3Biomass
The Role of Charcoal
Biomass is organic matter primarily in the form of wood, crop residues, and animal waste in that order of importance. Biomass as wood is readily available in temperate and tropical regions or collected with ultimately debilitating consequences in semi-arid areas. The great advantage of biomass is that it is free, and in temperate and tropical regions, freely available. Generally speaking, since biomass is “free,” it is inefficiently utilized as a residential or commercial fuel. For instance, about two-thirds of the energy content of wood is lost when transformed into charcoal in developing nations, about double that for producing charcoal in the developed world. Charcoal is made from wood through a process called pyrolysis where wood is heated in the absence of sufficient oxygen to support combustion. Organic gases and water are evaporated leaving a residue of nearly pure carbon. Released gases provide fuel for pyrolysis and for drying fresh wood before being transformed to charcoal. Any backyard barbecue hamburger-flipping aficionado can recite the virtues of charcoal over wood: higher heat content, cleaner burning, and conveniently transportable.
Modern research is underway to transform something as old and mundane as charcoal to a new product. The method of making biochar, a substance with a long history dating back to the aboriginal Indians in the Amazon basin, was lost when their civilization collapsed. Biochar, made from the residue of incomplete organic pyrolysis, was a key component in terra preta soils found in the Amazon. Terra preta soil was highly productive allowing large numbers of Indians to live in well-organized permanent communities where today few can survive on natural red clay soil depleted of nutrients by tropical rains. Terra preta improved soil texture for plant growth by retaining and slowly releasing water, natural fertilizers, and nutrients. It greatly reduced the risk of water table contamination compared to modern practices of spreading animal manure and commercial fertilizers. Biochar promoted plant growth on marginal land increasing agricultural output, which, through photosynthesis, sequestered carbon. Research is underway to try to replicate something the Indians were doing centuries ago.[1] One company has developed a fuel efficient process to convert municipal solid waste, sewage sludge, PVC, plastics, rubber tires, wood, food, animal, and agricultural waste to synthetic biofuels via depolymerization employing microwaves with biochar as the residual product.[2]
Biomass in Home Heating
Biomass is burned for heating homes in New England and other parts of North America and northern Europe. Biomass can be firewood split from logs or bark and edgings residue from a lumber mill. Fireplaces burning split logs provide an attractive background setting in living rooms of millions of homes. Unfortunately conventional fireplaces allow most of the heat to escape up the chimney. Some fireplaces may actually increase heating needs by acting as a heat pump transferring warm indoor air to the outside environment. When firewood is burned in homes in North America and Europe specifically for heating purposes, combustion takes place in specially designed space heaters where relatively little heat escapes along with the products of combustion to the outside. The reason for this is clear: firewood is not free. For those who gather their own firewood, the desire not to dedicate too much time wielding an ax or chain saw when a paying job or some other preferred activity is at stake provides the incentive to have a fuel efficient wood burner. Most wood burners intheUS are located in rural areas of New England and New York not served by natural gas pipelines.[3]The Northeastern US is not alone in consuming biomass for heating. Biomass supplies as much as 25 percent of heating requirements in Estonia, Latvia, Finland, and Sweden.[4]
Wood Pellets
Wood and wood pellets have displaced an estimated 5 billion gallons per year of heating oil and propane in the US. Wood pellets are gaining popularity as a home fuel at the expense of split wood and wood residues. Raw material for making wood pellets can be sawdust, wood chips, lumber mill scrap, and entire trees unsuitable for lumber, either green (freshly cut) or partially dry. To ensure proper operation of wood pellet stoves made by different manufacturers, the varied raw materials must be transformed to a standard product with consistent moisture and ash content, heat value, and burn characteristics. The first step to produce wood pellets is hammer mills reducing raw material to sawdust, then dried to a specific moisture content. Fuel for drying can be either sawdust as a biofuel or propane or natural gas as petrofuel (choice of fuel is critical in determining the impact of wood pellets on carbon emissions). Sawdust is heated by pressurization to release natural lignins and then passed through a high pressure extrusion die to manufacture pellets of correct diameter, length, and density with lignin acting as “glue” to ensure their integrity and durability.[5] While pellets have energy content near that of coal, energy consumed in producing pallets is higher than mining and shipping coal to a customer when all factors are taken into consideration including energy consumed in collecting wood waste. Nevertheless carbon emissions of wood pellets should be less than coal net of replacement plant growth.
Wood pellets are sold in 40 pound bags, 50 bags to a skid, delivered to homeowners. When needed, a bag must be carried and emptied into an automatic feeding furnace controlled by a thermostat. Despite this added effort, more than 800,000 homes in the US have switched to wood pellets for space heating. Some consume two tons of pellets a year in more moderate climates or where pellet stoves are supplementary to the primary means of heating. Other homes more dependent on pellets for heat and located in colder regions consume four tons of pellets (four delivered skids).
An alternative to a wood pellet stove is an anthracite coal stove. Anthracite coal has the highest energy content and lowest sulfur and nitrous oxides and particulate emissions of all coal types. Coal is delivered in bulk by the ton and normally stored in a sheltered space near the coal stove, which is replenished by a coal bucket. Coal stoves do not have an automatic feed feature of wood pellet stoves and require more attention. Wood pellet and anthracite coal stoves are space heaters that may supplement a heating oil furnace. However if properly placed on a lower floor location with inside doors and inter-floor vents open, it may be possible that the furnace never turns on throughout the winter.[6] About 100,000 homes a year without access to lower cost natural gas are abandoning heating oil and propane for pellet and, to a lesser degree, anthracite stoves. Anthracite stoves compete with wood pellet stoves and buyers must decide which type stove to purchase based on economic and operational considerations and fuel availability.[7]
Growth in wood pellet consumption is rapidly accelerating. US capacity to produce pellets tripled between 2008 and 2011 reaching an estimated 9.4 million tons annually in 2012 and is expected to be 15.6 million tons by 2016. While domestic consumption is increasing, real growth is booming exports from southeastern US, up to 2 million tons in 2012 from virtually nothing just a few years earlier. The market is utilities in the UK, the Netherlands, and Belgium burning wood pellets to meet renewable energy standards. Europe’s domestic market for pallets is 13 million tons in 2012, which includes both home and utility consumption. It is slated to continue to grow to 25–30 million tons to allow the European Union to achieve its goal of 20 percent renewables by 2020.[8] At that time, 68 percent of renewables in the EU will be biomass segmented with 52 percent for heating purposes, 11 percent biofuels, and 5 percent electricity demand, which is hearty growth![9] In response for this new demand for biomass, bioenergy forest plantations to produce up to 25 million tons of woody biomass are being proposed in the US Southeast. Choice of biomass includes pine, eucalyptus, sweetgum, hybrid poplar, and cottonwood, and other fast growing plant species. Annual forest plantation yields are expected to be 8–15 green tons per acre with harvesting occurring every 5–12 years providing a renewable and sustainable biomass resource for a number of bioenergy applications.[10]
Utilities in Korea are planning to import 5 million tons of wood pellets by 2020 from Australia, Vietnam, Indonesia, and North America to meet their renewable energy quotas.[11] A growing Asian market for wood pellets is spawning projects in northwest Canada to develop biomass farms for wood pellet manufacture. Carbon emission reduction not only has to cover fossil fuels consumed in harvesting and manufacturing wood pellets, but ship’s fuel consumed in moving pellets thousands of miles across the Atlantic or Pacific Oceans. Another area of biomass growth for heating is pellets made from switchgrass, which will be sold to greenhouses facing high fuel oil bills during the winter months. Monetary savings in switching to biomass can be significant.[12]
It goes without saying that pellet exports should be conducted on a sustainable basis that does not result in deforestation. Sustainability can be assured if sufficient land for tree farms is set aside whereby biomass converted to pellets is less than replacement growth. Biomass farms may be initially sustainable, but can become nonsustainable if depletion of soil nutrients is left unattended as trees are continually removed. Fertilizers applied to sustain tree growth that are made from fossil fuels would detract from biomass fuel’s capacity to reduce greenhouse gas emissions.
Two Processes for Making Ethanol
Two chief processes for making ethanol from corn have to do with the nature of the products: wet mill and dry mill. The wet mill process produces ethanol plus a variety of food products such as corn sweeteners, corn syrup, corn oil, and gluten feed. The dry mill process produces ethanol and a high-protein animal feed called wet or dried distillers’ grain. Dry mill plants have lower capital and operating costs, make up over 80 percent of ethanol plants, and are attractive investments for farmer cooperatives, entrepreneurs, and private investors. Wet mill plants produce higher-valued coproducts that justify their higher capital and operating costs and are owned by large food corporations who have access to supermarket shelf space.
Ethanol production is presented here in some detail to gain an appreciation that little in energy is done in a simple straightforward fashion. The process begins with truckload deliveries of corn kernels to an ethanol plant with a storage capacity of 7–10 days of production. They are screened to remove debris such as bits of corn stalks and then ground into coarse flour. Milled corn grain is mixed with water to a desired pH factor and cooked as hot slurry. An alpha-amylase enzyme is added and the slurry is heated to 180–190°F for 30–45 minutes to reduce its viscosity. Next is primary liquefaction where the slurry is pumped through a pressurized jet cooker at 221°F, held for five minutes, and then cooled by an atmospheric or vacuum-flash condenser. Then secondary liquefaction holds the mixture for 1–2 hours at 180°F–190°F to allow alpha-amylase enzyme to break down starch into short chain dextrins.After adjusting for pH and temperature, a second enzyme, glucoamylase, is added as the mixture is pumped into fermentation tanks. Now known as mash, the glucoamylase enzyme breaks down the dextrins into simple sugars.
Once starch is converted to sugar, the process is the same as making ethanol from juice extracted from sugarcane. Yeast is added to convert sugar to ethanol and carbon dioxide. Fermentation takes 50–60 hours ending up with a mash of 15 percent ethanol, grain solids, and yeast. Carbon dioxide is captured at some ethanol plants and sold to companies as dry ice for flash freezing and as condensed, pressured gas for carbonating soft drinks. Carbon dioxide may one day be pipelined to played-out oil reservoirs as a tertiary means of enhancing recovery, or to greenhouses to enhance plant growth, or to algal farms to produce more biofuels.
Fermented mash is pumped and heated in a multicolumn distillation system. The columns take advantage of differences in boiling points of ethanol and water to separate hydrous ethanol (95 percent ethanol, 5 percent water) equivalent to 190-proof alcohol. Hydrous ethanol can be consumed directly in pure ethanol burning automobiles. To produce anhydrous ethanol required for gasohol, hydrous ethanol passes through a molecular sieve that separates water and ethanol molecules yielding 200-proof anhydrous or waterless ethanol. Then a denaturant such as gasoline or other petroleum liquid is added to make ethanol unfit for human consumption before entering storage normally sized to hold 7–10 days’ production.
The difference between using corn and sugar as feedstocks once the corn is reduced to a sugar is in the residues. In Brazil, waste of distillation of sugar is vinasse that is applied to sugarcane fields as a form of fertilizer-recycling. In the US, stillage in the bottom of distillation tanks contains solids and yeast as well as water added during the distillation process. It is passed through centrifuges to separate thin stillage, a liquid with 5–10 percent solids and wet distillers’ grain. Some of the thin stillage, referred to as sweetwater, is routed back to the slurry tanks as makeup water, reducing the amount of fresh water required to support ethanol-making. The rest is sent through a multiple evaporation system to concentrate stillage into syrup with 25–50 percent solids. The syrup, high in protein and fat content, is mixed with wet distillers’ grain from centrifuges. With the syrup, wet distillers’ grain contains most of the nutritional value of the original corn or other grain feedstock plus waste residual yeast from fermentation.
Wet distillers’ grain has a limited shelf life, varying between four days to two weeks, and is expensive to transport with its high water content. Wet distillers’ grain, which is 33 percent solid, is sent to dairy farms or cattle feedlots located within 100 miles of the ethanol plant. Dairy cows can be fed rations of up to 43 percent wet distillers’ grain and beef cattle up to 37 percent. Alternatively wet distillers’ grain can be dried into dry distillers’ grain, but the drying process increases energy consumption for older, less efficient plants by as much as 50 percent. Dry distillers’ grain has a shelf life of several months, is less costly to transport, and is commonly used as a high-protein additive in cattle, swine, poultry, and fish feed. Evaporated thin stillage sold as condensed syrup, or thick stillage, can be blended or sprayed over distillers dried grains to produce distillers dried grains with solubles. Ruminant feed (dairy cows and cattle) can be up to 40 percent dry distillers’ grain and up to 10 percent for nonruminant feed (poultry and swine).
In 2014, US distillers grains were 60 percent dried, 27 percent wet, and 13 percent modified wet. Of total distillersgrains, beef cattle consumed 43 percent, dairy cattle 30 percent, swine 16 percent, poultry 10 percent, and other 1 percent. Ethanol plants produced 36 million metric tons of distillers grains plus 2 million tons of corn gluten feed and 1 million tons of corn gluten meal. Distillers grain exports of almost one third of production totaled 11.4 million tons of which China received 43 percent and Mexico 13 percent. Nations receiving 3–6 percent of exports in order of importance were Vietnam, S. Korea, Japan, Turkey, Thailand, Indonesia, and Canada.[13]
In the wet mill process, grain is first soaked or “steeped” in water and diluted sulfurous acid for 24–48 hours to separate components within the grain. After steeping, corn slurry passes through a series of grinders to separate corn germ from which corn oil is then extracted. Hydroclonic, centrifugal, and screen separators segregate fiber, gluten, and starch components. Steeping liquor is concentrated to heavy steep water in an evaporator, then co-dried with the fiber component and sold as corn gluten feed of 21 percent protein to the livestock industry. A portion of the gluten component is further processed for sale as corn gluten meal to poultry breeders with 60 percent protein content and no fiber. Gluten can find its way into the human food chain, which some find objectionable. Starch can be fermented into ethanol, or dried and sold as cornstarch, or processed into corn syrup.
A typical dry mill will produce 2.8 gallons of ethanol, 17.5 pounds of distillers’ dried grains, and 17 pounds of carbon dioxide in addition to thin stillage from a bushel of corn. Emerging technologies may increase marketable co-products in the form of germ separation prior to final grinding of the corn kernels and other fermentable products such as lactic acid, acetic acid, glycerol, and others. Progress is constantly being made to improve efficiency and productivity of ethanol production. Between the early 1980s and early 2000s, there has been a 50 percent decline in energy required to produce ethanol, an increase in production yield of 23 percent from 2.2 gallons per bushel of corn to 2.7 gallons per bushel and a cut in capital costs of building ethanol plant by 25–30 percent. Earlier plants used azeotropic distillation systems to dehydrate (removal of the last vestiges of water) which proved to be expensive, costly to operate, energy intensive, and hazardous. Today molecular sieves or molsieves are the most popular means to dehydrate ethanol. Molsieves, a bed of ceramic beads, absorb water molecules in vaporized ethanol. Molsieves are lower cost, easier to operate, less energy demanding, and more environmentally friendly compared to azeotropic distillation. Another improvement is energy recycling–thermal capture of waste energy via heat exchangers for heating purposes. Technology improvements for making enzymes, required for hydrolyzing starches to fermentable sugars, have increased yield fivefold. The practice of discarding spent yeast and replacing with a new batch, called “pitching,” has given way to ethanol plants propagating their own yeast. Ethanol plants that once purchased truckloads of yeast per month now need only a few pounds to start the propagation process. While plant automation has reduced the number of employees required to run an ethanol plant, the remaining workers must have a higher skill set. Automation has improved the efficiency and uniformity of the product, enhancing its quality. Since 1995, natural gas consumption to produce a gallon of ethanol is down 36 percent, electricity use down 38 percent, water consumption down 53 percent, ethanol yield (2.82 gallons of ethanol per bushel of corn) is up 12 percent. Moreover the yield for corn on a trend line basis has increased from 95 bushels per acre in 1980/1981 to 160 bushels per acre in 2012/2013. 2014/2015 was an exceptional good year for corn and estimated yield is 171 bushels per acre. Part of this is improved varieties of corn that require less fertilizer than in the past resulting in a downward trend in the size of the Gulf of Mexico hypoxic zone (waters with insufficient oxygen to sustain marine life from algae blooms fed by fertilizers and other forms of pollution run-off entering the Mississippi River). Moreover, the ethanol yield per bushel has grown from 2.51 gallons in 1995 to 2.82 gallons in 2012 with every expectation for further growth. The aggregate impact of these improvements has been to cut the cost of producing ethanol by nearly half exclusive of buying raw materials and shipping ethanol to market.