Lack of synergetic effects in the pyrolytic characteristics of woody biomass/coal blends under low and high heating rate regimes

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Abstract

Pyrolytic behaviours of woody biomass/coal mixtures were investigated under both low and high heating rate conditions over a range of temperatures between 200°C and 1400°C. Results obtained from this comprehensive investigation indicated that the pyrolytic characteristics of the mixtures follow those of the parent materials in an additive manner. Therefore, under inert (non-oxidising) conditions the two fuels undergo independent thermal conversion without any chemical interactions. As such, the yield of the major pyrolysis products (e.g. volatiles and char) is proportional to the percentage of woody biomass and coal in the mixture. This confirms the hypothesis made by a number of researchers about the lack of synergistic effects in the yield of pyrolysis products from blended coal and woody biomass. However, in this study, we show that even the compositions of the gaseous products from blended samples are linearly proportional to those of their parent fuels (lack of synergistic effects). These findings can potentially help to understand and predict the behaviour of woody biomass/coal blends in practical combustion systems.

Introduction

Fears of climate change and increasing concern over the global warming have prompted a search for new, cleaner methods of electrical power generation. Co-firing of biomass fuels (e.g. wood chips, straw, bagasse, peat and municipal solid waste) and coal is presently being considered as an effective means of reducing the global CO2 emissions [1], [2], [3], [4], [5], [6], [7], [8]. The rationale is that the photosynthesis of growing plants removes from the atmosphere, the CO2 generated from combustion processes and subsequently reduces the atmospheric build-up of carbon dioxide. The co-firing technology has attracted much attention in recent years and as a result, demonstrations have been performed in a number of utility installations across the world, including Australia, Europe and the United States. All types of combustion technologies have been used but a particular attention has been given to pulverised fuel (PF) boilers, as they constitute the bulk of power generation hardware in many countries. Despite the simplicity of the co-firing concept, its application in PF boilers is associated with many technical issues, which remain unresolved at least in part [2]. These issues can be categorised into four major groups [1], [2], [6]: (a) handling related issues, for instance, poor pulverisation or unwanted fires in pulveriser units; (b) combustion related issues, such as, flame stability; (c) ash-related issues, and (d) emissions. Despite recent progress there is still no fundamental understanding of the underlying mechanisms that cause such technical problems [2], [6]. Studies on fundamental issues such as pyrolytic behaviour of biomass/coal blends or biomass char reactivity are scarce.

Previous investigations on co-pyrolysis of biomass/coal mixtures have mostly concentrated on the mechanism of production of the gas-phase species. There has also been a handful of studies [9], [10], [11], [12] on the impact of synergistic effects (i.e. chemical interaction between the two fuels) on the yield of major pyrolysis products, in particular, volatile matter. However, much less attention has been given to the influence of synergistic effects on the composition of the pyrolysis products. In addition, most previous studies on synergistic effects focused on examining the impact of various controlling parameters (e.g. heating rate, temperature, blending ratio, etc.) in isolation.

As a result, these studies were not generally conclusive and often led to conflicting claims, for instant, with regards to the co-pyrolysis of sewage sludge/coal [10] and coffee/coal [9].

The objective of this study was to address these shortcomings through a comprehensive investigation of the pyrolytic behaviour of woody biomass/coal blends over a wide range of heating rates and temperatures relevant to PF boilers. The fundamental knowledge gained from this project is essential for the proper understanding of practical PF-based systems as experience has shown that development of good practical solutions can be hampered by not drawing upon fundamental knowledge. For example, the knowledge of the low heating rate pyrolysis of biomass/coal mixtures may help to prevent the accidental fires, which sometimes occur in fuel handling units (e.g. mills or pulverisers) of typical PF boilers during co-firing exercises [14], [15]. On the other hand, the knowledge of the high heating rate pyrolysis of biomass/coal blends, may help to understand and predict the impact of co-firing on the combustion-related phenomena in PF boilers (e.g. fuel ignition, flame stability, flame temperature, flame geometry, etc.).

Section snippets

Experimental

Two sets of experiments were carried out to investigate the pyrolytic behaviour of biomass/coal blends under the low and high heating rate conditions, respectively. The low heating rate experiments were conducted to understand the pyrolytic characteristics of coal, woody biomass, and their mixtures under conditions pertinent to the pulveriser units (i.e. mills) of typical PF boilers. That is a heating rate between 10°C/min and 50°C/min and a temperature between 200°C and 400°C. The high heating

Pyrolytic behaviours of individual fuel components

Fig. 3 demonstrates the production behaviour of volatiles produced from coal and woody biomass over a furnace temperature range of 200–1000°C under the low heating rate condition of 10°C/min. It can be seen that the devolatilisation process of the coal and sawdust under inert conditions can be divided into three stages. In the first stage (temperatures <400°C for coal and 300°C for biomass), the primary volatiles are formed. At this stage, hemicellulose which constitutes approximately 20–30%

Conclusions

Two sets of experimental investigations were conducted to examine the role and the relative importance of synergistic effects, if any, during the co-pyrolysis of woody biomass/coal blends under the low and high heating rate regimes. The experiments were carried out under conditions pertinent to typical PF boilers.

The results obtained from this study clearly show that under the non-oxidising (inert) conditions, the two fuels undergo independent thermal conversion without any chemical

Acknowledgements

The authors wish to acknowledge the financial support provided by the Coorperative Research Centre for Coal in Sustainable Development, which is funded in part by the Coorperative Research Centres Program of the Commonwealth Government of Australia.

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