Torrefaction of biomass can be described as a mild form of pyrolysis at temperatures typically ranging between 200 and 300°C in an inert and reduced environment. Common biomass reactions during torrefaction include devolatilization, depolymerization, and carbonization of hemicellulose, lignin, and cellulose. The torrefaction process produces a brown to black uniform solid product, as well as condensable (water, organics, and lipids) and Noncondensable gases (CO ₂ , CO, and CH ₄ ). Typically during torrefaction, 70% of the mass is retained as a solid product, containing 90% of the initial energy content, while 30% of the lost mass is converted into condensable and noncondensable products. The system’s energy efficiency can be improved by reintroducing the material lost during torrefaction as a source of heat. Torrefaction of biomass improves its physical properties like grindability; particle shape, size, and distribution; pelletability; and proximate and ultimate composition like moisture, carbon and hydrogen content, and calorific value. Compared to raw biomass, the carbon content and calorific value of torrefied biomass increases by 15–25% wt, while the moisture content decreases to <3% (w.b.). Torrefaction decreases the grinding energy by about 70%, and the ground torrefied biomass has improved sphericity, particle surface area, and particle size distribution. Torrefied biomass pelletization at temperatures of 225°C decreases the specific energy consumption and increases the capacity of the mill by a factor of 2. The loss of the OH functional group during torrefaction makes the material hydrophobic (i.e., loses the ability to attract water molecules) and more stable against chemical oxidation and microbial degradation. These improved properties make torrefied biomass particularly suitable for cofiring in power plants and as an upgraded feedstock for gasification.

Terrestrial lignocellulosic biomass has the potential to be a carbon neutral and domestic source of fuels and chemicals. However, the innate variability of biomass resources, such as herbaceous and woody materials, and the inconsistency within a single resource due to disparate growth and harvesting conditions, presents challenges for downstream processes which often require materials that are physically and chemically consistent. Intrinsic biomass characteristics, including moisture content, carbohydrate and ash compositions, bulk density, and particle size/shape distributions are highly variable and can impact the economics of transforming biomass into value-added products. For instance, ash content increases by an order of magnitude between woody and herbaceous feedstocks (from ∼0.5 to 5 %, respectively) while lignin content drops by a factor of two (from ∼30 to 15 %, respectively). This increase in ash and reduction in lignin leads to biofuel conversion consequences, such as reduced pyrolysis oil yields for herbaceous products as compared to woody material. In this review, the sources of variability for key biomass characteristics are presented for multiple types of biomass. Additionally, this review investigates the major impacts of the variability in biomass composition on four conversion processes: fermentation, hydrothermal liquefaction, pyrolysis, and direct combustion. Finally, future research processes aimed at reducing the detrimental impacts of biomass variability on conversion to fuels and chemicals are proposed. © 2015 Battelle Energy Alliance, LLC, contract manager for Idaho National Laboratory.

Torrefaction, a thermal pretreatment process, is gaining attention as it improves the physical properties and chemical composition of biomass for recycling. During torrefaction, biomass is heated slowly in an inert or oxygen-deficit environment to a maximum temperature of 300°C. The torrefaction process creates a solid uniform product with lower moisture and higher energy content than the raw biomass. During torrefaction, moisture and some volatile organic compounds volatilize from the biomass. Depending on stoichiometry and other conditions, non-condensable gas species, including CO and CO₂, are formed. The specific objectives of this research are to: 1) understand the impact of torrefaction on product quality in terms of the physical properties, chemical composition, and storage behavior of the biomass; 2) discuss the various reactors used for biomass torrefaction; and 3) develop a model for designing a moving bed torrefier, considering fundamental heat and mass transfer calculations. Torrefaction improves the physical properties, chemical composition, and energy and storage properties of biomass. Torrefaction of biomass at 300°C increases the energy content by about 30% as compared to the raw biomass. For example, when torrefied, the calorific value of the biomass increases from about 18–19 MJ/kg to about 20–24 MJ/kg. The torrefied material has a moisture content of about 1–3% wet basis (w.b.). The loss of the hydroxyl group during torrefaction makes the biomass hydrophobic. The brittle nature of the torrefied biomass makes it easier to grind. The devolatilization and carbonization reactions change the proximate and ultimate composition. The carbon content increases, whereas the hydrogen, oxygen, and nitrogen content of the biomass decreases. Despite its superior properties, the commercialization of torrefaction technology is slow due to challenges associated with reactor design and final product quality. The different types of reactors that are typically used for biomass torrefaction are the fixed bed, rotary drum, microwave, fluidized bed, and horizontal and vertical moving bed. The moving bed reactor has gained popularity among the different torrefaction reactor designs as it is easy to operate and scale. In addition, it helps produce a uniform torrefied product. In this paper, different moving bed torrefaction and gas recycle concepts are conceptualized to assess the features, advantages, and disadvantages of various design and operating concepts. These designs include example concepts for: 1) vertical and horizontal torrefaction reactors; 2) recycle of all or a portion of the torrefier off-gas; 3) counter and co-flowing gas and biomass in the torrefier; 4) controls for the system temperatures, pressures, flow rates, and gas compositions; and 5) the ability to sample the biomass feed, torrefied product, and gas streams for analysis as needed to investigate the thermal decomposition, physical behavior, and operational performance of the torrefaction system. The article also briefly describes the solid feed system, gas supply and recycle system, solid product management, torrefier gas monitoring, control system, and fugitive dust emissions control. The model presented in this paper includes a set of equations for basic calculations to configure the torrefaction reactor dimensions, such as diameter and height of the moving bed torrefier for different capacities based on target and calculated solids and gas velocities, residence times, and temperatures.

Decentralized biomass processing facilities, known as biomass depots, may be necessary to achieve feedstock cost, quantity, and quality required to grow the future U.S. bioeconomy. In this paper, we assess three distinct depot configurations for technical difference and economic performance. The depot designs were chosen to compare and contrast a suite of capabilities that a depot could perform ranging from conventional pelleting to sophisticated pretreatment technologies. Our economic analyses indicate that depot processing costs are likely to range from ∼US$30 to US$63 per dry metric tonne (Mg), depending upon the specific technology implemented and the energy consumption for processing equipment such as grinders and dryers. We conclude that the benefits of integrating depots into the overall biomass feedstock supply chain will outweigh depot processing costs and that incorporation of this technology should be aggressively pursued.

A flat die pellet mill was used to understand the effect of high levels of feedstock moisture content in the range of 28–38% (w.b.), with die rotational speeds of 40–60 Hz, and preheating temperatures of 30–110 °C on the pelleting characteristics of 4.8 mm screen size ground corn stover using an 8 mm pellet die. The physical properties of the pelletised biomass studied are: (a) pellet moisture content, (b) unit, bulk and tapped density, and (c) durability. Pelletisation experiments were conducted based on central composite design. Analysis of variance (ANOVA) indicated that feedstock moisture content influenced all of the physical properties at P < 0.001. Pellet moisture content decreased with increase in preheating temperature to about 110 °C and decreasing the feedstock moisture content to about 28% (w.b.). Response surface models developed for quality attributes with respect to process variables has adequately described the process with coefficient of determination (R2) values of >0.88. The other pellet quality attributes such as unit, bulk, tapped density, were maximised at feedstock moisture content of 30–33% (w.b.), die speeds of >50 Hz and preheating temperature of >90 °C. In case of durability a medium moisture content of 33–34% (w.b.) and preheating temperatures of >70 °C and higher die speeds >50 Hz resulted in high durable pellets. It can be concluded from the present study that feedstock moisture content, followed by preheating, and die rotational speed are the interacting process variables influencing pellet moisture content, unit, bulk and tapped density and durability