According to a report from the Environmental Integrity Project (EIP), carbon dioxide emissions from power plants rose 5.56 percent in 2010 compared to the prior year, the biggest annual increase since the Environmental Protection Agency began tracking emissions in 1995. The EIP also reported that coal-fired boilers provided 45 percent of the electric power in the United States in 2010, and were responsible for 81 percent of U.S. CO2 emissions from power plants.
In the past decade, carbon dioxide and other greenhouse gases (GHGs) have been subjected to intense study for their contribution to climate change. Electrical utilities are facing federal, state and local mandates for renewable power production. At the same time, utilities also are facing existing and upcoming regulations that demand emissions reduction from their current sources of power generation. In order to meet these mandates and regulations in a timely and inexpensive manner, electrical utilities are adding more “green” options to their power portfolios. These green options can include
| Advertisement |
|---|
|
|
Biomass co-firing
In the United States, many electrical utilities either utilize or are contemplating biomass co-firing with coal. Biomass co-firing is heralded as a simple and inexpensive way to reduce CO2 and other greenhouse gas (GHG) emissions from power plants. There are several environmental advantages to co-firing biomass. For example, the elemental composition of biomass is naturally low in sulfur and mercury, thereby reducing SOX and heavy metals emissions. Co-firing of biomass may also reduce nitrogen (NOX) emissions, due to the low elemental nitrogen content of biomass when compared to coal. Another advantage of biomass can be found in the carbon cycle. When compared to coal, the amount of net CO2 released into the atmosphere is substantially lower for biomass. This is due to the fact that biomass removes carbon from the atmosphere during its growth cycle. The carbon is then released back into the atmosphere during combustion, and removed again during the next growth cycle of the biomass.
While many advantages exist for biomass co-firing in the areas of renewable power and emissions reduction, the disadvantages of co-firing could possibly outweigh the advantages for certain types of biomass. Biomass has a lower bulk density than coal, which translates into higher transportation and storage costs. Depending on where it is sourced from, biomass usually has a higher moisture content than that of coal, implicating an increase in transportation and fuel processing costs, as well as a possible decrease in boiler output. For woody biomass, the heating value is roughly 2/3 that of coal. For annual herbaceous crops, the heating value is substantially lower than that of coal. A lower heating value for biomass can force boilers running at nameplate capacity to produce less steam, and therefore less electricity. The fibrous nature of biomass can greatly increase the fuel processing costs for pulverized coal boilers. Finally, the inherent alkali and alkaline earth metals present in biomass are known to foul and corrode boilers, as well as decrease the usable life of catalysts in downstream emission removal processes.
Co-fire pellets
Current research on pyrolysis at Iowa State University might allow electrical utilities to harness the advantages of biomass co-firing while avoiding the disadvantages.
Fast pyrolysis is the rapid decomposition of biomass in the absence of oxygen to yield three products: a solid known as biochar, a liquid bio-oil and a weak synthesis gas. Biochar can be used as a soil fertilizer or combusted for heat and power. Bio-oil can be upgraded to fuels or combusted for heat and power. The syngas product is usually combusted to provide heat for the pyrolysis reactor.
