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Effect of Hydrothermal Carbonisation on the Combustion and Gasification Behaviour of Agricultural Residues and Macroalgae: Devolatilisation Characteristics and Char Reactivity Daniel James Lane, Ewan Truong, Francesca Larizza, Rocky de Nys, and Philip J. van Eyk Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03125 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017
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Energy & Fuels
Effect of Hydrothermal Carbonisation on the Combustion and Gasification Behaviour of Agricultural Residues and Macroalgae: Devolatilisation Characteristics and Char Reactivity Daniel J. Lane1, Ewan Truong1, Francesca Larizza1, Rocky de Nys2, and Philip J. van Eyk1* 1
School of Chemical Engineering, The University of Adelaide, Adelaide, SA, 5005, Australia
2
MACRO – The Centre for Macroalgal Resources and Biotechnology, College of Science and
Engineering, James Cook University, Townsville, Qld, 4811, Australia *Corresponding author E-mail address:
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ABSTRACT
Hydrothermal carbonisation (HTC) can potentially improve the fuel quality of low-value biomass resources which are otherwise unsuitable for use in industrial heat and power applications. The effect of HTC pre-treatment on the combustion and gasification behaviour of two agricultural residues (grape marc and sugar cane bagasse) and a freshwater species of macroalgae was investigated, with emphasis on devolatilisation behaviour and char reactivity. HTC was carried out in a custom-built, laboratory-scale, batch reactor at three temperatures, 180, 220, and 260 °C, with a slurry density of 15 % w/w dry biomass in water. The volatile release behaviour of the collected products (hydrochars) and untreated biomass feedstocks were characterised by dynamic thermogravimetric analysis. Char reactivity was characterised by isothermal gasification of samples of the hydrochars and untreated biomass in a thermobalance in carbon dioxide, following fast pyrolysis of the samples in a separate fixed-bed, batch reactor. Hydrochars were more energy dense and contained lower concentrations of catalytic metals, particularly K and Na, than the untreated biomass feedstocks. HTC caused a significant reduction in the total release of volatiles and an increase in the yield of char during devolatilisation. The bulk of the volatile matter was released at higher devolatilisation temperatures for the hydrochars. These trends became more pronounced with increasing HTC temperature from 180 to 260 °C. The char components of grape marc and macroalgae became substantially less reactive following HTC. The char component of bagasse became more reactive following HTC at 180 °C but became less reactive following HTC at 220 and 260 °C. Activation energies for char gasification of HTC treated (220 °C) grape marc, bagasse, and macroalgae in CO2, were 177, 247, and 282 kJ mol-1 respectively. These values are 92%, 10%, and 52% greater than the values for the untreated biomass.
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1. INTRODUCTION Growing pressure to establish fuel security and ongoing global warming have brought recent attention to renewable sources of energy. Biomass is currently the most used source of renewable energy and its use is projected to continue to displace that of non-renewable fossil fuels in the future1. Combustion and gasification technologies have long been used to convert biomass to energy and fuels, and are relatively well established with many large-scale installations in operation worldwide. However, the types of biomass used in these processes today (e.g. wood and dedicated energy crops) are limited in availability and there are sustainability issues associated with their supply on an industrial scale2. This has motivated research into the utilisation of alternate sources of biomass in these thermochemical processes, particularly macroalgae and agricultural residues such as bagasse (sugar cane trash) and grape marc3-5. These biomass resources all have several advantages as an energy source. Grape marc and bagasse are both available at low-cost and in vast quantities within various agricultural regions around the world. In Australia alone, of the order of 11 million tonnes of bagasse and 170 thousand tonnes of grape marc are generated each year6, 7. Importantly, these two materials are generated at central locations and therefore do not need to be collected or harvested. Macroalgal biomass can be produced with high areal energy yields, with productivities as high as 70 dry tonnes ha-1 year-1 commonly reported in the literature8. This value compares favourably to values typically reported for wood (5 – 15 dry tonnes ha-1 year-1) and for dedicated energy crops (8 – 30 dry tonnes ha-1 year-1)
9, 10
. The cultivation of macroalgae can be carried out in both freshwater
and seawater based production systems11 as well as various types of contaminated wastewater12. This means that the production of macroalgae can be situated on non-arable land, which limits direct competition with the production of food crops, a major drawback of the production of
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wood and dedicated energy crops2. Despite these advantages, grape marc, bagasse, and macroalgae all remain underutilised in combustion and gasification processes. This is largely due to the low energy densities and unfavourable handling and transport characteristics of these materials. Furthermore, grape marc and macroalgae tend to contain high levels of certain inorganic impurities which can lead to operational problems during their conversion5, 13, 14. This may deter plant operators from feeding these materials into their combustion or gasification reactors. In order to increase the utilisation of grape marc, bagasse, and macroalgae in combustion and gasification processes some level of pre-treatment is required which improves the fuel quality of these materials. Hydrothermal carbonisation (HTC), involving the reaction of biomass in hot compressed water at temperatures and pressures typically within the ranges of 180 – 260 °C and 20 – 80 bar, is a rapidly developing pre-treatment technology for low-quality biomass fuels. Unlike other pretreatment technologies such as dry torrefaction, HTC is well suited to wet forms of biomass15 such as grape marc, bagasse, and macroalgae. Numerous studies have compared various structural and chemical properties of the solid product of HTC (referred to as “hydrochar”) with that of the untreated biomass16-18. It has been shown that HTC increases the energy density, grindability, and hydrophobicity of a diverse range of biomass materials16-19 to a greater extent than dry torrefaction17, 18. These changes reduce costs associated with biomass storage, handling, and transport. Several studies17,
20-23
have reported significant extractions of various inorganic
impurities during HTC, particularly K, Na, Cl, S, and P. It is well known that these elements are intimately connected with major operational issues in industrial combustion and gasification systems, including fouling, deposition, corrosion, and in the case of fluidised bed technologies, bed particle agglomeration24-26. A reduction in the concentrations of K, Na, Cl, S, and P can be
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expected to alleviate these issues and therefore promote smooth, uninterrupted combustion or gasification. It is clear that HTC causes several beneficial changes to the physiochemical properties of biomass materials. However, there is still limited understanding of the impact of these changes on the resultant combustion and gasification behaviour of the formed hydrochars. When a biomass particle is first introduced into an industrial combustor or gasifier the particle dries and devolatilises, forming a residual char. Proper characterisation of the release of volatiles is essential for effective design and operation of industrial reactors, especially with respect to the design of the biomass feeding system, reactor configuration, and distribution of combustion air (or gasification agent)27. A few recent studies have investigated the impact of HTC pre-treatment on the devolatilisation characteristics of wood18, 28-30 and microalgae31. In these studies, HTC was carried out in batch reactors and the devolatilisation behaviour of the formed hydrochars characterised by dynamic heating of the hydrochars in a thermobalance in inert gas atmospheres. Clear changes to the devolatilisation behaviour were reported following HTC. In all cases HTC pre-treatment resulted in a reduction in the total amount of volatiles released during devolatilisation. The peak rate and peak temperature of devolatilisation was also affected by HTC but in different ways depending not only on the HTC temperature but also on the biomass type. Given that grape marc, bagasse, and macroalgae all have different chemical compositions to that of wood and microalgae it could be expected that HTC pre-treatment will influence their resultant devolatilisation characteristics differently. Thus, further experiments are needed to determine the impact of HTC on the devolatilisation characteristics of these biomass materials. The solid char residue which remains after devolatilisation is either oxidised or reduced depending on the local gas atmosphere through heterogenous reactions between the char residue and gas phase reactants. This reaction is the slowest step in industrial combustion and
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gasification processes, and therefore, generally determines the total conversion of carbon in practical systems. For this reason there have been numerous studies on the reactivity of coal32-34 and biomass chars35-38. These studies have shown char reactivity to depend on the physical and chemical properties of the char which in turn depend on the inherent properties of the fuel39 as well as on the devolatilisation conditions40. Certain inorganic elements are known to have a significant impact on char reactivity. There is strong consensus in the literature that the alkali metals, K and Na, catalyse char reactivity. The alkaline earth metals, Ca and Mg, are understood to catalyse char reactivity, but to a lesser extent than the alkali metals37,
38
. Silicon has been
reported to reduce char reactivity by forming inactive alkali silicates37. The overall catalytic effect of the inorganic matter likely depends on the relative proportions of inorganic elements in the char. These proportions will almost certainly be different for hydrochars than for untreated biomass, given that the inorganic elements are extracted to different extents during HTC20, 21. Despite this knowledge, few studies have investigated the impact of HTC on the reactivity of biomass chars. Bach et al. recently published a series of papers on the effect of HTC pretreatment on the conversion of microalgae41, wood29,
42
and wood-derived biomass43 during
combustion in air by means of dynamic heating of samples of hydrochar and untreated biomass in a thermobalance, at slow, controlled heating rates (
