Environmental Engineering Reference
In-Depth Information
minor contaminants (Chastain, 2006). Production of landfill gas starts one or two years after
the waste has been disposed of and continues for 10 to 60 years depending on the volume of
biodegradable material deposited in the landfill (EPA, 2009). Instead of letting the landfill
gas to escape into the air, modern sanitary landfills contain gas extraction wells that allow the
gas to be collected and redirected to a network of pipes that route the gas to a flare or as a
source of renewable energy.
Landfill gas can be used to generate electricity or as a replacement for natural gas, coal,
or fuel oil in the production of direct or indirect thermal energy. Existing technologies for
electricity generation are internal combustion engines (from 100 kW to 3 MW), microturbines
(from 30kW to 250kW), and gas turbines (from 800kW to 10.5MW). On-site electricity
generation also allows heat recovery that boosts the whole efficiency of the generation system.
(See section titled “Combined Heat and Power” in this chapter.)
Before use, landfill gas needs cleaning. Impurities such as sulfur and siloxane compounds
need removal from the gas stream before reaching the point of use. Sulfur compounds produce
corrosion of the internal parts of engines and turbines and siloxanes—organic silicon com-
pounds that result from the decomposition of cosmetics, hairspray, soaps, creams, and dry-
cleaning products—are transformed into silicon dioxide, which create harmful abrasive
deposits on machinery parts (Chastain, 2006).
The main restrictions of using landfill gas as a renewable energy source is availability. If
a landfill exists in the vicinity of a food-processing plant, landfill gas is a great resource to
tap into. In the United States, there are some good examples of companies burning landfill
gas to produce electricity and heat. One case is the BMW plant in Greer, South Carolina.
In 2003, BMW retrofitted four KG2 gas turbines to burn landfill gas after installing a 15.2-km
(9.5-mile) pipeline from the Palmetto Landfill to the BMW plant. The Lanchester Landfill
in Narvon, Pennsylvania, installed a 21-km (13-mile) pipeline to distribute landfill gas to
four direct users.
It is important to mention that financial incentives, such as tax exemptions, low interest
loans, state grants, and other funding opportunities are available in the United States to finance
these types of projects. Also companies that develop landfill gas-to-energy projects can sell
RECs to help finance the cost of their projects.
Like landfill gas, biogas is released during the anaerobic decomposition of organic matter.
Facilities that have large quantities of organic waste can produce and collect biogas in anaerobic
digesters and use it in a similar fashion as landfill gas. Feedstocks for biogas production are
food wastes, food scraps, manure from animal husbandry, and biosolids in wastewater treat-
ment plants. Biogas components are similar to landfill gas; however, their composition varies
significantly depending on the feedstock used to feed the anaerobic fermentation. In the best
case, the percentage of methane in biogas can be up to 75 percent methane and 25 percent of
carbon dioxide, nitrogen, moisture, and hydrogen sulfide. (See section on anaerobic water treat-
ment in Chapter 9). Raw biogas is corrosive and needs cleaning similarly to landfill gas. Some
of the technologies available to upgrade the biogas and landfill gas are the use of scrubbers,
solid adsorption, and membrane separation. Depending on the technology, biogas and landfill
gas can be stripped from corrosive compounds and siloxanes or can be upgraded to “biometh-
ane” with contents of up to 99 percent methane (Bruijstens et al., 2008).
Biomass
Biomass is the organic material produced by plants (e.g., leaves, roots, seeds, and stalks) that
does not go into food products but instead has other applications. Theoretically, corn and
soybeans could be considered biomass; however, the term is reserved to materials with high
