Article pubs.acs.org/EF
Bioslurry as a Fuel. 5. Fuel Properties Evolution and Aging during Bioslurry Storage Mingming Zhang, Sui Boon Liaw, and Hongwei Wu* School of Chemical and Petroleum Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia ABSTRACT: This study investigates the evolution of fuel properties and aging of a series of bioslurry fuels prepared from fast pyrolysis bio-oil and biochar at different biochar loading levels (up to 20 wt %) for a storage period of 29 days. The results demonstrate that, at room temperature, the storage of bioslurry results in a reduction in the acidity [total acid number (TAN)], a reduction in the viscosity, and an increase in the water content of the bio-oil phase. In comparison to the blank bio-oil samples, the presence of biochar leads to more severe changes in the fuel properties of bioslurry. After 29 days of storage, the bioslurry fuels are still acidic. An increase in the biochar loading level further decreases the TAN and viscosity of bio-oil phases and increases the water content of bio-oil phases. The storage of bioslurry also results in undesired redistribution of alkali and alkaline earth metallic species between the biochar and bio-oil phase in bioslurry, via the leaching of these inorganic species from the biochar into the acidic bio-oil by two-step kinetics.
1. INTRODUCTION Biomass pyrolysis under various conditions is widely accepted to be an important technology for biomass utilization,1−7 and recent research on this topic has led to some interesting developments (just to list a few).8−14 Particularly, biomass fast pyrolysis4−7 converts biomass that is bulky and of poor grindability15,16 into bio-oil and biochar products of high volumetric energy densities. Bio-oil may be upgraded and/or refined to produce liquid fuels and chemicals, potentially taking advantage of the vast existing infrastructure for conventional petroleum refinery.2,17−25 One near-term application is the bioslurry concept via suspending fine biochar particles in biooil, initially attempted by commercial developers (e.g., Karlsruhe26). A series of recent studies by Wu et al.27−30 showed that bioslurry can be a promising strategy and potentially make an important contribution to the establishment of a bioenergy industry based on mallee biomass in Western Australia. Additionally, suspending fine biochar particles into bio-oil also addresses the potential issues (dusty and/or spontaneous combustion) associated with biochar direct transport. Part 1 of this series (10.1021/ef1008105)27 has shown that a bioslurry supply chain is economically viable. Part 2 of this series (10.1021/ef100957a)28 has demonstrated that the energy and carbon footprints of bioslurry fuel from mallee biomass are small. Part 3 (10.1021/ef1008117)29 and part 4 (10.1021/ef101535e)30 have shown that bioslurry fuels prepared from fresh bio-oil or a bio-oil-rich fraction from fresh bio-oil extracted by biodiesel have desired fuel and rheological properties, which meet the specifications for stationary applications, such as combustion and gasification. Bio-oil, as one of the two major components used for preparing bioslurry fuels, is well-known for its aging during storage at room temperature or short time at an elevated temperature.31−35 Bio-oil aging leads to changes in viscosity, water content, homogeneity, and acidity of bio-oil, which are important characteristics to be considered for fuel application.1,36 It is known that bio-oil aging is closely associated with underlying chemical reactions occurring in bio-oil and can also © 2013 American Chemical Society
be exacerbated at an elevated temperature and potentially catalyzed by inherent catalytic species [particularly alkali or alkaline earth metallic (AAEM) species].37,38 However, little work has been performed thus far on the aging, hence the changes in properties of bioslurry fuels during storage. Particularly, it is largely unknown how bioslurry fuel stability and rheological properties will evolve with time during storage. It is also unclear what roles the large quantity of biochar presented in a bioslurry can play in the aging and chemical stability of the bioslurry fuel. Therefore, this study continues the series of studies on bioslurry as a fuel and focuses on fuel properties evolution and the aging of bioslurry fuels during storage. The aging experiments of various bioslurry fuels at room temperature were carried out, considering a series of biochar loading levels. Sampling was periodically performed for the bioslurry samples in the process of aging at various lengths of periods up to 29 days. The collected bioslurry samples were separated into solid and liquid samples, which were subsequently subject to an array of analyses, including those for rheology, viscosity, water content, total acid number (TAN), and concentrations of AAEM species.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. As-received raw pine wood chips were cut and sieved to yield a biomass sample with the size fraction of 1−2 mm. The biomass sample was then sealed in plastic bottles and stored in a fridge prior to experiment. Pyrolysis experiments were then performed to produce biochar from the biomass sample using a labscale drop-tube/fixed-bed pyrolysis reactor similar to the one in a previous study.39 Briefly, the quartz reactor was preheated to a pyrolysis temperature of 500 °C, with a stream of ultrahigh-purity argon (purity > 99.999%) flowing through the reactor at 1 L/min. The biomass sample was fed into the reactor via a water-cooled feeding Received: September 19, 2013 Revised: October 30, 2013 Published: October 31, 2013 7560
dx.doi.org/10.1021/ef401888j | Energy Fuels 2013, 27, 7560−7568
Energy & Fuels
Article
probe at a feeding rate of ∼1 g/min, and the reactor was further held at the pyrolysis temperature for 10 min. Once the pyrolysis was completed, the reactor was lifted from the furnace to cool rapidly (with the ultrahigh-purity argon continuously flowing through the reactor) to room temperature under ambient conditions for sample collection. Multiple experiments were carried out for producing the amount of biochar required for this study. The experiments considered two bio-oils (bio-oils A and B), which were supplied by two different suppliers. Both bio-oil samples were produced from the fast pyrolysis of pine wood samples available to the suppliers using their own pilot-scale biomass fast pyrolysis reactor systems at 500 °C, respectively. In other words, different pine samples were used for the production of the two bio-oil samples and the biochar samples; however, this should not influence the conclusions drawn in this study. It was reported previously that the soot-like solid particles (typically with a content of
