Modelling and simulation of downdraft biomass gasifier: Issues and challenges

Modelling of a downdraft biomass gasifier involves modelling of several phenomena such as drying, pyrolysis, thermochemical reactions, fluid and solid flow and heat transfer. Transient modelling of such a gasifier taken up in the present work is more challenging due to widely different time scales of the phenomena leading to stiff equations which are highly non-linear. Transient modelling also requires selection of suitable global reactions and their kinetic constants. The kinetic data presented by different researchers for several of the important reactions were found to have inconsistencies in values and units, which posed a major challenge and had to be resolved through careful tracing back of the data to original references. There are also uncertainties in the value of specific surface area of char, which is a very important factor in modelling rates of heterogenous reactions. Most of the issues and challenges faced in transient modelling of a gasifier have not been discussed in the literature. The method of igniting the gasifier for transient simulation is another aspect on which literature is silent, as is the case with modelling radiation in the packed bed of such gasifiers. The present work highlights these issues and challenges in the context of development of a 1D transient model of a downdraft biomass gasifier along with a discussion on the approach used for addressing them. The simulations from the model developed with careful choice of information from the literature show good agreement with experimental results reported in the literature.

Automated summary

Modelling of a downdraft biomass gasifier involves complex phenomena such as different time scales, inconsistencies in kinetic data, uncertainties in specific surface area, etc. Challenges in transient modelling, including the lack of discussion on ignition methods and radiation modelling, have not been fully addressed.