System Application Benefits and Impacts

Electricity production from biomass is being used and is expected to continue to be used as base load power in th e electricity supply system. A near-term application for biomass gasification is with industrial-scale turbines fo r repowering of the pulp and paper and sugar cane industries. It has been estimated that roughly 70% of the powe r houses in the U.S. pulp and paper industry (which represents more than 30% of the world's capacity) will need to be replaced within the next 10-15 years [7]. A similar situation exists in the sugarcane industry. Repowering these plants with modern, efficient, gas turbine technology will substantially improve efficiency, reduce emissions, and provid e additional electrical power that can offset purchases or be exported to the surrounding area. A recent study [2] examined a variety of options for mill repowering and found BGCC to be the most economically attractive option. Use of BGCC in the sugarcane industry worldwide could increase the power available for export to the surroundin g community by an order of magnitude [8]. This is a significant benefit because many sugar mills are located i n developing regions with burgeoning electric power needs. It is worth noting that rapid developments are also bein g made in smaller turbine sizes as well, and the industrial and cogeneration markets (10-50 MW e output) should not be ignored.

As discussed in the Overview of Biomass Technologies, there is approximately 7 GW of grid-connected biomas s generating capacity in the U.S. [9], much of it associated with the wood and wood products industry, which obtains more than half its electricity and thermal energy from biomass. In comparison, coal-fired electric units account fo r 297 GW of capacity, or about 43% of total generating capacity. In 1994, U.S. biomass consumption was approximately 3 EJ, and represented about 3.2% of the 94 EJ of total primary energy consumption [9]. Electricity from biomas s represents about 1% of the total U.S. demand. The amount of electricity derived from this quantity of biomass coul d be roughly doubled if gasification/turbine based power systems were employed (average efficiency of existing capacity = 20%, efficiency of biomass/turbine systems = 35-40%).

Biomass-to-electricity systems based on gasification have a number of potential advantages. Projected proces s efficiencies are much higher than the direct combustion systems in commercial use today. Process efficiencies ar e comparable to high efficiency coal-based systems, but can be achieved at a smaller scale of operation. Thus, not only does biomass close the carbon cycle, but gasification based systems, due to their high efficiency, reduce CO 2 emissions per megawatt of power generated over conventional biomass power plants. Biomass is also lower in sulfur than is most U.S. coal. A typical biomass contains 0.05 to 0.20 weight % sulfur on a dry basis and has a higher heating value o f about 29.8 MJ/kg (8500 Btu/lb). This compares with coal at up to 2-3 dry weight %. The biomass sulfur content translates to about 51 to 214 mg SO2/MJ (0.12 to 0.50 lb SO2/MMBtu). The higher sulfur level is still less than th e regulated limit set in the current New Source Performance Standards (NSPS). Controlled NOx levels from biomas s plants will also be less than the NSPS standards.

Since gasifers operate at much lower temperatures than combustors, gasification allows a wider variety of feedstocks, such as high alkali fuels, than may be technically feasible for direct combustion systems. High alkali fuels such a s switchgrass, straws, and other agricultural residues often cause severe corrosion, erosion, and deposition problems o n heat transfer surfaces in conventional combustion boilers [10]. Gasification systems can easily remove the alkal i species from the fuel gas before it is combusted.

Future technology, such as gasification/fuel cell systems, holds the promise of efficiencies well above 50% even a t relatively small scales. Gasifier development potentially benefits other technology areas such as fuels and chemical s through development of gasifier technology which can also be used to generate syngas for chemical synthesis.

The emission data shown in Table 1 are taken from DeLong [1], and are based on alfalfa feed. These data were use d rather than estimates generated by the BIOPOWER model [4], since data in the DeLong study were provided b y equipment vendors, and the BIOPOWER model is more generic. Since wood is lower in nitrogen than alfalfa, it i s expected that the estimate of NOx emissions listed here is higher than actual. The ash produced is based on yearly plant feed, assuming biomass is 1.2% ash, as is common for wood. Essentially the same turbine technology is used for th e systems through 2010, so the emissions are assumed to be constant. Since advanced turbine systems have not yet been built, emission estimates for later systems were not made. The details of the steam-injected gas turbines (STIG) use d in the 2020-2030 cases are not available so boiler blowdown estimates were not made; however, a worst case scenario would have amounts the same as the 2005 case. Future plants will need to meet applicable Federal, state, and loca l emission requirements.

Table 1. Emissions from a high-pressure, direct gasification system.

Indicator

Base Year

Name

Units

1997

2000

2005

2010

2020

2030

Particulates (PM10)

g/Nm3

0.007

0.007

0.007

0.007

Nitrogen [email protected]% O2

g/GJ

64.5

64.5

64.5

64.5

Carbon Monoxide

g/GJ

20.6

20.6

20.6

20.6

Non-CH4 Hydrocarbons

g/GJ

9.6

9.6

9.6

9.6

Sulfur Dioxide

g/GJ

81.8

81.8

81.8

81.8

Ash

Mg/yr

2,912

2,912

3,883

3,883

4,271

4,271

Boiler blowdown

Mg/yr

6,989

6,989

9,319

9,319

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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