According to Cherubini et al.
], carbon dioxide (CO2
) emissions from biomass combustion are considered climate neutral if the bioenergy system is carbon (C) flux neutral, where the CO2
released from biofuel combustion equals the amount of CO2
sequestered in the biomass. Not all biofuels made from different feedstock materials have the same impact, as some may include large inputs of nitrate ion (NO3
), which has a direct correlation to their carbon footprint and can result in negative energy balances. According to Pimentel and Patzek [2
], Sheehan et al.
], and Shapouri et al.
], the net energy gain varies based on the type of feedstock material used.
Energy from biomass can be produced from different thermochemical (combustion, gasification, and pyrolysis), biological (anaerobic digestion and fermentation), or chemical (esterification) processes, where direct combustion can provide a near-term energy solution [5
]. The use of biomass for fuels applications on a commercial scale is limited, mainly due to biomass preparation, accumulation logistics, and economics [9
]. The major limitation of raw biomass is that it contains appreciable amounts of oxygen, nitrogen, sulfur, chlorine, and fluorine, which make it thermally unstable and produces tars and oils that can be problematic in conventional equipment used for coal combustion or gasification [11
]. In their studies on formulation, pretreatment, and densification options for cofiring higher percentages with coal, Tumuluru et al.
] identified that significant differences exist in the chemical compositions of biomass to coal; biomass has higher fractions of hydrogen, oxygen, and volatiles content, as well as lower carbon fractions and calorific value as compared to coal. These differences affect the combustion properties and limit the percentage of biomass that can be co-fired with coal. Chlorine in the biomass can have a detrimental impact on the boiler tube and can contribute to the corrosion of the boiler surface, depending on the concentration [12
]. Certain biomass fuels, however, may contain relatively high amounts of chlorine or fluorine. During combustion, chlorine is released as HCl and fluorine as HF. Both HCl and HF are highly corrosive and can destroy boiler surfaces [13
]. Studies on the effect of biomass fluorine with regard to combustion behavior and off-gas emissions are scarce.
1.1. Biomass Challenges
Biomass sources have a number of quality and logistical challenges in common. Inconsistent moisture is one of those primary challenges. Moisture in biomass feedstocks needs to be adjusted to suit the conversion process. Uncontrolled variations in moisture reduce the efficiency of the process and increase costs [14
]. Also, high moisture in the biomass leads to natural decomposition, resulting in loss of quality and storage issues, such as off-gas emissions. One major limitation of biomass is its low energy and bulk density as compared to conventional fossil fuels. This results in varied biomass performance for different end-use applications, as well as increasing transportation and handling costs. In addition, biomass has irregular shapes and contains a large amount of oxygen relative to carbon and hydrogen. These biomass properties result in many problems, such as high particle-size reduction energies, high transportation costs, feeding problems due to poor flowability, and lower heat and mass-transfer rates during thermochemical conversion processes (i.e.
, gasification and pyrolysis). Issues related to ash become increasingly important when biomass is considered for combustion along with coal. Straw and other herbaceous fuels like miscanthus or grass have a higher content of ash than wood because they uptake more nutrients during growth. In the case of wood fuels, the bark content in the fuel has an influence on the ash content as bark has a higher ash content as well as higher levels of mineral impurities, such as sand and soil [15
]. The concentrations of inorganic elements in biomass—including silica, sulfur, and alkali metals—are an important specification because they form alkali silicates or sulfates that melt or soften at temperatures as low as 700 °C [18
To overcome many of these challenges, biomass needs to be preprocessed before being converted into other products. Raw biomass can rarely be used in the conversion process without some form of preprocessing. Grinding is a commonly used preprocessing operation that helps achieve a consistent particle size. However, the performance of many grinders is limited by the moisture content of the biomass [14
]. High moisture content results in materials with inconsistent particle sizes that may not react consistently during the conversion processes, thereby reducing the efficiency of the process. In addition, the collection and transport efficiencies of biomass are low, as the material is typically loaded and unloaded in batches [14
]. Raw biomass that is thermally unstable when used in a thermochemical conversion process such as gasification can lead to the formation of condensable tars and result in problems like gas-line blockage [10
]. Preprocessing the biomass to improve its performance in either thermochemical or biochemical conversion processes is a good option. Common pretreatment methods include chemical, mechanical, and thermal-like acid pretreatment, or the ammonia fiber explosion method (AFEX), steam explosion, and torrefaction. Most of these pretreatment methods help break the amorphous and crystalline regions of the biomass and make the material more amenable to bio- or thermo-chemical conversions.
Biomass torrefaction has been recognized as a technically feasible method of converting raw biomass into a high-energy density, hydrophobic, compactable, grindable, and low O/C ratio solid suitable for commercial and residential combustion and gasification applications. Torrefaction is generally understood to be a thermal pretreatment technology where a group of products are formed from the partially controlled and isothermal pyrolysis of biomass occurring in the absence of oxygen at temperatures between 200 and 300 °C [19
]. Initial stages of torrefaction involve basic drying where only the surface moisture is removed; further drying results in the removal of moisture and other volatiles due to chemical reactions through the thermo-condensation process, which typically happens at temperatures above 160 °C. At this temperature, not only moisture and some volatiles but also some CO2
gets released [20
]. At 180 and 270 °C temperatures, the reaction is mostly exothermic, which results in hemicellulose degradation. At this point, the biomass begins to darken and give off additional moisture, carbon dioxide, and large amounts of acetic acid with some phenols that have low energy values [20
]. At temperatures ≥280 °C, the biochemical reactions become completely exothermic, resulting in the production of CO, CO2
, and CH4
gases, in addition to hydrocarbons like phenols, cresols, and other heavier products due to ring rupturing and scissoring of organic moieties [21
]. During torrefaction of lignocellulosic materials in particular, the major reactions of decomposition affect the hemicellulose, whereas lignin and cellulose decompose to a lesser extent [22
]. Hemi-cellulose degradation results in the loss of volatiles, which have low energy content, and most of the energy content of the biomass material is retained. Hemicellulose degradation also results in the loss of OH groups, making biomass material hydrophobic. According to Shafizedeh [22
] and Williams and Besler [23
], the major reactions of decomposition affect the hemicelluloses, whereas lignin and cellulose decompose to a lesser degree. Bourgois and Doat [21
] recommend not torrefying biomass above 300 °C, as it leads to extensive devolatilization and initiates pyrolysis.
Other advantages in pretreating biomass using torrefaction include: (a) a reduction in the feedstock natural variability, mainly due to differences in the biomass species resulting in different chemical compositions, climatic and seasonal variations, storage conditions, and time, which helps develop a uniform feedstock for bioenergy applications; (b) an improvement in physical attributes like reduced moisture content, better grindability, particle size, and sphericity; and (c) a reduction in oxygen and hydrogen content, which increases the percentage of carbon content, making the biomass more suitable as a fuel. In addition, many researchers have investigated the effect of torrefaction process time and temperature on the physical and chemical composition [19
Recent studies by Tumuluru et al.
] on pretreatment methods for bioenergy applications, such as torrefaction, indicated that torrefied material makes a good fuel for biofuels applications, such as co‑firing. In addition, it has been noted that torrefaction of biomass not only increases biomass energy properties, but produces some higher hydrocarbons as well, which can typically be used for producing chemicals or for improving overall energy efficiency [31
]. Some recent experimental and techno-economical studies on torrefaction include: (a) the effects of particle size, different corn stover components, and gas residence times on the torrefaction of corn stover by Medic et al.
]; (b) the techno-economic analysis of a production-scale torrefaction system for cellulosic biomass upgrading by Shah et al.
]; (c) biomass upgrading by torrefaction for the production of biofuels by van der Stelt et al.
]; (d) the study of particle size effect on biomass torrefaction and densification by Peng et al.
]; (e) recent advances in biomass pretreatment, torrefaction fundamentals, and technology by Chew and Doshi [36
]; and (f) studies by Tumuluru et al.
] on the response surface analysis of elemental composition and energy properties of corn stover during torrefaction. In general, response surface methodology (RSM) is the commonly used method to understand the effect of process variables on the product properties. RSM is a combination of mathematical and statistical techniques and is widely used to study process and product development data [38
Miscanthus (Miscanthus x giganteus) is classified as a C4 perennial grass and is a potential crop for bioenergy production, along with switchgrass. White oak wood is widely used by the furniture and housing industries. White oak sawdust is a byproduct available from the lumber industry and is a good bioenergy application source. Different types of torrefaction reactors (i.e., screw auger, fixed and fluidized bed, and bubbling sand bed) are generally used to carry out torrefaction studies. Of these, the fixed and fluidized bed reactors are most commonly used in laboratory-scale studies. No literature is available on chemical compositional changes in white oak sawdust samples during torrefaction in a bubbling sand bed reactor.
The aim of this present study is to understand the effect of torrefaction temperature and time on some chemical compositions of miscanthus and white oak sawdust samples. Specific objectives include: (a) torrefaction of miscanthus in the temperature range of 250–350 °C at 30–120 min to understand the changes in moisture, carbon, hydrogen, nitrogen, and volatiles content; and (b) developing response surface models for miscanthus data that can help predict the changes in chemical composition due to torrefaction. In the case of white oak sawdust samples, only two temperatures and one residence time (230 and 270 °C and 30 min) were selected, as our studies on corn stover [37
] and miscanthus indicated a significant weight loss when biomass is torrefied at temperatures greater than 300 °C and residence times of >30 min.