Science and Coal

Posts Tagged ‘range’

High oil prices and the desire to reduce energy dependence in the United States have brought coal-to-liquid (CTL) technology to the forefront of the discussion about alternative fuel sources. Nearly 100 years old, CTL processes have long been used by countries lacking access to oil, most notably Germany, where production peaked during the 1940s; South Africa, which has been using CTL technology for fuel since the 1950s; and, more recently, China, where the Shenhua Group LLC began trial operation of the world’s first direct CTL facility in December 2008, and intends to eventually produce 1 million tons of coal-based liquid fuel a year. The U.S. Government promoted the development of CTL technologies following the oil shocks of the 1970s, but shelved the projects after the price of oil fell during the 1980s. In the current economic and political environment of the United States, with oil prices surpassing $100 per barrel in summer 2008 and generally projected to rise in the long term, synthetic fuel derived from coal may once again become economically viable, and several projects are in the initial design phase around the country. From an environmental standpoint, however, the carbon dioxide (CO2) emissions produced throughout the lifecycle of coal-based liquid fuel make it a less desirable option.

Turning Coal into Liquid Fuel
Coal can be converted into liquid fuel using several liquefaction processes; these processes can be divided into two general categories. The first category, indirect liquefaction, is a multi-step procedure that first requires the gasification of coal to produce a “syngas.” This syngas is then converted to liquid fuel via two methods: the Fischer-Tropsch process or the Mobil process. In the Fischer-Tropsch process, which is much more common, the syngas is then cleansed of impurities and subjected to further chemical refinement to produce a sulfur-free diesel or gasolinei. The initial syngas can be derived from coal alone, or from a coal / biomass mixture. The process is the same when biomass is included, but the amount of CO2 emitted during the process decreases as the proportion of biomass increases. In the less-common Mobil process, the syngas can be converted to methanol, which is subsequently converted to gasoline via a dehydration sequence. Indirect liquefaction of coal during Fischer Tropsch produces a significant amount of CO2 that is removed from the fuel as a necessary step during the final stages of the process. However, recent research has suggested a modified Fischer-Tropsch method that could significantly reduce CO2 emissions during liquefaction.ii
The second category, direct liquefaction, requires creating a chemical reaction at high temperatures and then using hydrogen gas and a catalyst to produce a liquid fuel. Direct liquefaction usually produces low-quality liquid fuel that is expensive to make compliant with U.S. standards for purity. Therefore, although the process is used in China, it is not a viable option for meeting the United States’ liquid fuel requirements and will not be discussed for the remainder of this brief. Read the rest of this entry »

As the United States’ oil reserves dwindle, some say the nation will have to rely on synthetic petroleum fuel made from its large stores of coal.

A two-step chemical process augments a method of making cleaner-burning alternative fuel from coal and other carbon sources by transforming some of its waste products into diesel fuel, researchers from the University of North Carolina at Chapel Hill and Rutgers, the State University of New Jersey, report.

“Two percent of the United States’ energy reserves is in oil, 3 percent is in gas, and 95 percent is in coal,” said Dr. Maurice Brookhart, W.R. Kenan Jr. professor of chemistry in UNC’s College of Arts and Sciences. “Many people in the energy sector think that when oil starts to run out, coal will be a source of transportation fuel for some time before we perfect the science behind solar and hydrogen-based energy. Producing diesel fuels from coal is especially attractive since diesel engines are more efficient than gasoline engines.” Read the rest of this entry »

When burned, coal is the dirtiest of all fossil fuels but a range of technologies are being used and developed to reduce the environmental impact of coal-fired power stations.
Collectively, they are known as clean coal technology (CCT).

CARBON CAPTURE AND STORAGE

Despite the improving efficiency of coal-fired power stations, CO2 emissions remain a problem.
Carbon capture and storage (CCS) involves capturing the carbon dioxide, preventing the greenhouse gas entering the atmosphere, and storing it deep underground.

note:
1. CO2 pumped into disused coal fields displaces methane which can be used as fuel
2. CO2 can be pumped into and stored safely in saline aquifers
3. CO2 pumped into oil fields helps maintain pressure, making extraction easier

A range of approaches of CCS have been developed and have proved to be technically feasible. They have yet to be made available on a large-scale commercial basis because of the costs involved.

COAL PREPARATION

Coal arriving at a power plant contains mineral content that needs to be removed before it is burnt. A number of processes are available to remove unwanted matter and make the coal burn more efficiently.

Coal washing involves grinding the coal into smaller pieces and passing it through a process called gravity separation.

One technique involves feeding the coal into barrels containing a fluid that has a density which causes the coal to float, while unwanted material sinks and is removed from the fuel mix. The coal is then pulverised and prepared for burning.

GASIFICATION

Coal gasification plants are favoured by some because they are flexible and have high levels of efficiency. The gas can be used to power electricity generators, or it can be used elsewhere, i.e. in transportation or the chemical industry.

INTEGRATED COAL GASIFICATION COMBINED CYCLE PLANT

note:
1. Coal burnt to produce syngas
2. Syngas burnt in combustor
3. Hot gas drives gas turbines
4. Cooling gas heats water
5. Steam drives steam turbines

In Integrated Gasification Combined Cycle (IGCC) systems, coal is not combusted directly but reacts with oxygen and steam to form a “syngas” (primarily hydrogen). After being cleaned, it is burned in a gas turbine to generate electricity and to produce steam to power a steam turbine.

Coal gasification plants are seen as a primary component of a zero-emissions system. However, the technology remains unproven on a widespread commercial scale.

REMOVING POLLUTANTS

Burning coal produces a range of pollutants that harm the environment: Sulphur dioxide (acid rain); nitrogen oxides (ground-level ozone) and particulates (affects people’s respiratory systems).

There are a number of options to reduce these emissions:

Sulphur dioxide (SO2)
Flue gas desulphursation (FGD) systems are used to remove sulphur dioxide. “Wet scrubbers” are the most widespread method and can be up to 99% effective.

A mixture of limestone and water is sprayed over the flue gas and this mixture reacts with the SO2 to form gypsum (a calcium sulphate), which is removed and used in the construction industry.

Nitrogen oxides (NOx)
NOx reduction methods include the use of “low NOx burners”. These specially designed burners restrict the amount of oxygen available in the hottest part of the combustion chamber where the coal is burned. This minimises the formation of the gas and requires less post-combustion treatment.

Particulates emissions

Electrostatic precipitators can remove more than 99% of particulates from the flue gas. The system works by creating an electrical field to create a charge on particles which are then attracted by collection plates. Other removal methods include fabric filters and wet particulate scrubbers.

Turning coal into gasoline-like fuel has several advantages. It would use America’s vast coal reserves. It would reduce the nation’s thirst for foreign oil and help dampen spikes in energy prices. There’s just one problem: It is not “climate friendly” – at least, not yet.

Coal-to-liquids (CTL) fuels could end up emitting nearly double the carbon dioxide that the equivalent amount of gasoline does, mostly because of the way it’s manufactured. The CTL industry says new technology will fix the problem. But because such technology is not yet developed, it’s unclear whether CTL fuels would be competitive without state and federal subsidies, even competing against high-priced diesel, jet fuel, or gasoline, analysts say.

That’s where politicians come in. The National Mining Association has ramped up Capitol Hill lobbying, creating a new coalition and website, futurecoalfuels.org. Many in Washington are warming to the idea. CTL bills in the House of Representatives and the Senate have received strong backing. Read the rest of this entry »