The Role of Sorghum as a Bioenergy Feedstock

Chapter 9: The Role of Sorghum as a Bioenergy Feedstock

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Chapter 9 The Role of Sorghum as a Bioenergy Feedstock Carla E. Shoemaker and David I. Bransby Department of Agronomy and Soils, 202 Funchess Hall, Auburn University, Auburn, Al 36849, U.S.A. [email protected]; 334-844-3964 [email protected]; 332-844-3935

Introduction

Existing economies are tied to the flexibility of non-renewable fossil energy sources primarily in the form of petroleum, coal, and natural gas. Petroleum provides the single largest fraction of the world’s energy (40% of total world energy used) and for most countries a large percentage is imported from politically volatile locations (IEA, 2002). Petroleum is also considered one of the largest single contributors to trade deficits in many countries. Burning non-renewable fossil energy sources contributes to carbon dioxide and other greenhouse gas emissions in the atmosphere, raising concerns about global climate change. Non-renewable fossil energy sources are not sustainable therefore it is imperative that renewable, less-polluting energy sources be identified and implemented into current energy markets. Renewable energy sources are needed to address a range of important economic and environmental issues and to insure a continuous energy supply. Energy security and reducing greenhouse gas emissions are the most important priorities for most countries in the world. Energy security is a concern due to uncertainties in supply and sudden increases in market prices of non-renewable fossil energy sources. These uncertainties stem from geopolitical tensions, weather disturbances, the manipulative behavior of OPEC (Organization of Petroleum Exporting Countries), growth of developing countries like China and India, and declining nonrenewable fossil energy reserves. Many countries have embarked on programs to develop alternative energy sources to reduce their dependence on imported fossil energy supplies and reduce the foreign exchange burden. The United States has established the Energy Independence and Security Act of 2007 amending the Renewable Fuels Standard (RFS)-Energy Policy Act of 2005, in order to reduce its dependence on foreign fossil energy supplies, reduce greenhouse gas emissions, and provide meaningful economic opportunity. The RFS recommendation is to produce 36 billion gallons of biofuels by the year 2022 (RFA, 2010). The production of biofuel from plant-based biomass is becoming an attractive alternative to non-renewable fossil energy sources. The advantage of plant-based biomass material lies in its photosynthetic ability. Nature has designed a sophisticated solar conversion system that self-assembles from water, nutrients in the soil and carbon dioxide (CO2) in the air with energy input from the sun. Critical factors in utilizing biomass as an alternative energy source will be the ability of the plant to achieve high biomass yields, grow in diverse climates under various environmental conditions, and be economically converted into a bio-based product.

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Sustainable Alternative Fuel Feedstock Opportunities, Challenges and Roadmaps for Six U.S. Regions

Sorghum (Sorghum bicolor L. Moench) is an herbaceous annual grass of tropical origin that is planted from seed, stores an appreciable amount of sugar with modest water requirements, is tolerant of arid and saline growing conditions, and reaches maturity in 90 to 180 days. It is considered a crop with universal value because it can be grown in tropical, subtropical, temperate, and semi-arid regions of the world. It is adaptable to existing cropping systems, can serve as a secondary crop or a short cycle crop rotation and is used as a source of forage and silage for livestock production systems. The objective of this review is to highlight the unique characteristics of sorghum as a bioenergy feedstock.

Potential Use as Biofuel

Sorghum is characterized by a vastly diverse germ plasm in terms of phenotypic and morphological traits. Sorghums can be classified into four main groups depending on their production characteristics: grain sorghum, forage sorghum (FS), high-tonnage sorghum (energy), and sweet sorghum. Sorghums have generated interest as an alternative bioenergy feedstock since the 1970s. Sorghum cultivars are primarily processed for production of table syrups and livestock feed. Sorghum cultivars are now being considered as a candidate in the search for efficient energy crops due to an increased interest in the conversion of biomass to energy. Sorghum cultivars possess readily available fermentable sugars within the culm, therefore enzymatic conversion of starch to sugar is not necessary which gives sorghum an economical advantage over starch based crops. The juice from sorghum can be converted to alcohol using currently available, conventional fermentation technology. The bagasse can be utilized to generate electricity or steam as part of a co-generation scheme or as a biomass feedstock for cellulosic biofuel production. Research interest in sorghum cultivars has focused on the general aspects of sugar production, disease resistance, cultural practices, and agronomic aspects of production such as total biomass yield. Future research into multiple planting dates to increase annual biomass yields and extend seasonal availability of sorghum cultivars needs to be considered because sorghum cultivars have a short growing cycle and are adapted to a wide array of environmental conditions with less nutrient requirements. In the United States, it is estimated that annual combined sorghum production uses seven million hectares of farmland. Currently, sorghum grain makes up 4% of the total grain used for ethanol production in the U.S. (Renewable Fuels Association, 2007).

Sorghum

Sorghum is a C4 crop in the grass family and is characterized by its high photosynthetic efficiency. Sorghum is an annual crop with considerable variability in growth characteristics. Grain, sweet, and forage type sorghums are all compatible with current agricultural production systems. Crops with a four-carbon (C4) photosynthetic pathways produce 30% more dry matter (DM) per unit of water than three-carbon (C3) crops and are more adapted to semiarid production regions (Samson and Knopf, 1994). Sorghum plants have the ability to counterbalance production situations. Habyarimana et al. (2004) reported that lower plant density results in higher leaf weight per plant, higher grain weight per panicle and higher tillering ability. Sorghums have an extensive root system that can penetrate 1.5 to 2.5 meters into the soil and extend one meter away from the stem. The large amount of root material contributes to the build-up of soil organic carbon after removal of the aerial parts of the plant, and can alleviate concerns about depletion of soil organic matter resulting from the removal of stover (Wilhelm et al., 2004). Sorghum requires less fertilizer than corn to achieve high yield (Lipinsky and Kresovich, 1980), can tolerate a wide range of soil conditions, from heavy clay soils to light sand, with pH ranging from 5.0 to 8.5 (Smith and Frederiksen, 2000). Sorghums become dormant in the absence of adequate water but do not wilt readily and are more efficient than corn in utilizing phosphorus and potassium. These characteristics make sorghum suitable for cultivation as a crop in optimal conditions and on marginal lands.


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Production of biofuels from plant structural carbohydrates (cellulose, hemicellulose, and lignin) is predicted to increase energy output per unit land area by five percent when compared to biofuels produced from starch and sugar (Farrell et al., 2006; Somerville, 2007). Sorghum has high biomass yield and excellent nitrogen use efficiency (Gardner et al., 1994; Anderson et al., 1995; Bean et al., 2008; Putnam et al., 1991). After harvesting, most varieties will regrow or ratoon. The ability to form a ratoon enables multiple harvests per season in some climates although yields typically decrease in ratoon crops.

Sweet Sorghum

Sweet sorghum yields vary considerably depending on the varieties, location grown (soil, water, climate, pests and diseases), inputs, and agronomic practices. When considering sweet sorghum for biofuel production via conventional fermentation, biomass yield, juice yield, and sugar production per acre are the most important characteristics. Sweet sorghum cultivars are characterized by the accumulation of high levels of fermentable carbohydrates (FC) (15-23%) within the culm (Sarath, et al., 2008; Smith et al., 1987). Total FC are comprised of three main sugars; sucrose (70%), glucose (20%), and fructose (10%) variation in percentages depends on variety and environmental conditions (Prasad et al., 2007). Sweet sorghum requires less water and contains higher FC levels than corn, making it a favorable biofuel crop for semiarid temperate climate regions (Reddy et al., 2007). Sugar content in the juice increases with maturity, and is low prior to seed development. Sweet sorghum is typically seeded in widely spaced rows (3040 inches). The ideal seeding rate for most sweet sorghum varieties is 3-4 seeds per linear foot of row with a final stand of 2-3 plants per linear foot of row. If plant populations are too high, the stalks will be spindly and contain less juice. Sweet sorghum varieties can grow 14 feet tall and produce 20 to 50 tons of biomass (fresh weight) per acre. Putnam et al. (1991) studied the performance of 13 sweet sorghum cultivars and reported total DM yield (16.1 to 35.8 Mg ha-1), brix values of extracted juice (5.8 to 13.7), harvest stalk moisture (67 to 76%), and extracted FC yields (2.3 to 7.0 Mg ha-1) to vary significantly among cultivars. Sweet sorghum requires less than 50% total nitrogen to produce similar ethanol yields as corn (Anderson et al., 1995) and is capable of removing 62% of total nitrogen with no difference in DM yield (Bean et al., 2008). Reports have shown that sweet sorghum yielding 11-16 Mg ha-1 will remove nitrogen, phosphorus and potassium at the rate of 112, 45, and 202 kg ha-1, respectively (Undersander et al., 1990). Ethanol production from sweet sorghum (5,600 liters ha-1 year-1from 140 t ha-1 per two crop annum-1 @ 40 L t-1) is comparable to ethanol production from sugarcane (6,500 litres ha-1 from 85-90 t ha-1 per crop @ 75 L t-1). Yield and quality characteristics of sweet sorghum and sugarcane vary with weather conditions and planting dates (Hipp et al., 1970; Broadhead 1972). Poornima et al. (2008) recorded higher grain (2483 kg ha-1) and millable cane yield (37.17 t ha-1) for early planting dates of sweet sorghum cultivars due to favorable environmental conditions during the early growing season. Under favorable conditions, sweet sorghum is capable of producing up to 13.2 metric tons per hectare of total sugars, which is equivalent to 7682 liters of ethanol per hectare (Murray et. al., 2009).

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As a bioenergy crop, sweet sorghum could be used to provide grain starch for hydrolysis; stem juice for direct fermentation; and bagasse as cellulosic feedstock for fermentation or boiler fuel (Saballos, 2008).

Forage Sorghum

Forage sorghums are coarse, fast growing; warm season grasses that provide livestock feed in midsummer. Sugar in the stalk is not the primary focus. Sorghum varieties are currently bred to produce high biomass yields (Juerg et al., 2009). Forage sorghum utilized as silage, hay and direct grazing represents approximately five million acres of production. In 2009 over 254,000 acres of sorghum were harvested producing an average of 13.7 tons of silage per acre (USDA National Agricultural Statistics Service, 2008). Forage sorghum has the potential to grow tall (6 to 15 ft) and can produce a large amount of vegetative growth. Compared with corn, forage sorghum is cheaper to produce, has comparable yields and slightly lower forage quality for silage. These qualities give forage sorghum potential for use in biofuel production (Oliver et al. 2005a; Oliver et al., 2005b). The primary sorghums used for forage are grouped as forage sorghums, sudangrass, and sorghum-sudangrass hybrids with each having specific growth characteristics. Typically forage sorghums are used for silage or for a single hay cutting. Sudangrass is used for grazing, multiple hay cuttings and silage. Sorghum-sudangrass hybrids are a cross between sorghum and sudangrass with smaller diameter stems, high tillering capacity, rapid re-growth potential and low grain yield and are best suited for hay and grazing. Optimum growth occurs in sustained elevated temperatures of 75 to 80 degrees Fahrenheit. Most forage sorghums are grown in the southern Great Plains (Kansas, Nebraska, and Texas). Introduction of traits such as brown midrib and photoperiod-sensitivity have expanded the use of forage sorghums. The brown midrib (BMR) forage genotypes usually contain less lignin and have reduced lignin chemical composition (Oliver et al. 2005a; Oliver et al., 2005b). The reduced lignin content of BMR sorghum increases its energy conversion efficiency and its nutritive value as a livestock feed (Gressell 2008). The BMR mutant genes most commonly used are BMR-6, BMR-12 and BMR-18, and refer to the reddishbrown pigmentation of the leaf mid-rib (McCollum et al., 2005; Sarath et al., 2008). Decreased DM yield, plant height, and tillering with increased lodging are potential negative characteristics of BMR sorghum (Pedersen et al., 2005). McCollum et al. (2005) reported that BMR forage sorghum varieties produced less total DM (16.8 Mg ha-1) than non-BMR forage sorghum (19.0 Mg ha-1). Lodging potential of BMR forage sorghum can be minimized by adjusting the seeding rate and managing nitrogen fertilization. Bean et al. (2003) concluded that lodging of the ‘BMR 100’ hybrid increased 25% with increased nitrogen fertilizer rates (56 to 112 kg ha-1). Increasing the seeding rate of ‘BMR 100’ hybrid from 74,100 to 148,200 seeds ha-1 also increased lodging by 56.6%.


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High-Tonnage (Energy) Sorghum

High-tonnage (energy) sorghum can produce increased levels of cellulose material, with reduced grain production depending upon the environment. These are usually classified as hybrid photoperiodsensitive forage sorghums (PS). Photoperiod-sensitive sorghums delay the flowering which in turn will delay the decline in forage quality providing flexibility in harvest management (McCollum et al., 2005). Photoperiod-sensitive sorghum will not produce grain at most latitudes in the United States. The PS characteristic allows these sorghum hybrids to accumulate vegetative dry matter for longer periods throughout the growing season. Photoperiod-sensitive hybrids can be derived from the cross of two photoperiod-insensitive parental sorghum lines (Rooney et al., 2007). Reported yields of PS sorghum on dry land ranged from 7.6 to 17.5 Mg ha-1 and under irrigation ranged from 15.4 to 21.3 Mg ha-1. Dual purpose forage sorghum dry matter yields during the same trial years ranged from 6.4 to 13.7 Mg ha-1 on dry land and 14.3 to 19.5 Mg ha-1 under irrigation (Roozeboom et al., 2004; Roozeboom et al., 2005). McCollum et al. (2005) reported dry matter yields of PS sorghum to be 26% to 43% greater than nonBMR and BMRs forage sorghums with less overall water use. Blumenthal et al. (2007) reported dry matter yields of PS sorghum to be 30 Mg ha-1 in southern Texas. Photoperiod-sensitive sorghum has characteristically high lignin content in the stalks, which minimizes lodging but decreases nutritive quality as a livestock feed. Although cellulosic biomass is receiving growing attention as a bioenergy feedstock, the concept is not well understood for sorghum biomass because scientific information on using forage sorghums for biofuel production is limited.

Grain Sorghum

Grain sorghum production for the United States ranks fifth and totaled 383 million bushels in 2009 with an average price of $5.90 per cwt (NASS 2010). The Great Plains states produce the largest volume of grain sorghum; however it is also grown from Georgia to California and South Texas to South Dakota. The two top producing states are Kansas and Texas harvesting nearly 84 percent of the U.S. sorghum crop with a total value of $1,050 million. Texas led the nation in area planted and silage production. Nebraska, Oklahoma, South Dakota and Colorado also produced quantities of grain sorghum (NASS 2010). Leading sorghum producers around the world include the United States (18.68%), Nigeria (17.12%), India (11.27%) and Mexico (9.81%) (Sorghum U.S. Grains Council). Grain sorghum use in the United States is primarily as a livestock feed with a higher feed to gain ratio and a lower average daily gain compared to corn. Ethanol production has become an important new market for grain sorghum due to the classification of grain sorghum as an advanced biofuel feedstock in the 2008 Farm Bill. According to the World Agricultural Supply and Demand Estimate report ethanol production will account for 26 percent of domestic grain sorghum usage. Currently more than one third of the U.S. grain sorghum crop is processed through an ethanol plant making the renewable fuels industry the fastest growing valueadded industry for sorghum.

Genetics

Numerous genetic methods can be employed to improve biomass properties. The choice of the method depends on the trait of interest, the biochemical process being targeted, and the plant species. Biomass quality is heavily influenced by the content and composition of lignin, cellulose, and hemicellulose. Biomass yield can be manipulated through genetics and through standard crop management practices that include plant height, stalk diameter, number of leaves, disease and pest resistance, and lodging susceptibility.

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Modification of plant biomass through the application of genetic, genomic, and plant breeding approaches to exploit both intraspecific and interspecific variations can aid in the development of bioenergy crops that support physical and chemical characteristics for high biomass yield and quality composition. In the U.S., sorghum has been researched for biofuel for more than 30 years (Lipinsky, 1977), with primary research, development, and breeding starting in the late 1970s through the mid1980s (Murray et al., 2009). Sorghum contains both cultivated and wild races and possesses a significant amount of genetic diversity for traits of agronomic importance (Hart et al., 2001). Approximately 4,000 cultivars of sweet sorghum are distributed throughout the world (Grassi et al., 2004), providing a diverse genetic base for the development of highly productive cultivars within various climate regions. Hybrids can be developed by crossing grain-type seed parents and sweet-type pollen parents resulting in higher biomass yields and sugar content when compared to the original grain-type parents (Hunter and Anderson, 1997). Hybrids can also co-produce grain at levels approaching the yields of the grain-type seed parent (Miller and McBee, 1993). Various biological techniques, including tissue culture (Baskaran and Jayabalan, 2005), genetic transformation (Godwin and Seetharama, 2005), molecular markers, genomics, and proteomics have been successfully exploited in sorghum (Dillon et al., 2005). Knowledge regarding genetic control of perennially has contributed to its promise as a bioenergy crop (Paterson, 2008). The sorghum genome has recently been sequenced, providing a better understanding of genetic and biochemical traits which will assist in developing a better genomics-assisted breeding program in sorghum (Paterson et al., 2009). Most of the bioenergy associated traits like biomass, carbohydrates, and stem juiciness are complex as shown by their continuous variation within a population, indicating that there are several genes responsible for the observed variability. However, there are limited studies regarding the genetics of sorghum carbohydrates and biomass production.

Production Challenges

The United States is considered the world’s top sorghum producer. Sorghum production challenges are focused on optimizing performance of the crop on marginal lands, developing avenues for the efficient conversion of biomass to biofuel, and developing better technologies for harvesting and processing. The costs associated with transportation of the crop to the mill will be the major limiting factor in the profitability of sorghum production for biofuels. Varieties that have higher sugar contents per ton of biomass will be more efficient to process and haul to the mill. A study by Memphis Bioworks showed that sweet sorghum processing plants should be located within six miles from production fields in the Delta region (Tripp et al., 2009). To utilize the bagasse for livestock feed, cattle should also be in the vicinity. Harvest timing greatly influences total biomass and fermentable carbohydrate contents of sweet sorghum. Lodging combined with rapid fermentable juice degradation after extraction or killing freeze, are issues that must be considered for harvesting and conversion process management. Almodores et al. (2007), Broadhead (1972), and Zhao et al. (2009) found that total dry matter yield, brix value, and sucrose content of sweet sorghum was highest when harvested at physiological maturity. Tsuchihashi and Goto (2004) reported single pass juice extraction rates using a triple-roll mill to be approximately 50 percent. Extracted juice storage issues can be overcome by fermenting the extracted juice immediately, or storing it at temperatures near freezing to impede microbial activity and breakdown of fermentable carbohydrates. Currently, the only commercially viable harvest method for sweet sorghum is removing the entire crop with a forage harvester and transporting it to a mill or biofuel facility. Using this method, transportation costs and proximity to the facility will play a large role in determining sorghum production profitability.


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One of the primary disadvantages of sorghum and other plants rich in soluble sugars is seasonal availability and storage expense. Research has determined that net production cost of fermentable carbohydrate from sweet sorghum calculated at the farm-gate, can be well below typical cost of fermentable carbohydrates derived from corn grain (Bennett and Anex, 2008). Eight harvest-transport-processing options were modeled to determine the economic feasibility of sorghum to biofuel. Included in the model were 4-row self-propelled and 2-row tractor-pulled forage harvesters, two different modes of in-field transport, fresh processing, on farm ensilage and at-plant ensilage. Monte Carlo simulation and sensitivity analysis are used to account for system variability and compare scenarios. Transportation costs were found to be significant ranging from $33 to $71 Mg1 FC, with highest costs associated with at-plant ensilage scenarios. Economies of scale benefit larger milling equipment and boiler systems reducing fermentable carbohydrate costs by more than 50% with increasing annual plant capacity from 37.9 to 379 million liters. Ensiled storage of high moisture sweet sorghum in bunkers can lead to significant losses of fermentable carbohydrates (>20%) and result in systems with net fermentable carbohydrate costs well above those of corn-derived carbohydrates. Despite relatively high transport costs, seasonal, fresh processed sweet sorghum is found to produce fermentable carbohydrates at costs competitive with corn grain derived carbohydrates (Bennett and Anex, 2009). Costs of producing sorghum for silage vary based on the cultivar, soil conditions, harvesting, and agricultural practices. In the North Florida region, silage sorghum yield ranges from 3.5 to 6.3 dry tons per acre, and production costs, including harvesting, ranges from $50 to $90 per dry ton (Hewitt, 2006). Based on results from growing and converting sorghum to ethanol (McBee et al., 1988), 1 acre of sorghum can yield up to 7.59 tons of oven-dried stem and about 1,240 pounds of grain. Costs associated with conversion include transportation to processing facilities, juice extraction, and processing to biofuel. Without a conversion facility to obtain reliable data, any estimates may be speculative.

Summary

The development and use of biofuels from renewable resources is beneficial to the environment while encouraging capital investment and promoting economic development. Sorghum is a water sipping, highly sustainable cropping option for producers in semi-arid regions with limited irrigation capacity or dry land producers with unpredictable rainfall. Compared to many other crops, sweet sorghum has high water and nutrient use efficiencies and is considered environmentally sustainable. In limited water supply regions sorghum can conserve an important natural resource while offering more yield and sustainability. Sorghum has the potential to be an excellent diversified biofuel crop able to fill the needs of multiple bioenergy conversion process across many environments with reduced energy requirements. Environment, energy inputs, harvesting logistics, specific energy conversion processes, and economics will ultimately dictate which crops are used for renewable fuel production.

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