Comparative study of pyrolysis of algal biomass from natural lake blooms with lignocellulosic biomass
Highlights
► Nuisance algae from algal blooms can be converted into biofuels through pyrolysis. ► Existing pyrolysis technologies may be directly applicable to algal feedstocks. ► Algal biochar has high N-content and may suitable as a soil amendment.
Introduction
Thermo-chemical conversion of lignocellulosic biomass is an area of intense research, development and commercialization activity with several pilot scale facilities currently engaged in scale-up and demonstration (Vispute et al., 2010). Bio-oils from biomass pyrolysis could be upgraded to liquid hydrocarbon fuels through well-established hydro-treating and/or hydro-cracking processes (Elliot et al., 2009, Lange, 2007). Alternatively, bio-oil could be converted to hydrogen by steam reforming or directly combusted in boilers for heat and electricity (Garcia-Perez et al., 2007). Syngas (CO + H2) from biomass pyrolysis could also be converted to liquid fuels by Fisher-Tropsch processes or combusted directly for power generation (Lange, 2007). The solid residue remaining after biomass pyrolysis, bio-char, is useful as a soil amendment and is known to improve soil quality by increasing the retention time of nutrients and agrochemicals (Lehmann and Joseph, 2009). Since bio-char is virtually non-biodegradable, such applications also result in carbon sequestration. Bio-char could also be used as a feedstock for producing valuable materials such as carbon fibers, activated carbon and carbon nano-tubes (Lehmann and Joseph, 2009). Steam reformation/gasification of bio-char into hydrogen or syngas is also possible (Chaudhari et al., 2003).
Algae hold promise as a biofuel feedstock that can complement lignocellulosic biomass (Posten and Schaub, 2009, Smith et al., 2010). Traditionally, algae have been considered for their lipid-production potentials. However, high-lipid containing algae grow slowly and require stringent and controlled cultivation. In open systems, slow growing lipid-rich cultures are likely to be contaminated and rapidly overtaken by fast-growing “rogue algae” that are generally deficient in lipids. Pyrolysis is an especially attractive option for fuel production from such lipid-lean algae (DOE, 2010) and could also be applied to post-extraction residues from lipid-rich algae. A schematic showing various processing options for conversion of both lipid-rich and -lean algae is shown in Fig. 1.
Algal blooms, commonplace in several water bodies, are one potential source of algal biomass. Ordinarily, algal blooms are detrimental to local ecosystems as well as economy (Hoagland and Scatasta, 2006). It is estimated that in the US, annual economic losses of nearly $2.2 billion and $100 million, respectively, are incurred due to eutrophication of freshwater systems and coastal algal blooms (Dodds et al., 2009, Hoagland and Scatasta, 2006.). Although the overall concentrations of algae in such large water bodies is relatively low, some promising low cost technologies for recovery of algal biomass from sources such as lakes have recently been reported (Smith, 2011). Thus, it might be possible to recover nuisance algae and profitably use the biomass as feedstock for renewable bio-fuels and products. An additional benefit would be mitigation of eutrophication (Hudnell, 2008).
The few studies that have previously reported on pyrolysis of microalgae show the feasibility of production of bio-oils and bio-char but have focused on treatment of laboratory-grown cultures (Miao et al., 2004, Miao and Wu, 2004, Du et al., 2011). However, pyrolytic behavior of naturally-occurring lipid-lean microalgal assemblages is not known. Some studies have reported pyrolysis of other aquatic flora, especially macroalgae (Ross et al., 2008, Bae et al., 2011, Muradov et al., 2010, Demirbas, 2006, Demirbas, 2010, Li et al., 2011). However most of these higher plant species contain larger amounts of more thermostable structural carbohydrates unlike microalgae. Additionally, the studies that report results of pyrolysis of microalgae or other aquatic species do not include comparisons of results with lignocellulosic biomass for which a much larger body of literature and economic feasibility studies are available (Wright et al., 2010). In this study we report the results of pyrolysis of microalgae from freshwater blooms and select lignocellulosic feedstocks (corncobs, woodchips and rice husk). A comparative assessment of the product yields from all biomass types, pyrolyzed under similar conditions, is presented.
Section snippets
Raw materials and sample preparation
Lignocellulosic biomass feedstocks (corncobs, woodchips and rice husk) were supplied by Red-Lion Bio-energy (Toledo, OH). Prior to use in pyrolysis experiments, these feedstocks were milled to a particle size of −80 mesh using a laboratory Wiley mill (Model 4, Thomas Scientific, Swedesboro, NJ) and dried in an oven at 45 °C for 24 h.
Samples of algal biomass were collected from two distinct blooms in Maumee bay of Lake Erie on 31st July 2009. It was determined that the Lyngbya sp. dominated in one
Feedstock characterization
The results of proximate analysis of feedstocks are shown in Table 1. The volatile matter and fixed carbon content values of the lignocellulosic feedstocks reported here (corncobs, woodchips and rice husk) compare well with values previously reported in the literature (Cheng, 2010). The ash content of algae samples was observed to be higher than that of lignocellulosic biomass (except rice husk) and is consistent with previous observations of high ash content in aquatic flora from natural
Conclusions
Pyrolysis could be a viable alternative for processing nuisance algae, such as those obtained from eutrophic water bodies. N-rich bio-char could be a valuable product from such processes, although presence of N-compounds in bio-oils would likely diminish their fuel value. Alternatively, pyrolysis could also be applied to algae residues after oil extraction from lipid-rich biomass (Fig. 1). Since algae pyrolysis can also produce liquid fuels with appreciable yields, technologies being developed
Acknowledgements
The authors would like to thank Center for Innovative Food Technology (CIFT) for financial support through a subcontract from US-AFRO. The authors would also thank Red Lion Bio-energy for providing lignocellulosic biomass and Dr. Thomas Bridgeman for algal feedstocks.
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