Shifting of the flame front in a small-scale commercial downdraft gasifier by water injection and exhaust gas recirculation

The present work investigates the enhancement of the load and fuel flexibility of biomass stratified downdraft gasifiers by introducing two novel water injection and exhaust gas recirculation methods. When downdraft gasifiers operate with a partial load or extremely dry fuel, the flame front moves upward. Therefore, these novel methods can be used to establish a secure operating point by shifting the hottest zone back into the desired area. Unlike previous research on this topic, the present work presents practically feasible methods that can be used in small-scale commercial gasifiers. To our knowledge, applying the exhaust gas of an internal combustion engine to shift the flame front in a downdraft gasifier is a highly novel method. The techniques are evaluated in a wood-chip driven, commercially available, 85-kW downdraft air gasifier. A gas analyzer and a flame ionization detector were used to determine the product gas quality. The results show, that (1) both methods are able to shift the flaming pyrolysis zone downward without worsening the gas quality. The water injection method even increased the LHV on dry basis due to intensified hydrogen production. The exhaust gas recirculation slightly reduced the fuel consumption and thus also the LHV of the product gas. Furthermore, (2) these methods are demonstrated to act rapidly, are completely reversible, and can thus be used to respond to rapid process changes; and (3) they can easily and practically be applied. Hence, the results are highly promising to enable a future load- and fuel-flexible operation of downdraft gasifiers.