor with and without a reactor extension was fed with fuel with different water contents (8–24 m%). During operation, the bed temperatures are measured as a function of the reactor height. In addition, permanent gases (CO, CO₂, H₂, CH₄) were measured, and an online tar measurement was ensured. An optimal temperature profile in the gasifier and extremely low tar values in the product gas were determined for all fuel water contents investigated.

By taking this simple approach, the operating limits for the permissible water content of the fuel could be extended from 15 m% to 22 m%, allowing cheaper fuel to be used without significantly reducing the gas quality. These results will help to make small-scale biomass gasification more attractive and economical.