Renewable Energy - Bioenergy

The term bioenergy refers to energy that is generated using biomass—organic material such as wood, agricultural waste from plants and animals, seaweed and algae, and municipal solid waste (MSW) or garbage. These raw materials can be converted into liquid or gaseous biofuels or used directly to provide heat, electricity, or combined heat and power. Figure 10.5 shows common materials used to make bioenergy. The by-products of biomass conversion can be used for fertilizers and chemicals.

Bioenergy from wood, waste, and alcohol fuels accounted for an estimated 2.9 quadrillion British thermal

TABLE 10.1
Energy production by source, 1949–2002
(Quadrillion Btu)

	Fossil fuels							Renewable energy¹
Year	Coal	Natural gas (dry)	Crude oil²	Natural gas plant liquids	Total	Nuclear electric power	Hydroelectric pumped storage³	Conventional hydroelectric power	Wood, waste, alcohol⁴	Geothermal	Solar	Wind	Total	Total
1949	11.974	5.377	10.683	0.714	28.748	0	5	1.425	1.549	NA	NA	NA	2.974	31.722
1950	14.060	6.233	11.447	0.823	32.563	0	5	1.415	1.562	NA	NA	NA	2.978	35.540
1951	14.419	7.416	13.037	0.920	35.792	0	5	1.424	1.535	NA	NA	NA	2.958	38.751
1952	12.734	7.964	13.281	0.998	34.977	0	5	1.466	1.474	NA	NA	NA	2.940	37.917
1953	12.278	8.339	13.671	1.062	35.349	0	5	1.413	1.419	NA	NA	NA	2.831	38.181
1954	10.542	8.682	13.427	1.113	33.764	0	5	1.360	1.394	NA	NA	NA	2.754	36.518
1955	12.370	9.345	14.410	1.240	37.364	0	5	1.360	1.424	NA	NA	NA	2.784	40.148
1956	13.306	10.002	15.180	1.283	39.771	0	5	1.435	1.416	NA	NA	NA	2.851	42.622
1957	13.061	10.605	15.178	1.289	40.133	(s)	5	1.516	1.334	NA	NA	NA	2.849	42.983
1958	10.783	10.942	14.204	1.287	37.216	0.002	5	1.592	1.323	NA	NA	NA	2.915	40.133
1959	10.778	11.952	14.933	1.383	39.045	0.002	5	1.548	1.353	NA	NA	NA	2.901	41.949
1960	10.817	12.656	14.935	1.461	39.869	0.006	5	1.608	1.320	0.001	NA	NA	2.929	42.804
1961	10.447	13.105	15.206	1.549	40.307	0.020	5	1.656	1.295	0.002	NA	NA	2.953	43.280
1962	10.901	13.717	15.522	1.593	41.732	0.026	5	1.816	1.300	0.002	NA	NA	3.119	44.877
1963	11.849	14.513	15.966	1.709	44.037	0.038	5	1.771	1.323	0.004	NA	NA	3.098	47.174
1964	12.524	15.298	16.164	1.803	45.789	0.040	5	1.886	1.337	0.005	NA	NA	3.228	49.056
1965	13.055	15.775	16.521	1.883	47.235	0.043	5	2.059	1.335	0.004	NA	NA	3.398	50.676
1966	13.468	17.011	17.561	1.996	50.035	0.064	5	2.062	1.369	0.004	NA	NA	3.435	53.534
1967	13.825	17.943	18.651	2.177	52.597	0.088	5	2.347	1.340	0.007	NA	NA	3.694	56.379
1968	13.609	19.068	19.308	2.321	54.306	0.142	5	2.349	1.419	0.009	NA	NA	3.778	58.225
1969	13.863	20.446	19.556	2.420	56.286	0.154	5	2.648	1.440	0.013	NA	NA	4.102	60.541
1970	14.607	21.666	20.401	2.512	59.186	0.239	5	2.634	1.431	0.011	NA	NA	4.076	63.501
1971	13.186	22.280	20.033	2.544	58.042	0.413	5	2.824	1.432	0.012	NA	NA	4.268	62.723
1972	14.092	22.208	20.041	2.598	58.938	0.584	5	2.864	1.503	0.031	NA	NA	4.398	63.920
1973	13.992	22.187	19.493	2.569	58.241	0.910	5	2.861	1.529	0.043	NA	NA	4.433	63.585
1974	14.074	21.210	18.575	2.471	56.331	1.272	5	3.177	1.540	0.053	NA	NA	4.769	62.372
1975	14.989	19.640	17.729	2.374	54.733	1.900	5	3.155	1.499	0.070	NA	NA	4.723	61.357
1976	15.654	19.480	17.262	2.327	54.723	2.111	5	2.976	1.713	0.078	NA	NA	4.768	61.602
1977	15.755	19.565	17.454	2.327	55.101	2.702	5	2.333	1.838	0.077	NA	NA	4.249	62.052
1978	14.910	19.485	18.434	2.245	55.074	3.024	5	2.937	2.038	0.064	NA	NA	5.039	63.137
1979	17.540	20.076	18.104	2.286	58.006	2.776	5	2.931	2.152	0.084	NA	NA	5.166	65.948
1980	18.598	19.908	18.249	2.254	59.008	2.739	5	2.900	2.485	0.110	NA	NA	5.494	67.241
1981	18.377	19.699	18.146	2.307	58.529	3.008	5	2.758	2.590	0.123	NA	NA	5.471	67.007
1982	18.639	18.319	18.309	2.191	57.458	3.131	5	3.266	2.615	0.105	NA	NA	5.985	66.574
1983	17.247	16.593	18.392	2.184	54.416	3.203	5	3.527	2.831	0.129	NA	(s)	6.488	64.106
1984	19.719	18.008	18.848	2.274	58.849	3.553	5	3.386	2.880	0.165	(s)	(s)	6.431	68.832
1985	19.325	16.980	18.992	2.241	57.539	4.076	5	2.970	2.864	0.198	(s)	(s)	6.033	67.647
1986	19.509	16.541	18.376	2.149	56.575	4.380	5	3.071	2.841	0.219	(s)	(s)	6.132	67.087
1987	20.141	17.136	17.675	2.215	57.167	4.754	5	2.635	2.823	0.229	(s)	(s)	5.687	67.608
1988	20.738	17.599	17.279	2.260	57.875	5.587	5	2.334	2.937	0.217	(s)	(s)	5.489	68.951
1989	21.346	17.847	16.117	2.158	57.468	5.602	5	^R2.837	3.062	^R0.317	0.055	^R0.022	^R6.294	^R69.364
1990	22.456	^R18.326	15.571	2.175	^R58.529	6.104	0.036	^R3.046	^R2.662	^R0.336	0.060	^R0.029	^R6.133	^R70.729
1991	21.594	18.229	15.701	2.306	57.829	6.422	0.047	^R3.016	2.702	^R0.346	0.063	^R0.031	^R6.158	^R70.362
1992	21.629	18.375	15.223	2.363	57.590	6.479	0.043	2.617	2.847	0.349	0.064	0.030	5.907	69.933
1993	20.249	18.584	14.494	2.408	55.736	6.410	0.042	2.892	2.804	0.364	0.066	0.031	6.157	68.262

TABLE 10.1
Energy production by source, 1949–2002

	Fossil fuels							Renewable energy¹
Year	Coal	Natural gas (dry)	Crude oil²	Natural gas plant liquids	Total	Nuclear electric power	Hydroelectric pumped storage³	Conventional hydroelectric power	Wood, waste, alcohol⁴	Geothermal	Solar	Wind	Total	Total
1994	22.111	19.348	14.103	2.391	57.952	6.694	0.035	2.683	2.939	0.338	0.069	0.036	6.065	70.676
1995	22.029	^R19.082	13.887	2.442	^R57.440	7.075	0.028	3.205	3.068	0.294	0.070	0.033	6.669	^R71.156
1996	22.684	^R19.344	13.723	2.530	^R58.281	7.087	0.032	3.590	3.127	0.316	0.071	0.033	7.137	^R72.472
1997	23.211	19.394	13.658	2.495	58.758	6.597	0.041	3.640	3.006	0.325	0.070	0.034	7.075	72.389
1998	23.935	19.613	13.235	2.420	59.204	7.068	0.046	3.297	2.835	0.328	0.070	0.031	6.561	72.787
1999	23.186	19.341	12.451	2.528	57.505	7.610	0.062	3.268	^R2.885	0.331	0.069	0.046	^R6.599	^R71.652
2000	22.623	^R19.662	12.358	2.611	^R57.254	7.862	0.057	2.811	^R2.907	0.317	0.066	0.057	^R6.158	^R71.218
2001	^R23.053	^R20.227	12.282	^R2.547	^R58.109	8.028	0.090	^R2.201	^R2.678	^R0.311	^R0.065	^R0.068	^R5.324	^R71.372
2002^P	22.554	19.561	12.314	2.561	56.990	8.145	0.089	2.668	2.756	0.304	0.064	0.106	5.899	70.946
¹End-use consumption and electricity net generation.
²Includes lease condensate.
³Pumped storage facility production minus energy used for pumping.
⁴Alcohol is ethanol blended into motor gasoline.
⁵Included in "Conventional Hydroelectric Power."
R = Revised. P = Preliminary. NA = Not available. (s) = Less than 0.0005 quadrillion Btu.
Note: Totals may not equal sum of components due to independent rounding.
SOURCE: "Table 1.2. Energy Production by Source, 1949–2002," in Annual Energy Review 2002, U.S. Department of Energy, Energy Information Administration, Washington, DC, October 2003

FIGURE 10.3
Renewable energy consumption by type, 2003

units (Btu) of energy in 2003, comprising 47 percent of renewable energy consumed. (See Figure 10.3.)

Biomass Conversion

There are two types of biomass conversion processes: thermochemical conversion and biochemical conversion. Thermochemical conversion uses heat to produce chemical reactions in biomass. Direct combustion is the easiest and most commonly used method. Materials such as dry wood or agricultural wastes are chopped and burned to produce steam, electricity, or heat for industries, utilities, and homes. Wood burning in stoves and fireplaces is one example. The burning of agricultural wastes is also becoming more widespread. In Florida, for example, sugar cane producers use the residue from the cane to generate much of their energy.

Pyrolysis, also called gasification or carbonization, uses heat to break down biomass to yield liquid, gaseous, and solid fuels. Converting wood to charcoal is an example of this process.

The second type of conversion process, 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 (without oxygen) 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. The DOE estimates that many thousands of biofuel plants are in use in Korea, and perhaps half a million plants operate in China.

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

Wood

Wood energy was the first energy source in America's industrialization. Wood, the most commonly used biofuel, is still used to heat millions of homes every year. In 2003 it provided 2.1 quadrillion Btu, accounting for 34 percent of renewable energy consumption (See Figure 10.3.)

When wood is widely used as a fuel in an area, deforestation can occur, resulting in the possibility of soil erosion and mud slides. Burning wood, as with the burning of fossil fuels, also pollutes the environment.

Ethanol and Methanol—Important Agricultural By-Products

Ethanol is a colorless, nearly odorless, flammable liquid derived from fermenting plant material that contains carbohydrates in the form of sugar. U.S. ethanol production has grown steadily since 1980, reaching 2.81 billion gallons in 2003, according to the U.S. Department of Energy. Production capacity is largely located in the farming regions of the upper Midwest. (See Figure 10.6.)

Most of the ethanol manufactured for use as fuel is derived from corn, wood, and sugar. A mixture of 10 percent ethanol and 90 percent gasoline is usable in any internal combustion engine without the need to modify the motor. Although the DOE claims that the demand for alcohol/gasoline blends is increasing because alcohol can substitute for lead as an octane booster, there is little question that the development of ethanol depends more on the continued support of farm state legislators than any economic benefit.

Ethanol is difficult and expensive to produce in bulk. Methanol-blend fuels have also been tested successfully. (Methanol is methyl alcohol.) Using methanol instead of diesel fuel virtually eliminates sulfur emissions and reduces other environmental pollutants usually emitted from trucks and buses. Burning biofuels in vehicle engines creates a "carbon cycle" in which the earth's vegetation can in turn make use of the products of combustion and, therefore, reduce net greenhouse gases. (See Figure 10.7.) Producing methanol from biofuels, however, is costly.

Some scientists believe ethanol made from wood, sawdust, corncobs, or rice hulls could liberate the alcohol FIGURE 10.4
Renewable energy consumption by major sources, 1973–2002 fuel industry from its dependence on food crops such as corn and sugar cane. Worldwide, there are enough corncobs and rice hulls left over from annual crop production to produce more than 40 billion gallons of ethanol.

Advocates of wood-derived ethanol believe that the eventual result of wood-to-ethanol conversion research could create 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 and, in the meantime, contribute to the prevention of global warming by continuing their carbon dioxide processing function. Other scientists warn that a huge demand for transportation fuels could create a demand for wood that might accelerate the destruction of old-growth forests and endanger ecosystems. Without careful attention to forestry practices, ethanol production might aggravate rather than solve the fuel problem.

Municipal Solid Waste

Each year millions of tons of municipal solid waste (MSW), or garbage as it is commonly called, are buried in landfills and city dumps. This method of disposal is not only costly but is becoming increasingly difficult as some landfills across the nation are near capacity. Many communities have discovered that they can solve two problems at once by constructing waste-to-energy (WTE) plants. Not only is garbage burned and reduced in volume by 90 percent, energy in the form of steam or electricity is generated in a cost-effective way. The potential energy benefit is significant.

WASTE-TO-ENERGY PLANTS.

The two most common WTE plant designs are the mass burn (also called direct combustion) and the refuse derived fuel (RDF) systems.

Most WTE 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.

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 garbage produces heat, and that heat is used to produce steam. The steam can be used directly for industrial needs or heat can be sent through a turbine to power a generator to produce electricity.

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, 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.

The major obstacle to increasing the use of municipal WTE plants is their effect on the environment. Noise from trucks, fans, and processing equipment at RDF plants can FIGURE 10.5
Biomass to bioenergy be unpleasant for nearby residents. The emission of particles into the air is controlled by electrostatic precipitators, and most gases can be eliminated by proper combustion techniques. There is concern, however, about the amounts of dioxin (a very dangerous air pollutant) that are often emitted from these plants.

LANDFILL GAS RECOVERY.

Landfills contain a large amount of biodegradable matter. Gas is created because of the lack of oxygen that helps the growth of methagens—types of bacteria that produce methane gas and carbon dioxide. In the past, as landfills aged, these gases built up and leaked out. This gas leakage prompted some communities to drill holes and burn off the flammable and dangerous methane.

The energy crisis of the 1970s made this 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. Depending on the extraction rates, most existing sites can produce gas for about 20 years.

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 burned to produce electricity. In its purest form methane gas is equivalent to natural gas and can be used in exactly the same way.

The advantages of tapping gas from a landfill go beyond the energy provided by the methane. When internal pressure forces methane gas to seep into the air, it carries very unpleasant odors into the surrounding neighborhoods. The released methane can also be a danger because, if it accumulates and is accidentally ignited, it can explode. Extracting the methane gas for energy eliminates both of these problems.