Science and Coal

Posts Tagged ‘atmosphere’

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.

New research, reported this week in the online early edition of the Proceedings of the National Academy of Sciences, finds that coal burning, primarily in North America and Europe, contaminated the Arctic and potentially affected human health and ecosystems in and around Earth’s polar regions.

The study, titled “Coal Burning Leaves Toxic Heavy Metal Legacy in the Arctic,” was conducted by the Desert Research Institute (DRI), Reno, Nev. and partially funded by the National Science Foundation.

Detailed measurements from a Greenland ice core showed pollutants from burning coal–the toxic heavy metals cadmium, thallium and lead–were much higher than expected. The catch, however, was the pollutants weren’t higher at the times when researchers expected peaks.

“Conventional wisdom held that toxic heavy metals were higher in the 1960s and ‘70s, the peak of industrial activity in Europe and North America and certainly before implementation of Clean Air Act controls in the early 1970s,” said Joe McConnell, lead researcher and director of DRI’s Ultra-Trace Chemistry Laboratory. Read the rest of this entry »

The world has taken a step closer to “clean coal,” thanks to new technology that actually uses CO2 to make power generation more efficient.
The research by scientists at Columbia University means that millions of tons of CO2 could be prevented from entering the atmosphere and instead used to turn coal, biomass and municipal waste into cleaner fuel.

This remarkable double hit is based on a well-established process called “gasification” that is already used to clean “dirty” fuels by heating them with steam and turning them into a mixture of hydrogen and carbon monoxide, known as syngas. In turn, that is then burned in power stations or used to create transport fuels.

But until now this process has demanded very large amounts of energy and water, and produced substantial CO2 emissions. Read the rest of this entry »

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 »