Modelling and simulation of downdraft biomass gasifier: Issues and challenges

Highlights

• 1-D transient model of a biomass gasifier developed and validated against experimental results.

• Inconsistencies in kinetic parameters are found and traced back to original publications and verified for consistency.

• Multiple time scales encountered in the model are addressed.

• Range of specific surface area of char in literature – reconciled by lab scale testing on char from experimental gasifier.

• Two methods of ignition of gasifier modelled and compared, and the more realistic one chosen for use.

Abstract

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.

Introduction

As humanity gropes for sustainable solutions to the developmental problems of the world, biomass gasification technology emerges as a promising alternative for both thermal and power applications. While the gasification technology dates back to World War II, the understanding of science behind gasification is still a matter of research. The term “biomass gasification” is used to denote conversion of solid biomass to a gaseous fuel through a thermochemical route. The resulting gaseous fuel mixture consisting of CO, H₂ and small amounts of CH₄ is known as producer gas and its calorific value generally varies in the range of 4–6 MJ/Nm³ [1]. Downdraft gasifiers are widely used due to their high gasification efficiency, low tar content and hence cleaner quality of producer gas as compared to other gasifiers of same power rating [[2], [3], [4]].

Modelling of biomass gasification involves representation of chemical and physical phenomena constituting biomass gasification in mathematical form to predict the gasifier performance. Modelling can help in providing an insight into the factors affecting the gasifier performance. In the past four decades, a reasonably large volume of literature has been published on modelling and predicting the performance of gasification systems [1,[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. Most of these researchers have addressed the effect of one or more operating parameters on the performance of the gasifier with specific feedstock using either steady state or transient formulation.

The phenomena that are observed in a biomass gasifier, viz., drying, pyrolysis, oxidation and reduction reactions, flow of gases and solids and heat transfer within solid bed and between solid and gas, are transient in nature. Most of the past studies focused on steady state modelling rather than transient modelling. Though after initial ignition, the gasifier may be considered to be in a pseudo-steady state, to capture the factors which may hinder ignition, transient modelling becomes important. Most researchers presenting transient models do not give details of how the ignition is simulated and hence to identify a suitable method for the same can be a challenge in itself.

In terms of modelling, a steady-state model is simpler to handle, while transient one is more challenging, owing to the widely different time scales of the phenomena involved. While biomass drying and flow of solids are slow phenomena, chemical reactions of oxidation are extremely fast, and fluid flow and heat transfer phenomena have a time scale in between the two. The orders of magnitude of the time scales can vary from a few seconds to 10⁻¹⁰ s leading to a set of stiff equations which are also highly non-linear and hence need a special solver.

Transient modelling also requires the chemical reactions to be modelled using the kinetic formulation. Using global reaction mechanisms with limited number of species is relatively easier than dealing with hundreds of reactions involving several intermediate radicals. However, selection of the appropriate set of global reactions is not easy since in literature, there is some variation in the set of reactions used. For many of the reactions, the kinetic data used by different researchers is not consistent. Thus, this aspect poses its own challenges.

Besides the need for consistent kinetic constants, modelling of heterogenous reactions is very critical due to a very strong dependence of reaction rate on char surface area. The literature presents a wide range of values for this parameter and identifying the suitable value for the conditions under consideration is non-trivial.

Since the gasifier operates at high temperatures of more than 1000 K, radiation becomes a very important mode of heat transfer. However, many researchers do not refer to use of radiation modelling. Again, choice of an appropriate model for radiation and the grid required for grid independence are questions one has to grapple with.

Thus, even 1-D modelling of a complex system such as a gasifier poses several challenges and issues which need to be addressed appropriately. The focus of the present work is to highlight each of these issues and how they were resolved by the authors while developing a 1D transient model of a downdraft biomass gasifier. The overall model of the system involves modelling each phenomenon. Thus, in this article, each of the issues mentioned above has been explained in the context of the relevant model. While challenges were faced at various stages, the major issues vis-à-vis literature emerged in modelling of thermochemical phenomena. For comprehensive understanding of the context and the issues, section 2 presents the summary of the mathematical model and modelling of the phenomena other than the thermochemical reactions highlighting the challenges wherever they are applicable. Section 3 presents the modelling of thermochemical phenomena with the corresponding issues along with the approach used by the authors to resolve those issues.