Search

It is important to know the limitations of the supercritical water gasification (SCWG) in terms of behavior of different biomasses, especially when determining whether SCWG is a suitable conversion process for a certain biomass. Ten different biomasses (eight different plant species, of which two were grown in two different sites) were processed to evaluate this aspect. Moist and dry, woody and grassy biomasses were gasified in the same experimental setup under similar conditions. Only small differences could be seen in the gasification experiments. The carbon gasification efficiency was 60.3 ± 5.1 %, the gas compositions were very similar. Solid deposits formed in all experiments in the same temperature zone of the reactor containing coke, salt building elements and heavy metals, sometimes leading to plugging. Nevertheless, an experimental duration of 6 h could be achieved for the dry biomasses. The experiment with the moist biomass Reed Canary Grass was ended early due to plugging of the feed tubing which is due to the different size reduction procedure for moist biomasses resulting in bigger biomass particles. This emphasizes the importance of sufficient size reduction prior to the experiment. Potassium addition as a homogeneous catalyst, in form of potassium hydroxide, has proven to be beneficial regarding gasification efficiency, but poses a threat regarding plugging due to salt deposits in the system.

The energetic assessment of biomass conversion processes is important for evaluating their application potential. Process energy efficiency is often evaluated based on simulation results for processes under idealized conditions. The present work uses data from a laboratory plant to perform an evaluation of the supercritical water gasification (SCWG). For this purpose, experiments were conducted with two types of feedstock, Reed Canary Grass and ethanol. Under the present lab-plant configuration, a cold gas efficiency of up to 79% could be reached, which is comparable to the gasification of biomass in fluidized-bed gasifiers or entrained-flow gasifiers. Based on the obtained data on the produced substances and their distribution, a scale up to pilot plant size was conducted. A model was derived from the available data to energetically assess different SCWG plant sizes based on real laboratory results. This model can be transferred to other feedstocks and other process designs to approximate the optimal size for the used biomass feedstock. The importance of heat recuperation in this process is described in detail based on pilot-scale data.