Mallee wood fast pyrolysis: Effects of alkali and alkaline earth metallic species on the yield and composition of bio-oil
Highlights
► Removing alkali and alkaline earth metals from wood does not affect bio-oil yield. ► However, bio-oil composition is affected by these inorganic species in biomass. ► Acid-soluble inorganic species are closely involved in pyrolysis reactions.
Introduction
Biomass is among the cheapest renewable energy sources [1]. It is also the only renewable resource that can be used directly to produce liquid fuels. Thermochemical conversion is commonly considered as the best technology route for the production of second generation biofuels and chemicals using lignocellulosic biomass. The pyrolysis of biomass would produce a liquid bio-oil product [2], [3], [4], [5]. While bio-oil may find other uses [4], [5], the upgrading of bio-oil to produce liquid biofuels and chemicals has attracted much attention. In addition to bio-oil, the pyrolysis of biomass also produces bio-char for carbon bio-sequestration and soil conditioning.
Both pyrolysis conditions and biomass feedstock properties can affect the yields and properties of bio-oil [2], [3], [4], [5]. A large number of studies [4], [5] have been carried out to investigate the effects of heating rate, temperature and holding time on the yields of bio-oil. As part of our long-term efforts to develop technologies for the production of biofuels and chemicals from mallee biomass that has been planted to control dryland salinity in Western Australia [6], [7], we have investigated [8], [9], [10] the pyrolysis of mallee biomass in a fluidised-bed pyrolyser under fast heating rate conditions. Pyrolysis temperature is one of the most important parameters influencing the yield of bio-oil from mallee wood [8]. As an important feedstock property, increasing mallee wood particle size led to a significant reduction in bio-oil yield and an increase in the water content of the produced bio-oil [10].
An important feature of biomass is the presence of finely dispersed inorganic species, especially alkali and alkaline earth metallic (AAEM) species, e.g. as carboxylates and KCl in its structure. Removing AAEM species can increase the bio-oil yield [11], [12], [13], [14]. These inorganic species in biomass, especially AAEM species, can also volatilise [15], [16], [17], [18], [19], even at low temperatures, and finally be condensed together with organic vapour as part of the bio-oil product.
The difficulties and thus the costs of bio-oil upgrading in the production of liquid biofuels and chemicals depend largely on the quality of bio-oil produced during pyrolysis. Indeed, bio-oil properties [9] are as important a consideration as the yield of bio-oil itself. The composition of bio-oil also largely determines the ultimate use of bio-oil. While high concentrations of hydrolysable sugars in the bio-oil may be favourable for the production of bio-ethanol through fermentation [20], high yields of mono-phenols will make the bio-oil an attractive feedstock to produce adhesives [21]. The bio-oil composition is clearly the most important consideration in optimising the conditions for the hydroprocessing of bio-oil [22] to produce hydrocarbon-based liquid fuels. The presence of inorganic species in bio-oil also greatly influences the subsequent use of bio-oil. For example, the AAEM species are also associated with the accelerated ageing of bio-oil [19], [23], thus contributing to modifications of the physical properties of bio-oil during storage.
To reduce the contents of AAEM species in bio-oil, one can either remove the AAEM species from the vapours before condensation (e.g. using a hot filtration technique [19]) or remove the AAEM from the biomass itself. Washing biomass with water and acids [12], [14], [24], [25] can be an effective way to achieve substantial removal of AAEM species from biomass. While it is well established that the removal of AAEM species from biomass can increase the overall bio-oil yield [11], [12], [13], [14], the effects of removing AAEM on the composition/properties of bio-oil is still imperfectly understood.
As the continuation of our efforts to understand the effects of process variables and feedstock characteristics on the yield and quality of bio-oils from mallee [8], [9], [10], this study aims at investigating the changes in bio-oil composition and properties with the extent of the removal of AAEM species from mallee wood prior to pyrolysis. The removal of AAEM species from wood in this study is not necessarily meant to be an economic means of biomass pre-treatment. Instead, it is carried out as a part of fundamental research to understand the pyrolysis behaviour of woody biomass whose AAEM contents may change from one species to another or from one region to another.
Section snippets
Biomass sample
The woody fraction of mallee Eucalyptus loxophleba (ssp. lissophloia) used as feedstock was provided by the Western Australian Department of Environment and Conservation. Trunks were shredded to coarse particles before being milled to smaller particle sizes with a Fritsch Laboratory Cutting Mill “Pulverizette 15”. The obtained biomass was then sieved and particles ranging from 180 to 425 μm were conserved at −9 °C to avoid microbial degradation prior to use. All biomass samples were dried in an
Removal of AAEM species
Fig. 1 shows the contents of AAEM species in the mallee wood before and after washing with water and acid for various lengths of time. Calcium was by far the most abundant AAEM species in the mallee wood. Water washing treatments, regardless of their length, were generally quite successful at removing large fractions of the AAEM species, with observed reductions of 50 to nearly 100% (see Fig. 1). In particular, the mono-valent K was especially easy to be removed by water washing. However, the
Conclusions
The washing of a mallee wood sample with water and a dilute acid has shown that AAEM species exist in the wood in two different forms: water-soluble and water-insoluble but acid-soluble. The pyrolysis of the washed wood in a fluidised-bed reactor has given similar yields of bio-oil and bio-char. However, the composition of bio-oil was drastically affected by the removal of AAEM species, especially the water-insoluble but acid-soluble AAEM species. Unlike the water-soluble AAEM species, the
Acknowledgements
Australian Government funding through the Australian Research Council (DP0556098), the International Science Linkages Program and the Asia-Pacific Partnership on Clean Development and Climate program supported this study. M. Garcia-Perez and Z. Wang are very grateful to the US National Science Foundation (CBET-0966419) and to the Washington State Agricultural Research Center for their financial support. M. He thanks the China Scholarship Council for supporting his study in Australia.
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