Lighting the Way: Toward a Sustainable Energy Future

  • AuthorInterAcademies Council
  • Release Date1 October 2007
  • Copyright2007
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3.4 Biomass
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The conversion of sunlight into chemical energy supports nearly all plant and animal life on Earth. Biomass is one of humanity’s oldest energy resources and, according to available estimates, still accounts for approximately 10 percent of global primary energy consumption today. Precise data do not exist, but as much as one-third of the world’s population relies on fuel wood, agricultural residues, animal dung, and other domestic wastes to meet household energy needs. Such traditional uses of biomass are estimated to account for more than 90 percent of the biomass contribution to global energy supply, most of which occurs outside the formal market economy and predominately in developing countries. In these countries, traditional biomass has been estimated to account for more than 17 percent of total primary energy consumption. Modern uses of biomass to generate electricity and heat or as a source of fuels for transportation are estimated to account for less than 10 percent of total biomass energy consumption worldwide.

Because biomass is a renewable resource that can achieve low or near-zero carbon emissions (provided appropriate conversion technologies are used and feedstocks are sustainably managed), expanded reliance on biomass in modern applications is widely viewed as playing an important role in the transition to more sustainable energy systems. Biomass merits particular attention because, in the near to medium term, it offers the most promising alternatives to petroleum-based liquid fuels for the transportation sector. By contrast, biomass use in traditional applications often has negative impacts on public health and the environment and is frequently conducted in a manner that cannot be considered sustainable or renewable (in the sense that it avoids degrading or depleting the underlying resource base over time). Aggregated energy data rarely distinguish between different types of biomass uses: it is difficult to tell from available statistics, for example, what portion of the estimated biomass contribution consists of forest and agricultural waste collected manually by small communities versus large-scale production of charcoal from native forests to supply industries and cities.60

In general, traditional uses of biomass, primarily for cooking in many parts of Africa, Asia, and Latin America, are quite inefficient and frequently result in the depletion of natural resources. Reliance on biomass fuels can lead to deforestation, for example, and in doing so can become a net source of greenhouse gas emissions. Moreover, in traditional applications, the quality of energy services provided using biomass resources (mostly lighting and heating) is generally poor and exacts a high price in terms of the human work necessary to collect and transport the fuel. This work can have the effect of excluding entire populations—especially girls and women—from the formal economy. And the health impacts associated with high levels of indoor air pollution typically pose a particular risk for the most vulnerable members of a community (women, children, and the elderly). Despite these drawbacks, billions of people continue to rely on dung, crop residues, and wood for the simple reason that these fuels are the most accessible and least costly energy resources available to them. Dry biomass is easily stored. Its use has cultural roots in many societies. And without it, many countries would have to increase energy imports, and many poor households would have to expend a greater share of their limited resources on purchasing other commercial forms of energy. Progress in delivering modern energy to rural areas has been slow, but significant opportunities exist to improve or displace traditional methods of using biomass energy with attendant benefits in terms of human health and conservation. Various technology options for improving combustion efficiency and reducing emissions are available at relatively modest cost: a modern cooking stove, for example, can yield efficiency improvements of 10–30 percent for a cost of US$5–10. Switching from traditional biomass to biogas, kerosene, propane (liquid petroleum gas), or even electricity can raise cooking stove efficiency substantially at a cost of US$20–60 per unit (refer back to Box 1.2 in Chapter 1).

Modern uses of biomass, however, offer a far greater array of possibilities for reducing dependence on fossil fuels, curbing greenhouse gas emissions, and promoting sustainable economic development. A range of biomass energy technologies, suitable for small- and large-scale applications, are available. They include gasification, combined heat and power (cogeneration) schemes, landfill gas, energy recovery from municipal solid wastes, or biofuels for the transportation sector (ethanol and biodiesel).

Recent interest in biomass energy has focused primarily on applications that produce liquid fuels for the transportation sector. Figure 3.8 outlines potential pathways to future biofuels production. Given growing concerns about global petroleum supply adequacy and the current lack of diversity in available fuel options for the transport sector, such fuels represent the highest-value use of biomass energy at present. Ultimately, the most promising biomass applications of all are likely to involve integrated systems where, for example, biomass is used as both fuel and feedstock in the co-production of liquid transportation fuels and electricity.

Of all available options, sugarcane ethanol is the most commercially successful biomass fuel in production today. Sugarcane ethanol has a positive energy balance and has benefited from supportive government policies in several countries, including Brazil that currently meets roughly 40 percent of its passenger vehicle fuel needs (one-third of its total transportation energy demand) with sugarcane ethanol (Macedo and other, 2004; Goldemberg and others, 2003). Globally, a substantial near-term opportunity exists to expand sugarcane ethanol production: almost 100 countries harvest sugarcane and state-of-art conversion technologies are available. Moreover, experience in Brazil suggests that the adverse environmental impacts associated with large-scale sugarcane ethanol production can be significantly mitigated by experience and legal enforcement of environmental regulations. Ethanol is also being produced on a commercial scale from corn in the United States, which has subsidized ethanol for a number of years and more recently adopted a federal renewable fuels mandate to promote alternatives to petroleum-based transportation fuels (USDOE, 2006; Perlack and others, 2005). Another type of biomass-based transport fuel—biodiesel—has recently become commercially available as a result of programs in Europe and North America, but this option offers limited potential for reducing production costs and its viability is likely to continue to depend on external incentives like agricultural subsidies. In addition, adherence to fuel specifications and effective quality control are important factors for ensuring the commercial viability of biodiesel. Recent technology advances have involved efforts to diversify the biodiesel supply chain by, for example, using bioethanol instead of coal methanol as a feedstock. Biogas energy from anaerobic digestion at landfills, sewage treatment facilities, and manure management sites, is considered a ‘low-hanging

 

 

 

 

 

 

 

 

Figure 3.8 Potential pathways for biofuels production

fruit’ option in the context of carbon credits available through the international Clean Development Mechanism (CDM). This form of biomass energy not only displaces fossil-fuel combustion but reduces emissions of methane, a more potent greenhouse gas than carbon dioxide.

Commercially available technologies for converting biomass to usable forms of energy vary in terms of scale, fuel quality, and cost. Large-scale technologies that are already on the market include fixed bed combustion, fluidized beds, dust combustion, biomass and coal co-firing, municipal solid wastes energy recovery as well as several types of systems for gasification, pyrolysis, etc. Many of these technologies are not yet commercially available in developing countries, however, and require financial support—as well as local capacity building—if they are to be deployed more widely.

The future of modern biomass
As with some other renewable energy options, the theoretical potential for biomass energy is enormous. Of the approximately 100,000 terawatts of solar energy flow that reach the Earth´s surface, an estimated 4,000 terawatts reach the world’s 1.5 billion hectares of existing crop lands. Assuming that modern biomass technologies could achieve 1 percent energy conversion efficiency, these existing crop lands could in theory yield 40 terawatts of usable energy flow, or more than 3 times the current global primary energy supply flow of 14 terawatts. This exercise is not intended to imply that all arable land should be converted for energy-production purposes but only to illustrate that there is scope for a significant expansion of the modern biomass energy contribution, given that this contribution was estimated at only 0.17 gigawatts in 2003 (Somerville, 2005; Macedo, 2005).

There are numerous areas in developing countries where the harvesting of improved biofuel feedstocks can be substituted for the present foraging of indigenous plants. The efficient use of these biomass feedstocks for the local co-production of heat, electricity, and transportation fuel would also have a profound impact on the ability of rural populations to access modern, cleaner forms of energy. Energy solutions that can be deployed with modest capital investments will be a crucial element of an effective energy strategy. It will also be crucial—as part of any large-scale expansion of biomass energy production—to manage competing demands for food production and habitat preservation. In areas where the resource base is sufficiently abundant to support both food and energy crops, or in cases where it is feasible to make complementary use of the same feedstocks (e.g., using residues from food crops for energy production), land constraints may not emerge as a significant issue. In other areas, however, the potential for energy production to displace food production may generate concern—especially if food production serves the local population, while energy production is primarily for export.61

Some of the most promising opportunities for addressing these concerns and expanding the contribution of modern biomass energy involve cutting-edge advances in the biological and chemical sciences, including the development of crops designed for energy production through genetic selection or molecular engineering, specialized enzymes, and even the artificial simulation of natural biological processes such as photosynthesis. Breakthroughs from new frontiers in biomass energy, in any of the several areas of current research described in, could have profound implications for the future of biomass energy technologies. As with other renewable resource options, the magnitude of the biomass contribution will depend on how much progress can be achieved in key areas:

    Reducing costs; Mitigating environmental impacts like water usage, chemicals (pesticides or fertilizers) added, biodiversity losses; and Minimizing pressure on scarce land resources in terms of competing requirements for food and fiber production and habitat preservation.
    Box 3.2 Frontiers in biofuels production
    At present, the biofuels industry is primarily based on the production of ethanol via the fermentation of sugars or starches and on the production of biodiesel derived from plant oils. The use of lignocellulosic (woody or fibrous) biomass materials—as opposed to starches or sugars—is, thought however to hold far greater potential for maximizing the efficient conversion of sunlight, water, and nutrients into biofuels. Perennial plants such as grasses or fastgrowing trees appear particularly attractive for large-scale sustainable biofuel production for several reasons: (a) no tillage is required for approximately 10– 15 years after first planting, (b) longlived roots can be developed to establish symbiotic interactions with bacteria to acquire nitrogen and mineral nutrients, resulting in order-of-magnitude less nitrate runoff and soil erosion, and (c) some perennials withdraw a substantial fraction of mineral nutrients from above-ground portions of the plant before harvest. Wild-type grasses such as miscanthus have produced up to 26 dry tons per acre (sufficient to produce 2,600 gallons of ethanol per acre) on non-irrigated, non-fertilized land in the United States (Long, 2006). This is approximately five times higher than the average yield from sugarbeet or starch feedstocks such as corn (the latter in dry weight). In general, biodiesel yields from most types of feedstock—except palm oil—are smaller.
    Present methods of producing ethanol from cellulosic feedstock proceed in three steps:
    (a) Thermochemical pretreatment of raw biomass to make complex cellulose and hemicellulose polymers more accessible to enzymatic breakdown;
    (b) Application of special enzyme cocktails that hydrolyze plant cell-wall polysaccharides into a mixture of simple sugars; and
    (c) Fermentation, mediated by bacteria or yeast, to convert these sugars to ethanol
    The energy-rich lignin that is separated from the cellulose and hemicellulose can then be either burned to power the biorefinery or converted to syngas and then to Fischer-Tropsch fuels.
    Current methods depend on complex, energy-intensive steps where pretreatment is incompatible with enzymatic deconstruction. As a result, additional neutralization steps are necessary, adding to overall cost and reducing overall process efficiency. In future bio-refineries, depolymerization (saccharification) and fermentation processes may be consolidated into a single step using a mixture of organisms in converting biomass to ethanol. Significant improvements in reducing energy inputs, enzyme costs, and the number of processing steps are highly likely if a total systems approach to biofuels production is taken.
    Applying advances from rapidly developing areas of science and technology such as synthetic biology and high throughput functional genomics holds out promise for rapidly improving feedstocks and the conversion of those feedstocks into biofuels. Possible areas of research that would increase biomass production and its conversion into fuel are listed in Table 3.6. Cellulosic materials such as rice and wheat straw, corn stover, and other crop and forest residues can serve as sources of cellulosic feedstock.
    The development of photosynthetic microbes that produce lipids or hydrocarbons also has great potential for biofuels production. While plant production of useable biomass is unlikely to exceed an overall solar conversion efficiency of 1–2 percent, algae can convert solar power at efficiencies in excess of 10 percent. A combination of anaerobic and aerobic microbial processes can be separately optimized so that a fuel precursor can be produced in an anaerobic environment and the final product in an aerobic setting. Efficient algae cultivation that would take full advantage of the high quantum efficiency of these micro- organisms would, however, require capital intensive infrastructure.

Solutions that simultaneously address all of these hurdles involve expanding the land available for biomass energy production; integrating biomass energy development with sustainable agricultural and forestry practices; improving crop productivity with regard to land, water, and nutrient use; and developing advanced production and conversion technologies. Biofuels produced from lignocellulose rather than starches appear more promising, both in terms of minimizing potential conflicts between food and energy production and in terms of maximizing environmental benefits (including greenhouse gas reductions) relative to fossil-fuel use.

Significant improvements have, of course, already been achieved worldwide with regard to agricultural productivity. Between 1950 and 1999, the land area used to grow cereal crops increased by 17 percent. During this same time, cereal-crop output rose by 183 percent, thanks to productivity improvements. The introduction of new strains of plant species has diversified crop cultures, allowing for efficient harvesting in different types of soils, climates, and water conditions and also achieving better yields.

The European Union and the United States are conducting intensive R&D to improve the cost competitiveness of commercial ethanol production. Current efforts are focused on promoting the efficient recovery of sugars through the hydrolysis of cellulose and hemicellulose fractions of biomass, as well as better sugar fermentation. Researchers are investigating a large number of possible process arrangements for different crops in hopes of reducing ethanol production costs by as much as one-third within five years (Macedo, 2005).

With rising oil and natural gas prices and with the new incentives generated by emerging carbon markets, landfill gas, sugarcane bagasse, biodiesel, managed forest wood, and waste-to-energy schemes are also becoming attractive options. Based on current trends in technology development, costs for biomass energy recovery are expected to decline by up to two-thirds in 20 years, even as a broader mix of biomass-based products—including not only energy products, but also chemical feedstocks—becomes commercially viable (Macedo, 2005).

Progress in developing biomass energy alternatives, besides relieving pressure on finite fossil-fuel resources, would reduce the cost of mitigating carbon emissions. Sugarcane ethanol, for example, has a positive net energy balance of eight to one and a near-zero present carbon-mitigation cost. As a means of avoiding greenhouse gas emissions, bioethanol could soon achieve negative costs as it becomes cheaper than gasoline—even without government subsidies—in some markets. On the other hand, much of the ethanol and biodiesel commercially produced in the OECD

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

countries at present has carbon mitigation costs in the range between US$60–400 per ton of carbon dioxide equivalent if upstream energy and chemical inputs are accounted for. Fertilizer use to grow biomass feedstocks, for example, can produce emissions of nitrous oxide, an extremely potent greenhouse gas—thereby offsetting some of the climate benefits associated with avoided petroleum use. Similarly, converting biomass to liquid fuels requires energy and—depending on the conversion efficiency of the process and the energy sources used—can also produce significant offsetting emissions. Improving the performance of biomass fuels from a climate mitigation perspective therefore depends on reducing these inputs.

Toward that objective, significant R&D efforts are now being focused on the development of commercially viable methods for producing ethanol from cellulosic feedstocks, which could substantially reduce costs and enhance associated greenhouse gas reductions. Interest is also growing in the development of integrated systems that would allow for the co-production of energy feedstocks with other agricultural outputs as a means of achieving significant cost savings and environmental benefits. For example, biodiesel production may make sense only if it uses seeds that are non-edible (by both humans and animals) as a feedstock or if it can be coupled with the cultivation of animal food.

Other potentially promising examples of integrated systems involve gasification processes that could allow for the co-production of multiple valuable outputs, including electricity, liquid transportation fuels, and chemicals. Gasification technology can be used with multiple feedstocks, including energy crops, animal waste, and a wide range of organic materials, as well as coal and other carbonaceous fuels. In general, the process involves producing a synthesis gas (composed primarily of carbon monoxide and hydrogen) from any carbon- and hydrogen-containing material; the synthesis gas can then be used to drive highly efficient turbines and as a feedstock for manufacturing a variety of synthetic chemicals or fuels. Small-scale gasification technology may eventually emerge as a promising option for improving energy access in isolated regions. Meanwhile, the most important use of locally available biomass residues may be in combination with modern combustion technologies as a replacement for diesel oil, which is now commonly used in old and inefficient diesel engines. Potential technologies for directly converting biomass for these purposes include thermal-chemical and catalysis processes.

Today’s biotechnology industry is beginning to look beyond established production processes to more advanced options such as ethanol hydrolysis and fermentation, biodiesel enzymes, higher carbon fixation in roots, and improved oil recovery (Somerville, 2005). Advances in genetic engineering have already allowed for the development of disease-resistant strains and for crops that are viable in environments (such as degraded lands) that were previously considered unsuitable for cultivation, as well as for crops with reduced requirements in terms of chemical inputs and water. New cutting-edge technologies under development include lignocellullosic bioprocessing techniques that would allow for the co-production of fuels and chemicals in ‘bio-refineries’ and genetic modifications to biomass feedstocks to facilitate the application of process technologies that could achieve 70–90 percent energy conversion efficiencies (Box 3.2).

In summary: Biomass
The biomass industry is market driven and will pursue productivity improvements accordingly. Private actors will also want to remove trading barriers—both tariff-related and technical—to the wider use of their products. More sophisticated markets, public pressure, international agreements, and tighter environmental controls are forcing biofuels producers to develop socially and environmentally sound practices that reduce water and chemical requirements, preserve ecosystems, reduce greenhouse gas and conventional pollutant emissions, and generate high-quality jobs. Nevertheless, subsidies and other incentives may be necessary to advance biomass technologies in the early stages. Such subsidies should be progressively removed as biomass-energy industries move up the learning curve. Brazil’s successful effort to develop sugarcane ethanol as an alternative transportation fuel, which is today fully competitive with gasoline in international markets, provides a useful paradigm in this regard.

At the same time, enthusiam for biomass alternatives to petroleumbased transportation fuels must be tempered: government inducements and mandates to promote energy independence should not overly distort market forces that moderate the competition between biofuels, food production, and other land-uses—nor should they jump ahead of the technology needed to achieve large-scale biofuels production in an environmentally sustainable and economically sensible manner.

Document Date: October 1, 2007
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