Renewable Energy - Biomass Energy

Biomass refers to organic material such as plant and animal waste, wood, seaweed and algae, and garbage. The use of biomass is not without environmental problems. Deforestation can occur from widespread wood use if forests are clear-cut, resulting in the possibility of soil erosion and mudslides. Burning wood, like burning fossil fuels, also pollutes the environment. Biomass can be burned directly or converted to biofuel by thermochemical conversion and biochemical conversion.

Direct Burning

Direct combustion is the easiest and most commonly used method of using biomass as fuel. Materials such as dry wood or agricultural wastes are chopped and burned to produce steam, electricity, or heat for industries, utilities, and homes. Industrial-size wood boilers are operating throughout the country, and the Department of Energy (DOE) maintains that many more will be built during the next decade. The burning of agricultural wastes is also becoming more widespread. In Florida, sugarcane producers use the residue from the cane to generate much of their energy.

Wood burning in stoves and fireplaces is another example of direct burning of biomass for energy, in this case heat. In the United States, residential use of wood as fuel generated 359 trillion Btu in 2003. In comparison, the generation of Btu from burning wood in the home in the 1980s was about 850–950 trillion Btu according to the Energy Information Administration's Annual Energy Review 2003.

FIGURE 6.1

Wood burning in stoves and fireplaces is another example of direct burning of biomass for energy, in this case heat. In the United States, residential use of wood as fuel generated 359 trillion Btu in 2003. In comparison, the generation of Btu from burning wood in the home in the 1980s ranged from about 850–950 trillion Btu annually according to the Energy Information Administration's Annual Energy Review 2003.

Thermochemical Conversion

Thermochemical conversion involves heating bio-mass in an oxygen-free or low-oxygen atmosphere, transforming the material into simpler substances that can be used as fuels. Products such as charcoal and methanol are produced this way.

Biochemical Conversion

Biochemical conversion uses enzymes, fungi, or other microorganisms to convert high-moisture biomass into either liquid or gaseous fuels. Bacteria convert manure, agricultural wastes, paper, and algae into methane, which is used as fuel. Sewage treatment plants have used anaerobic

TABLE 6.1

Energy consumption by source, selected years, 1949–2003
(Quadrillion btu)
	Fossil fuels							Renewable Energy¹
Year	Coal	Coal coke net imports	Natural gas²	Petroleum^3,4	Total	Nuclear electric power	Hydroelectric pumped storage⁵	Conventional hydroelectri power	Wood, waste, alcohol^4,6	Geothermal	Solar	Wind	Total	Electricity net imports	Total^4,6
¹Electricity net generation from conventional hydroelectric power, geothermal, solar, and wind; consumption of wood, waste, and alcohol fuels; geothermal heat pump and direct use energy; and solar thermal direct use energy.
²Natural gas, plus a small amount of supplemental gaseous fuels that cannot be identified separately.
³Petroleum products supplied, including natural gas plant liquids and crude oil burned as fuel. Beginning in 1993, also includes ethanol blended into motor gasoline.
⁴Beginning in 1993, ethanol blended into motor gasoline is included in both "Petroleum" and "Wood, waste, alcohol," but is counted only once in total consumption.
⁵Pumped storage facility production minus energy used for pumping.
⁶"Alcohol" is ethanol blended into motor gasoline.
⁷Included in "Conventional hydroelectric power."
R = Revised.
P = Preliminary.
NA = Not available.
(s) = Less than 0.0005 and greater than −0.0005 quadrillion Btu.
Note: Totals may not equal sum of components due to independent rounding.
Web Page: For data not shown for 1951–1969, see http://www.eia.doe.gov/emeu/aer/overview.html.
SOURCE: "Table 1.3. Energy Consumption by Source, Selected Years, 1949–2003 (Quadrillion Btu)," in Annual Energy Review 2003, U.S. Department of Energy, Energy Information Administration, Office of Energy Markets and End Use, September 7, 2004, http://www.eia.doe.gov/emeu/aer/pdf/aer.pdf (accessed September 28, 2004)
1949	11.981	−0.007	5.145	11.883	29.002	0.000	⁷	1.425	1.549	NA	NA	NA	2.974	0.005	31.982
1950	12.347	0.001	5.968	13.315	31.632	0.000	⁷	1.415	1.562	NA	NA	NA	2.978	0.006	34.616
1955	11.167	−0.010	8.998	17.255	37.410	0.000	⁷	1.360	1.424	NA	NA	NA	2.784	0.014	40.208
1960	9.838	−0.006	12.385	19.919	42.137	0.006	⁷	1.608	1.320	0.001	NA	NA	2.929	0.015	45.087
1965	11.581	−0.018	15.769	23.246	50.577	0.043	⁷	2.059	1.335	0.004	NA	NA	3.398	(s)	54.017
1970	12.265	−0.058	21.795	29.521	63.522	0.239	⁷	2.634	1.431	0.011	NA	NA	4.076	0.007	67.844
1971	11.598	−0.033	22.469	30.561	64.596	0.413	⁷	2.824	1.432	0.012	NA	NA	4.268	0.012	69.289
1972	12.077	−0.026	22.698	32.947	67.696	0.584	⁷	2.864	1.503	0.031	NA	NA	4.398	0.026	72.704
1973	12.971	−0.007	22.512	34.840	70.316	0.910	⁷	2.861	1.529	0.043	NA	NA	4.433	0.049	75.708
1974	12.663	0.056	21.732	33.455	67.906	1.272	⁷	3.177	1.540	0.053	NA	NA	4.769	0.043	73.991
1975	12.663	0.014	19.948	32.731	65.355	1.900	⁷	3.155	1.499	0.070	NA	NA	4.723	0.021	71.999
1976	13.584	(s)	20.345	35.175	69.104	2.111	⁷	2.976	1.713	0.078	NA	NA	4.768	0.029	76.012
1977	13.922	0.015	19.931	37.122	70.989	2.702	⁷	2.333	1.838	0.077	NA	NA	4.249	0.059	78.000
1978	13.766	0.125	20.000	37.965	71.856	3.024	⁷	2.937	2.038	0.064	NA	NA	5.039	0.067	79.986
1979	15.040	0.063	20.666	37.123	72.892	2.776	⁷	2.931	2.152	0.084	NA	NA	5.166	0.069	80.903
1980	15.423	−0.035	20.394	34.202	69.984	2.739	⁷	2.900	2.485	0.110	NA	NA	5.494	0.071	78.289
1981	15.908	−0.016	19.928	31.931	67.750	3.008	⁷	2.758	2.590	0.123	NA	NA	5.471	0.113	^R76.342
1982	15.322	−0.022	18.505	30.232	64.037	3.131	⁷	3.266	2.615	0.105	NA	NA	5.985	0.100	^R73.253
1983	15.894	−0.016	17.357	30.054	63.290	3.203	⁷	3.527	2.831	0.129	NA	(s)	6.488	0.121	^R73.101
1984	17.071	−0.011	18.507	31.051	66.617	3.553	⁷	3.386	2.880	0.165	(s)	(s)	6.431	0.135	^R76.736
1985	17.478	−0.013	17.834	30.922	66.221	4.076	⁷	2.970	2.864	0.198	(s)	(s)	6.033	0.140	^R76.469
1986	17.260	−0.017	16.708	32.196	66.148	4.380	⁷	3.071	2.841	0.219	(s)	(s)	6.132	0.122	^R76.782
1987	18.008	0.009	17.744	32.865	68.626	4.754	⁷	2.635	2.823	0.229	(s)	(s)	5.687	0.158	^R79.225
1988	18.846	0.040	18.552	34.222	71.660	5.587	⁷	2.334	2.937	0.217	(s)	(s)	5.489	0.108	^R82.844
1989	19.070	0.030	19.712	34.211	73.023	5.602	⁷	2.837	3.062	0.317	0.055	0.022	6.294	0.037	^R84.957
1990	19.173	0.005	19.730	33.553	72.460	6.104	−0.036	3.046	2.662	0.336	0.060	0.029	6.133	0.008	^R84.668
1991	18.992	0.010	20.149	32.845	71.996	6.422	−0.047	3.016	2.702	0.346	0.063	0.031	6.158	0.067	^R84.595
1992	19.122	0.035	20.835	33.527	73.519	6.479	−0.043	2.617	2.847	0.349	0.064	0.030	5.907	0.087	^R85.949
1993	19.835	0.027	21.351	433.841	75.055	6.410	−0.042	2.892	^4,R2.803	0.364	0.066	0.031	^R6.156	0.095	^4,R87.578
1994	19.909	0.058	21.842	34.670	76.480	6.694	−0.035	2.683	2.939	0.338	0.069	0.036	6.065	0.153	89.248
1995	20.089	0.061	22.784	34.553	77.488	7.075	−0.028	3.205	3.068	0.294	0.070	0.033	6.669	0.134	91.221
1996	21.002	0.023	23.197	35.757	79.978	7.087	−0.032	3.590	3.127	0.316	0.071	0.033	7.137	0.137	94.224
1997	21.445	0.046	23.329	36.266	81.086	6.597	−0.041	3.640	3.006	0.325	0.070	0.034	7.075	0.116	94.727
1998	21.656	0.067	22.936	36.934	81.592	7.068	−0.046	3.297	2.835	0.328	0.070	0.031	6.561	0.088	95.146
1999	21.623	0.058	23.010	37.960	82.650	7.610	−0.062	3.268	2.885	0.331	0.069	0.046	6.599	0.099	96.774
2000	22.580	0.065	^R23.916	38.404	^R84.965	7.862	−0.057	2.811	2.907	0.317	0.066	0.057	6.158	^R0.115	^R98.905
2001	^R21.952	^R0.029	^R22.906	38.333	^R83.221	^R8.033	−0.090	2.201	^R2.640	0.311	0.065	0.068	^R5.286	0.075	^R96.378
2002	^R21.980	^R0.061	^R23.662	^R38.401	^R84.104	^R8.143	^RP 0.088	^RP2.675	^R2.791	^R0.328	^P0.064	^RP0.105	^RP5.963	0.078	^R98.026
2003	^P22.707	^P0.051	^P22.507	^P39.074	^P84.338	^P7.973	^P−0.088	^P2.779	^P2.884	^P0.314	^P0.063	^P0.108	^P6.150	^P0.022	^P98.156

(oxygen-free) digestion for many years to generate methane gas. Small-scale digesters have been used on farms, primarily in Europe and Asia, for hundreds of years. Biogas pits (a biomass-based technology) are a significant source of energy in China.

Another type of biochemical conversion process, fermentation, uses yeast to decompose carbohydrates, yielding ethyl alcohol (ethanol) and carbon dioxide. Sugar crops, grains (corn, in particular), potatoes, and other starchy crops commonly supply the sugar for ethanol production.

Ethanol and Methanol

Ethanol (ethyl alcohol) is a colorless, nearly odorless, flammable liquid derived from fermenting plant material that contains carbohydrates in the form of sugar. Most of the ethanol manufactured for use as fuel in the United States is derived from corn, wood, and sugar. Gasohol is a product formed by mixing ethanol and gasoline. There are three types of gasohol: 10% gasohol, which is a mixture of 10% ethanol and 90% gasoline; 7.7% gasohol, which is at least 7.7% ethanol but less than 10%; and 5.7% gasohol, which is at least 5.7% ethanol but less than 7.7%. The Federal Highway Administration estimated that in 2002 nearly 21 billion gallons of gasohol were used by Americans, up from 17.4 billion gallons in 2001 and 16.3 billion gallons in 2000.

Automobiles can run on gasohol and can be built to run directly on ethanol or on any mixture of ethanol and gasoline. However, ethanol is difficult and expensive to produce in bulk. The development of ethanol as a fuel source may depend more upon the political support of legislators from farming states and a desire for some independence from foreign oil rather than upon savings at the gas pump.

Some scientists believe ethanol made from wood, sawdust, corncobs, or rice hulls could liberate the alcohol fuel industry from its dependence on food crops, such as corn and sugarcane. Worldwide, enough corncobs and rice hulls are left over from annual crop production to produce more than forty billion gallons of ethanol.

Advocates of wood-derived ethanol believe that it could eventually be a sustainable liquid fuel industry that does not rely on pollution-generating fossil fuels. For instance, if new trees were planted to replace those that were cut for fuel, they would be available for later harvesting while at the same time alleviating global warming with their carbon dioxide-processing function. However, other scientists warn that an increased demand for wood for transportation fuels might accelerate the destruction of old-growth forests and endanger ecosystems.

Methanol (methyl alcohol) fuels have also been tested successfully. Using methanol instead of diesel fuel virtually eliminates sulfur emissions and reduces other environmental pollutants usually emitted from trucks and buses. Producing methanol from biofuels, however, is costly.

Burning biofuels in vehicle engines is part of the "carbon cycle" in which the earth's vegetation can, in turn, make use of the products of automobile combustion. (See Figure 6.2.) Automobile combustion generated from fossil fuels, however, contains pollutants. In addition, generating excessive amounts of carbon dioxide from either fossil fuels or biofuels is thought to add to global warming because this gas acts as a "blanket," trapping heat between the earth and the atmosphere.

Municipal Waste Recovery

Each year millions of tons of garbage are buried in landfills and city dumps. This method of disposal is becoming increasingly costly as many landfills across the nation near capacity. Many communities discovered that they could solve both problems—cost and capacity—by constructing waste-to-energy plants. Not only is the garbage burned and reduced in volume by 90%, but also energy in the form of steam or electricity is generated in a cost-effective way, and the potential energy benefit is significant. Use of municipal waste as fuel has increased steadily since the 1980s. According to EIA figures, municipal waste (including landfill gas, sludge waste, tires, and agricultural by-products) generated 88 trillion Btu of energy in 1981, which grew to 558 trillion Btu by 2003.

The two most common waste-to-energy plant designs are the mass burn (also called direct combustion) system and the refuse derived fuel (RDF) system.

MASS BURN SYSTEMS. Most waste-to-energy plants in the United States use the mass burn system. This system's advantage is that the waste does not have to be sorted or prepared before burning, except for removing obviously noncombustible, oversized objects. The mass burn eliminates expensive sorting, shredding, and transportation machinery that may be prone to break down.

In mass burn systems, waste is carried to the plant in trash trucks and dropped into a storage pit. Large overhead cranes lift the garbage into a furnace feed hopper that controls the amount and rate of waste that is fed into the furnace. Next, the garbage is moved through a combustion zone so that it burns to the greatest extent possible. The burning waste produces heat, and that heat is used to produce steam. The steam can be used directly for industrial needs or can be sent through a turbine to power a generator to produce electricity.

REFUSE DERIVED FUEL (RDF) SYSTEMS. RDF systems process waste to remove noncombustible objects and to create homogeneous and uniformly sized fuel. Large items such as bedsprings, dangerous materials, and flammable liquids are removed by hand. The trash is then shredded and carried to a screen to remove glass, rocks,

FIGURE 6.2

and other material that cannot be burned. The remaining material is usually sifted a second time with an air separator to yield fluff. The fluff is sent to storage bins before being burned, or it can be compressed into pellets or briquettes for long-term storage. This fuel can be used as an energy source by itself in a variety of systems, or it can be used with other fuels, such as coal or wood.

PERFORMANCE OF WASTE-TO-ENERGY SYSTEMS. Most waste-to-energy systems can produce two to four pounds of steam for every pound of garbage burned. A 1,000-ton-per-day mass burn system will burn an average of 310,250 tons of trash each year and will recover two trillion Btu of energy. In addition, the plant will emit 96,000 tons of ash (32% of waste input) for landfill disposal. An RDF plant produces less ash but sends almost the same amount of waste to the landfill because of the noncombustibles that accumulate in the separation process before burning.

DISADVANTAGES OF WASTE-TO-ENERGY PLANTS. The major problem with increasing the use of municipal waste-to-energy plants is their effect on the environment. The emission of particles into the air is partially controlled by electrostatic precipitators, and many gases can be eliminated by proper combustion techniques. There is concern, however, about the amounts of dioxin (a very dangerous air pollutant) and other toxins that are often emitted from these plants. Noise from trucks, fans, and processing equipment at these plants can also be unpleasant for nearby residents.

Landfill Gas Recovery

Landfills contain a large amount of biodegradable matter that is compacted and covered with soil. Bacteria called methanogens thrive in this oxygen-depleted environment. They metabolize the biodegradable matter in the landfill, producing methane gas and carbon dioxide as byproducts. In the past, as landfills aged, these gases built up and leaked out. This gas leakage prompted some communities to drill holes in landfills and burn off the methane to prevent dangerously large amounts from exploding.

The energy crisis of the 1970s made landfill methane gas an energy resource too valuable to waste, and efforts were made to find an inexpensive way to tap the gas. The first landfill gas recovery site was finished in 1975 at the Palos Verdes Landfill in Rolling Hills Estates, California.

In a typical operation, garbage is allowed to decompose for several months. When a sufficient amount of methane gas has developed, it is piped out to a generating plant, where it is turned into electricity. In its purest form, methane gas is equivalent to natural gas and can be used in exactly the same way. Depending on the extraction rates, most sites can produce gas for about 20 years. The advantages of tapping gas from a landfill go beyond the energy provided by the methane, as extraction reduces landfill odors and the chances of explosions.