Operational experience with miscanthus feedstock at the bioliq® fast pyrolysis plant

Abstract

Fast pyrolysis is a thermochemical process for direct liquefaction of lignocellulosic biomass that offers flexibility in terms of feedstock variability since all biomass macromolecules are converted. In contrast to this expectation, it is also obvious that there are limits to feedstock flexibility once a fast pyrolysis plant has been built. This relates to the simple fact that any pretreatment is to a certain extent specific for one type of biomass, but also to the observation that liquid yield can vary widely depending on the feed. Different bio-oil yields directly affect heat management and volume flows in an installation. Laboratory equipment has very limited suitability to assess flexibility of a large scale implementation. To address this shortcoming, a change in feedstock with significantly different bio-oil yield was conducted in the bioliq® pilot unit that was designed for wheat straw with a feedstock capacity of 500 kg h−1. Miscanthus was used in two experimental campaigns that each lasted two weeks. While steady state operation succeeded, feedstock capacity had to be reduced significantly to around 200 kg h−1 to allow for a better control of pressure fluctuations in the hot gas section. An evaluation of volume flow rates reveals that these fluctuations are not due to an increased volatile yield. It is concluded that plugging issues caused by an increased reactivity of the produced volatiles likely caused the observations and limitations to plant capacity. The specific heat demand for the reactor of the fast pyrolysis unit was estimated to be approx. 0.83 and 0.76 MJ kg−1 for both miscanthus campaigns and ca. 0.67 MJ kg−1 for wheat straw. Heat supply to the reactor was not a limiting factor when changing the feedstock because the low feedstock rate outweighed the increase in specific heat demand. The yield distribution from miscanthus has been compared to experiments conducted in parallel in a smaller process development unit with a feedstock capacity of 10 kg h−1. The results impressively show the influence of ash content on the yield distribution and also the importance to focus on (earth) alkali metals to better describe their catalytic activity during biomass pyrolysis.