Biochemical Conversion of Biomass

Biochemical conversion of biomass refers to processes that decompose the original biomass into useful products. Commonly, the energy product is either in the liquid or in the gaseous form; hence, it is called “biofuel” or “biogas,” respectively. Biofuels are very promising for the transportation sector, while biogas is used for electricity and heat production. Normally, biofuels are obtained from dedicated crops (e.g., biodiesel from seed oil), while biogas production results from concerns over environmental issues such as elimination of pollution, treatment of waste, and control of landfill greenhouse gas (GHG) emissions.

Biogas from Anaerobic Digestion

Biogas is produced most commonly by anaerobic digestion of biomass. Anaerobic digestion refers to the bacterial breakdown of organic materials in the absence of oxygen. This biochemical process produces a gas called biogas, principally composed of methane (30%-60% in volume) and carbon dioxide. Such a biogas can be converted to energy in the following ways:

• • Biogas converted by conventional boilers for heating purposes at the production plant (house heating, district heating, industrial purposes).
• • Biogas for CHP generation.
• • Biogas and natural gas combinations and integration in the natural gas grid.
• • Biogas upgraded and used as vehicle fuel in the transportation sector.
• • Biogas utilization for hydrogen production and fuel cells.

An important production of biogas comes from landfills. Anaerobic digestion in landfills is brought about by the microbial decomposition of the organic matter in refuse. Landfill gas is, on average, 55% methane and 45% carbon dioxide. With waste generation increasing at a faster rate than economic growth, it makes sense to recover the energy from that stream, through thermal or fermentation processes.

Biofuels for Transport

A wide range of chemical processes may be employed to produce liquid fuels from biomass. Such fuels can find a very high level of acceptance by the market, thanks to the relatively easy adaptation to existing technologies (i.e., gasoline and diesel engines). The main potential biofuels are outlined below.

• • Biodiesel is a methyl-ester produced from vegetable or animal oil to be used as alternative to conventional petroleum-derived diesel fuel. Compared to pure vegetable or animal oil, which can be used in adapted diesel engines as well, biodiesel presents lower viscosity and slightly HH V.
• • Pure vegetable oil is produced from oil plants through pressing, extraction, or comparable procedures, crude or refined but chemically unmodified. Usually, it is compatible with existing diesel engines only if blended with conventional diesel fuel, at rates not higher than 5%-10% in volume. Higher rates may lead to emission and engine durability problems.
• • Bioethanol is ethanol produced from biomass and/or the biodegradable fraction of waste. Bioethanol can be produced from any biological feedstock that contains appreciable amounts of sugar or other matter that can be converted into sugar, such as starch or cellulose. Also, ligno- cellulosic materials (wood and straw) can be used, but their processing into bioethanol is more expensive. Application to modified spark ignition engines is possible.
• • Bio-ETBE (ethyl-tertio-butyl-ether) is ETBE produced on the basis of bioethanol. Bio-ETBE may be effectively used for enhancing the octane number of gasoline (blends with petrol gasoline).
• • Biomethanol is methanol produced from biomass. Methanol can be produced from gasification syngas (a mixture of carbon monoxide and hydrogen) or wood dry distillation (old method with low methanol yields). Virtually all syngas for conventional methanol production is produced by steam reforming of natural gas into syngas. In the case of biomethanol, a biomass is gasified first to produce a syngas from which the biomethanol is produced. Application to spark ignition engines and fuel cells is possible. Compared to ethanol, methanol presents more serious handling issues, because it is corrosive and poisonous for human beings.
• • Bio-MTBE (methyl-tertio-butyl-ether) is a fuel produced on the basis of biomethanol. It is suitable for blends with petrol gasoline.
• • Biodimethylether (DME) is dimethylether produced from biomass. Bio-DME can be formed from syngas by means of oxygenate synthesis. It has emerged only recently as an automotive fuel option. Storage capabilities are similar to those of LPG. Application to spark ignition engines is possible.

Benefits from Biomass Energy

There is quite a wide consensus that, over the coming decades, modern biofuels will provide a substantial source of alternative energy. Nowadays, biomass already provides approximately 11%—14% of the world’s primary energy consumption (data vary according to sources).

TABLE 4 Benefits in Reduction of GHG Emissions

Source: Risoe National Laboratory.¹⁶¹ Note: +, positive; neutral.

There are significant differences between industrialized and developing countries; in particular, in many developing countries, bioenergy is the main energy source, even if it is used in very low efficient applications (e.g., cooking stoves have an efficiency of about 5%—15%). Furthermore, inefficient biomass utilization is often associated with the increasing scarcity of hand-gathered wood, nutrient depletion, and the problems of deforestation and desertification.

One of the key drivers to bioenergy deployment is its positive environmental benefit regarding the global balance of GHG emissions. This is not a trivial matter, because biomass production and use are not entirely GHG neutral. In general terms, the GHG emission reduction as a result of employing biomass for energy is as reported in Table 4.

Since the energy cost associated with collection and transport of biomass is a significant portion, bioenergy is a decentralized energy option whose implementation presents positive impacts on rural development by creating business and employment opportunities. Jobs are created all along the bioenergy chain, from biomass production or procurement, to its transport, conversion, distribution, and marketing.

Bioenergy is a key factor for the transition to a more sustainable development.

Potential for CO[sub(2)] Emission Reduction

When biomass is used for energy production, the carbon contained in it is ultimately transformed into C0₂. In fact, such a biomass-derived C0₂ does not contribute to global warming, as it equals the C0₂ absorbed by the biomass during its growth; the relatively short time of such carbon cycle makes the biomass a carbon-neutral energy resource.

Abundant resources and favorable policies¹⁷! enable bio-power to expand in Northern Europe (mostly cogeneration from wood residues), in the United States, and in countries producing sugar cane bagasse (e.g., Brazil).

In the short term, cofiring remains the most cost- effective use of biomass for power generation, along with small-scale, off-grid use. In the mid-long term, gasification plants and biorefineries for biofuel production could expand significantly (mainly ethanol, lignocellulosic ethanol, biodiesel). International Energy Agency projections suggest that the biomass share in electricity production may increase from the current 1.3% to some 3%-5% by 2050, depending on assumptions.¹⁸¹ This is a small contribution compared to the estimated total biomass potential, but biomass are also used for heat generation and to produce fuels for transport.

Today, biomass supplies some 50 EJ/yr (1 EJ = 10^ls joules [J] = 10¹⁵ kilojoules [kj] = 24 million tons of

equivalent [Mtoe]) globally, which represents 10% of global annual primary energy consumption (Figure 2). This is mostly traditional biomass used for cooking and heating.

Based on this diverse range of feedstocks, the technical potential for biomass is estimated in the literature to be possibly as high as 1500 EJ/yr by 2050, although most biomass supply scenarios that take into account sustainability constraints indicate an annual potential of between 200 and 500 EJ/yr (excluding aquatic biomass). Forestry and agricultural residues and other organic wastes (including municipal solid waste) would provide between 50 and 150 EJ/yr, while the remainder would come from energy crops, surplus forest growth, and increased agricultural productivity.

FIGURE 2 Share of bioenergy in the world primary energy mix. (Source: International Energy Agency.¹⁹¹)

Projected world primary energy demand by 2050 is expected to be in the range of 600 to 1000 EJ, compared to about 500 EJ in 2008.^|9) Scenarios looking at the penetration of different low-carbon energy sources indicate that future demand for bioenergy could be up to 250 EJ/yr. This projected demand falls well within the sustainable supply potential estimate, so it is reasonable to assume that biomass could sustainably contribute between a quarter and a third of the future global energy mix. Whatever is actually realized will depend on the cost competitiveness of bioenergy and on future policy frameworks, such as GHG emission reduction targets.

Given the C0₂-neutral nature of biomass, and assuming that biomass will primarily substitute fossil fuels, the potential for reduction in C0₂ emission in 2050 can then be estimated as the same figure (i.e., around 20%-30% of anthropogenic C0₂), compared to a business-as-usual scenario. Definitely, biomass will play a determinant role towards a C0₂-free development.

Conclusions

Biomass refers to a very wide range of substances produced by biological processes. In the energy field, special focus has been and will be placed on vegetable biomass, such as wood and agricultural byproducts, because of the energy potential as well as economic and environmental benefits. Size and humidity standardization of biomass is a necessary step to make it suitable for effective domestic and industrial exploitation: chips, briquettes, and pellets are modern examples of standard solid fuels.

Biomass can be converted into energy in three pathways: combustion, thermochemical processing, and biochemical processing. The combustion of solid biomass for the production of heat or electricity and heat is the most viable technology, while pyrolysis and gasification still face economic and reliability issues. Among biochemical processes, anaerobic digestion is often used to reduce the environmental impact of hazardous waste and landfills. Biochemical processes are also concerned with the conversion of biomass into useful fuels for transportation, such as biodiesel, bioethanol, and biomethanol. All of them can effectively contribute to the transition to a more sustainable transportation system at zero GHG emissions.

Biomass represents a viable option for green energy resources of the 21st century.

References

• 1. European Biomass Industry Association, available at http:// www.eubia.org/.
• 2. DOE Biomass Research and Development Initiative, available at http://www.bioproducts- bioenergy.gov/.

• 3. Overend, R.P.; Milne, T.A.; Mudge, L.K. Fundamentals of Thermochemical Biomass Conversion; Elsevier Applied Science Publishers Ltd.: New York, 1985.
• 4. Bridgewater, A.V. The technical and economic feasibility of biomass gasification for power generation. Fuel J. 1995, 74 (6-8), 557-564.
• 5. Franco, A.; Giannini, N. Perspectives for the use of biomass as fuel in combined cycle power plants. Int. J. Thermal Sci. 2005, 44, (2), 163-177.
• 6. Risoe National Laboratory, Denmark, available at http:// www.risoe.dk/.
• 7. Biomass for Power Generation and CHP, International Energy Agency (IEA), Energy Technology Essentials, ETE03, Jan 2007.
• 8. World Energy Outlook; International Energy Agency (IEA), 2002.
• 9. Bioenergy—A Sustainable and Reliable Energy Source, International Energy Agency (IEA), available at http://www.ieabioenergy.com, 2009.