Acetic acid and lithium chloride effects on hydrothermal carbonization of lignocellulosic biomass
Introduction
Energy independence has been a goal in the United States since the oil crisis in the 1970’s. Interest in “homegrown” fuels has fluctuated directly with the price of imported non-renewable fuels. However, using potential food, such as corn, to produce energy seems unethical in a world of limited caloric resources. As a non-food renewable resource, lignocellulosic biomass could be used as an important starting point to produce biofuels indefinitely. Enhanced biofuels can be produced from low-value biomass via two methods. With biochemical conversion, biomass is broken down by enzymatic or chemical processes and then converted to ethanol through fermentation. In thermochemical conversion, biomass is broken down to intermediates using heat and upgraded to fuels using both heat and pressure with catalysts present. However, lignocellulosic biomass exists in many diverse forms. Crops such as hard or soft woods, switch grass, or miscanthus can be grown, while waste products from food production, such as rice hulls, corn stover, straws, and bagasse, can be obtained easily and cheaply. The great variation in solids handling for these different feedstocks causes difficulties in further processing. Also, the seasonal nature of these plant-based materials requires good storage properties. Pretreating biomass by heating it in an inert environment can improve the usefulness of these feedstocks (Prins et al., 2006a, Prins et al., 2006b, Yu et al., 2008). Pretreatment can be performed as a dry process or as a wet process. The dry process is called dry torrefaction or mild pyrolysis. When done in hot compressed water, the process is often referred to as wet torrefaction, biomass hydrolysis, or hydrothermal carbonization. Relatively simple processes, both pretreatments increase not only a biomass’s density to decrease transportation costs, but also its hydrophobic behavior for simpler storage. Torrefaction’s solid product is easily crushable, regardless of the particular initial feedstock, leading to a more uniform feed for processing. In addition, the process improves the solid product’s heating value on a weight basis, with increased carbon percentage, making it more suitable for co-firing in coal power plants. Although pretreatment produces some carbon dioxide, carbon is taken from the atmosphere and fixed in the original biomass, so the process is carbon neutral.
Dry torrefaction typically is performed between 225 and 300 °C with reaction times of between 30 min and several hours (Prins et al., 2006a, Prins et al., 2006b, Sadaka and Negi, 2009). Wet torrefaction or hydrothermal carbonization is run at slightly lower temperatures (180–260 °C), with liquid water used, necessitating high pressures of up to 4.6 MPa gauge. The reactions involved do not proceed significantly at temperatures lower than 180 °C. Although reaction time for hydrothermal carbonization can be several hours, the initial 20 min appear to generate the vast majority of product (Knezevic et al., 2010). Hydrothermal carbonization is preferred for several reasons. At temperatures of 227 °C to 327 °C, the ionic product of water is maximized, leading to the possibility that it could act as an acid or base catalyst for reactions. Also, the dielectric constant of water is much lower at these temperatures than ambient temperatures, causing it to behave more like a non-polar solvent (Yu et al., 2008). Hydrothermal carbonization when used at similar temperatures to dry torrefaction is more effective in providing energy densification by reducing the mass of the solid product, and increasing its higher heating value (HHV) (Yan et al., 2009). The characteristics of hydrothermal carbonization, including the shorter reaction time at lower temperature and reduced equilibrium moisture content, which reduces degradation over time, indicate its greater feasibility for a seasonal feedstock (Yan et al., 2009).
Using hydrothermal carbonization, the energy densification ratio, the ratio of the heating value of the pretreated solid fuel product to that of the original biomass, can be increased by 3% to 47%, depending on the type of biomass and reaction conditions. Reaction temperature has been found to be the most significant variable in changing the solid product qualities, with higher temperatures decreasing mass yield and increasing HHV. Biomass pretreated by hydrothermal carbonization has increased fixed carbon and atomic carbon, implying it has become a fuel similar to low rank coal. In addition, reduced equilibrium moisture content in pretreated biomass indicates a more hydrophobic nature leading to better storage properties (Acharjee, 2010).
Hydrothermal carbonization produces not only a solid fuel for use or subsequent conversion, but also other potential high-value products. Glucose, among other simple sugars, and 5-hydroxymethyl furfural (5-HMF) can be precipitated in significant quantities from the aqueous product stream. The aqueous stream also contains volatile acids. For example, when loblolly pine was pretreated at 230 °C for 5 min, 0.025 g of acetic acid was produced per gram of original wood. Formic acid (0.0085 g) and lactic acid (0.0026 g) were produced as well (Yan et al., 2010). These acids lower the pH in the reaction system as they are produced.
The role of pH in hydrothermal carbonization has not been fully explored. The reaction rate at 300 °C for solutions of glucose, which can serve as a model for biomass, was observed at pH levels of 1 to 14, but no clear trends emerged (Knezevic et al., 2009). An initial pH of greater than 7 leads to a liquid rather than solid product (Ando et al., 2000, Hu et al., 2008). Many hydrothermal carbonization processes report the addition of such acids as citric acid (Hu et al., 2008, Titirici and Antonietti, 2007), acrylic acid (Demir-Carkan, 2009), and sulfuric acid (Lu et al., 2009a, Lu et al., 2009b). The effect of acetic acid was chosen for investigation because it is the primary acid produced in hydrothermal carbonization (Yan et al., 2010), thus reducing the complexity of the reaction system. Also, acetic acid could be recycled to treat fresh batches of biomass, minimizing cost. A reaction temperature of 230 °C may be preferable because the degradation of glucose, a valuable product of hydrothermal carbonization, rapidly increases above 230 °C (Yu et al., 2008). In addition, higher temperatures increase the pressure necessary to maintain liquid water, requiring sturdier reactor walls and making the process more hazardous and expensive. It could be expected that adding lithium chloride would reduce pressure in aqueous systems, as well as having a possible catalytic effect and increasing the activity of the [H+] ions in the system, reducing pH. Thus, investigating the effects of both acetic acid and lithium chloride on the hydrothermal carbonization process at 230 °C was deemed useful.
Section snippets
Biomass
As a typical lignocellulosic biomass, loblolly pine was acquired from Alabama, USA. On a mass basis, it consists of 11.9% hemicelluloses, 54.0% cellulose, 25% lignin, 8.7% extractives and 0.4% ash (Yan et al., 2010). Pine samples were milled to the desired particle size of 14–28 mesh (1.168–0.589 mm in diameter) before hydrothermal carbonization. Before treatment, the moisture content was measured to be 5.2% by weighing a sample, then measuring weight loss after drying for 24 h at 105 °C.
Acetic acid
Glacial
Lithium chloride effect on reaction pressure
Pressure of the standard reaction with no additives is approximately 4.6 MPa gauge at 260 °C, and 2.6 MPa gauge at 230 °C. These pressures correspond to the vapor pressure of water, and are largely unaffected by the presence of biomass. With the addition of 1 g of LiCl per g pine, gauge pressure at 230 °C is reduced to 1.8 MPa, while 2 g of LiCl per g pine reduces the reaction gauge pressure to 1.2 MPa. Zeng et al. (2006) describes the vapor pressure of aqueous LiCl at high temperatures, which is
Conclusions
Addition of acetic acid and/or LiCl to hydrothermal carbonization of lignocellulosic biomass contributes to increased fuel density of the solid product. HHV is increased and mass yield is reduced when 0.4 g of acetic acid is added per g pine. Addition of LiCl reduces reactor pressure and increases the energy densification ratio. Added at a ratio of 1 g LiCl per g pine, LiCl addition results in greatly increased HHV and reduced mass yield. Cellulose degradation is promoted by the addition of
Acknowledgements
This work was supported by the US Department of Energy, Award DE-FG36-01GO11082. The authors acknowledge the assistance of Tapas Acharjee (UNR), Dr. Glenn Miller (UNR), and Kevin Schmidt (UNR).
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