The electric power sector uses wood and biomass-derived wastes to generate electricity for sale to the other sectors.
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Understanding the degree of biomass compositional variability is crucial to developing a robust conversion process. However, in addition to understanding compositional variability, it is useful to know where this variability originates. Kenney et al. Briefly, some of the major sources of biomass compositional variation derive from local agronomic conditions [ 21 ], drought [ 22 ], harvest season and year [ 23 ], and harvest method [ 24 ].
A further analysis of the sources of biomass variability and its impact on conversion processes has been compiled by Williams et al. As can be seen in Tables 1 — 5 , biomass has a broad range of compositional variability, even within an individual feedstock.
This variation has a substantial impact on biomass conversion to fuels and value-added chemicals that varies depending on the chosen conversion process. The following section investigates how feedstock quality impacts four common conversion processes: biochemical fermentation, direct combustion, pyrolysis, and hydrothermal liquefaction HTL.
General impacts of feedstock physical and chemical properties will be discussed before a more in-depth look at each conversion process. The physical properties of biomass have a myriad of effects on its conversion to fuels and chemicals. Arguably, the two most important physical properties of biomass, regardless of conversion process, are particle size and moisture content. Practically all conversion methods require some degree of size reduction.
Biochemical conversion processes can accept a greater range of particle sizes, and the final size needed tends to be dependent on the processing system utilized [ 26 , 27 ]. On the thermochemical side, hydrothermal liquefaction is much more insensitive to particle size due to high heating rates in the liquid media [ 28 ], but a significant amount of size reduction is needed to pump biomass sludges in a continuous system [ 29 ].
Pyrolysis uses particles smaller than 0. Optimal combustion particle size is often larger and varies for different biomass types at approximately 6 mm for straw, 4 mm for Miscanthus , and 2—4 mm for wood [ 31 ].
While particle size is obviously important, others have argued that moisture content is likely the single most problematic property affecting feedstock supply and biorefining operations [ 20 ]. Moisture increases heating rates during steam pretreatment for biological conversion [ 32 ], reduces bio-oil quality and thermochemical conversion [ 33 ], and causes low thermal efficiency in combustion processes [ 34 ].
Aside from particle size and moisture content, other physical properties of interest include bulk density, elastic properties, and microstructure. Biomass chemical properties also have a large influence on best conversion process and the quality of the final product. The three primary chemical components of interest in biomass conversion are ash content, volatiles, and lignin. High ash content generally has a negative effect on biomass conversion across the board by reducing the effectiveness of dilute acid pretreatment for biological processes [ 35 ] and increasing char yields and fouling in thermochemical processes such as HTL [ 36 ], pyrolysis [ 37 ], and combustion [ 38 ].
However, there exist several strategies for ash removal including leaching and air classification [ 39 ]. Volatiles are generally represented by light organic acids such as acetic acid and furans. The furan fraction of the volatiles can reduce fermentation efficiency in biological processes [ 40 ] and lower energy content and stability in bio-oils produced by thermochemical processes [ 41 ]. Lignin, on the other hand, can have a variety of effects on biomass conversion depending on the process chosen.
Lignin generally has a negative effect on ethanol production by blocking enzyme access to cellulose [ 42 ] but can increase oil yields for pyrolysis [ 43 ] and heating values for combustion [ 34 ] during thermochemical conversion. Ethanol production from biomass occurs via two primary steps: depolymerization of the cellulose and hemicellulose to fermentable sugars and fermentation of these sugars to ethanol.
Biomass conversion to ethanol has been evaluated in many reviews [ 42 — 45 ] which vary focuses from pretreatment and enzymatic hydrolysis [ 42 ] to optimization of the cellulase enzyme for improving sugar conversion to ethanol [ 44 ] and evaluation of current and future economic aspects of fuel ethanol production [ 46 ].
This work will build upon these previous reviews to explain how biomass compositional variability can influence fermentation processes for fuel production.
Mixed rangeland grasses are a prime example of a feedstock with high compositional variability. These grasses are an emerging alternative to traditional energy crops. Mixed rangeland grasses also preserve natural habitat and typically require less maintenance than traditional energy crops.
However, the naturally high variability of these grasses can lead to reduced product yields in biochemical conversion processes. Adler et al. Ethanol yields are maximized when there is increased targeted coverage of C 4 prairie grass energy crops, such as switchgrass, which sequester more carbon than typical C 3 conservation grassland varieties [ 47 ].
This preference for C 4 grasses illustrates how the production of ethanol using fermentation is typically much more dependent on biomass carbohydrate content. Given the compositional data above, fermentation is better matched to herbaceous crops than lignocellulosic material due to the higher carbohydrate content of grasses. Additionally, fermentation processes are typically more tolerant of the higher ash contents of herbaceous feedstocks [ 48 ]. However, it should be repeated that high alkali metal content from excess soil collected during harvest can increase acid neutralization during pretreatment and lower the xylan digestibility for corn stover, consequently lowering ethanol yields [ 35 ].
Despite compositional variability generally being a disadvantage in feedstock processing, there exists at least one aspect to variability that could be advantageous. Changes in structural carbohydrate content with anatomical fraction in corn stover significantly affect glucose yield. After hydrolysis, glucose concentration can be three times greater in the cobs, leaves, and husks than stalks [ 49 ].
Additionally, the corn cobs, leaves, and husks respond better than stocks to simultaneous saccharification and fermentation SSF despite having similar glucan levels [ 50 ]. Therefore, selective fermentation of specific anatomical fractions could increase process efficiency if a cost-effective separation process could be devised and there is a value-added coproduct that could be produced from the stalks.
The advantage of separating biomass by anatomical fraction extends to other biomass types as well. While biomass as a feedstock exhibits a significant amount of compositional variability, illustrated in the tables above, the different options for thermochemical conversion are almost diverse. Thermochemical conversion operations utilize reactions using both solids pyrolysis and combustion and liquids hydrothermal liquefaction.
Products from thermochemical processes also span a wide range of states from solid biochar , through liquid bio-oil , all the way to gas syngas. The wide variety of processing options and product outputs, along with short reaction times on the order of seconds , allows thermochemical conversion operations to utilize a wide array of diverse process inputs.
The combustion of biomass, which is still common in developing countries, has been used for thousands of year to do everything from managing agricultural lands to producing heat and energy for industrial processes [ 52 ].
Currently, developed countries use nonrenewable fossil fuels such as oil, coal, and natural gas as a primary source of energy; however, these energy supplies could be depleted in the next 40—50 years [ 53 ]. In an effort to reduce the rate at which these nonrenewable resources are being depleted and reduce environmental impact, there is a shift toward the combustion of renewable biomass and other waste products such as paper and plastics.
Literature reviews focus on the combustion of biomass as an energy source both with [ 54 — 56 ] and without [ 30 , 53 , 57 , 58 ] torrefaction as a pretreatment to improve combustion efficiencies and material grinding and storage properties. One of the major problems with combusting biomass in a traditional coal plant is slagging, a mineral buildup due to the higher ash content in biomass than in coal.
This problem means that low-ash content biomass, such as woody feedstocks, is better to use than herbaceous materials which have intrinsic ash contents about five times greater than woody materials in combustion applications.
While biomass combustion does present problems with slagging, it does have the benefit of reducing harmful greenhouse gas emissions as compared to coal [ 59 ], and the energy produced can be incorporated directly into the current energy grid without infrastructure changes.
Pyrolysis is a thermochemical process that starts with a solid and can be tuned to produce either a solid biochar or a liquid bio-oil. However, this chapter will focus on the production of bio-oil and the effects of biomass composition on the resulting oil yields and quality.
Pyrolysis of biomass to produce fuels has been thoroughly reviewed in the academic literature [ 33 , 61 — 64 ]. The pyrolysis process is well suited for low-moisture-content material with low ash and high lignin content, meaning that pyrolysis processes favor lignocellulosic feedstocks. In addition to decreasing oil yields, the alkali metals common in herbaceous crops can also have damaging effects on reactors and reduce catalyst lifetimes [ 65 ].
However, more recent studies have taken into account not only the production of pyrolysis oil but also the upgrading of that oil to the final fuel for a range of feedstocks including pines, poplars, switchgrass, and corn stover. Hydrothermal liquefaction HTL is a unique thermal conversion process that utilizes biomass and water slurries. This makes HTL particularly well suited to turning high water content material, such as algae, municipal solid wastes, or grasses into bio-based oils.
Additionally, HTL bio-oils tend to be higher quality than pyrolysis oils because they have less oxygen. However, the oil yields for HTL are lower than pyrolysis and the oxygen content is still higher than crude oil [ 67 ]. Performing the dissolution of biomass in a water media also saves energy on drying the feedstock, and the high heat transfer rates in a liquid media reduce particle size reduction requirements [ 36 ].
Due to the liquid nature of the reaction media and the high temperatures, these reactors often operate at high pressures 5—40 MPa to keep the reaction media as a liquid or supercritical fluid. Since the operating conditions and products of hydrothermal reactors are so diverse, the reviews of this material span a wide range.
In recent years, improvements in satellite technology produced finer-scale data and increasing emphasis on global sampling efforts both made this recent work possible. As Elizabeth Pennisi at Science reports, the research of this latest work spent three years poring over the scientific literature to come up with an accurate measurement for each kingdom of life. While the findings represent their best efforts, they do acknowledge that their calculations could still use refining, and the numbers might change with updated survey data.
As Pennisi reports, finding the weight of life on Earth was not the research team's ultimate goal. Instead, they are trying to determine the dominant proteins on the planet.
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