Comparative assessment of process stability in a stratified downdraft gasifier under different operating strategies

Stratified downdraft gasifiers are a promising technology for decentralized biomass-based combined heat and power generation, but their operation is strongly affected by fuel properties and operating conditions. While several measures have previously been proposed to extend the operable range of such systems, their influence on process stability has not yet been systematically compared. In this work, the process stability of a commercial stratified downdraft gasifier was assessed under different operating strategies, namely reactor extension, air preheating, exhaust gas recirculation (EGR), and load modulation. The analysis combines a stability-focused re-evaluation of previously published datasets with newly generated load-modulation experiments. Stability was quantified using the temporal standard deviations of the air mass flow rate, total hydrocarbon content ( T H C ), and summarized bed temperature fluctuations. The results show that the investigated strategies influence process stability through different mechanisms. For wet fuel operation, increasing the fuel bed height by reactor extension proved to be the most effective stabilization strategy, as it reduced fluctuations of air flow from 1.5 kg h − 1 to 1.2 kg h − 1 , hydrocarbons, and bed temperatures. Air preheating enabled operation with wetter fuels as well, but acted mainly as a compensatory measure and did not improve stability to the same level, as it increased T H C fluctuations from 2000 ppmv w . b . to up to 4000 ppmv w . b . . For very dry fuels, EGR improved the process stability and restored the thermal behavior of the reactor to levels comparable to near-optimal fuel conditions. Load modulation affected the process differently: reduced load decreased short-term fluctuations of the air mass flow rate and hydrocarbon content, indicating a calmer gasification regime. Overall, the results provide a comparative stability assessment of relevant operating strategies for stratified downdraft gasification and show that stable operation over a broad range of fuel properties requires different stabilization approaches.