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Greener coal

This page created 2005/06/09, modified 2015/01/31
Contact: email daveclarkecb@yahoo.com

Introduction

Contents

of this page

Greener coal
Coal liquefaction
Coal gasification
Water in coal
Composition of Leigh Creek coal
Notes
Solar assisted coal
Links
 
Coal is, weight-for-weight, the fossil fuel that produces the most greenhouse polluting carbon dioxide for each unit of energy it delivers, (see EnUnits.html) with the possible exception of oil shale. However, the reality in the early twenty-first century is that we cannot practically close down all our coal burning power stations in a few years.

There is no such thing as clean coal, but now, and in the next few years, in addition to finding alternatives to coal, we should be looking at ways of making coal burning more efficient. If we can get another megawatt-hour of electricity out of every tonne of coal we burn, that means we need burn less coal to get the same amount of electricity.

(This page uses several technical units. Energy units, definitions and conversions are available on an additional page. That page also gives figures for the amount of energy that can be obtained from a list of fuels.)

In Australia at least, the operators of coal fired power stations have two unfair economic advantages over sustainable electricity:

1. They are able to dump their wastes into the atmosphere at no costs to themselves (see No level playing field and Fossil fuel electricity in perspective);

2. They are heavily subsidised by the government. (While sustainably generated electricity is sold at a higher price than fossil fuel generated electricity – and this is a form of subsidy – it does not receive the numerous hidden subsidies that fossil fuel power gets.)

Removing the subsidies and applying a carbon tax would 'level the playing field'. It would no longer be necessary to sell sustainably generated power at a higher price than fossil fuel generated power.




Acknowledgements

Frank Nizynski of the Australian Coal Association provided me with links to several useful sources.

Rod Brunker of HRV Flinders (the operators of the Leigh Creek coal mine) provided me with a list of percentages of the major constituents of Leigh Creek coal.

The librarian (no name given) of the Conservation Council of SA also provided information.




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Coal liquefaction

Coal liquefaction, the production of liquid petroleum fuels from solid coal, seems to be little discussed in Australia, at least up to the time of writing (June 2005). The process was developed in oil-hungry Germany in World War Two and later used in the economically embargoed apartheid South Africa.

As petroleum becomes progressively more scarce and expensive the production of liquid fuels from coal will become steadily more economically attractive. It will be essential for the sustainability of the world environment that liquefaction of coal is combined with sequestration of the carbon dioxide produced in the process.

Carbon dioxide is produced as a by-product of the liquefaction process. It is again produced when the liquid fuel made from the coal is burned. So coal liquefaction is more bad news for greenhouse/climate change.

Wikipedia states that the break-even oil price where liquefaction of coal becomes viable is around US$25 to US$35/barrel. Elsewhere I have seen a price of US$60/barrel quoted. As I write (June 2005) the oil price is around US$55/barrel.

China, with its rapidly growing economy, heavy reliance on imported oil, and large reserves of coal, is the world leader in coal liquefaction technology. It is currently building a coal liquefaction plant with a capacity of a million tonnes per year. This is scheduled to come into production by 2007.

Ideally, coal liquefaction and carbon-containing liquid fuels should not be used because of the resulting greenhouse damage. However, if they must be used in the short-term, and that appears to me to be inevitable, society should be minimising the damage.

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Coal gasification

Coal gasification is the production of combustible gases, such as carbon monoxide, hydrogen, and methane from coal. The gases produced are used as fuels, gaseous fuels being generally more convenient to use than solid coal.

The coal is milled, dried and pulverized. It is then fed into a gasifier reactor, and at high pressure and temperature it is converted into synthesis gas ("syngas", mainly carbon monoxide and hydrogen).

The Queensland Government-owned Stanwell Corporation and Shell are working on a study into the feasibility of "Integrated Gasification Combined Cycle" with sequestration of the waste carbon dioxide. The study is due for completion in October 2005.






Water in coal

Wet, or 'green' wood, doesn't burn well; the same applies to coal

Few people who have had any significant experience with wood fires, whether camp fires in the bush, or slow combustion fires in the home, would be unaware that green wood is greatly inferior as a fuel to dry wood. Green wood is wood straight from the tree. It contains a high proportion of water which must be boiled-off as the wood is burned. This reduces the value of the wood as a fuel in several ways:
  • You buy the wood by weight. If it is green then some of what you are buying is water;
  • It adds 'dead weight'. When you carry or transport the wood you are transporting water that is of no value;
  • A fire built of green or wet wood does not burn well;
  • A fire built of green wood produces more smoke;
  • Most importantly, a significant part of the energy of the burning wood is wasted in boiling-off the water.
As a guide to the comparitive amounts of energy that are involved, Wikipedia states that green wood contains about 10 megajoules of energy per kilogram of wood (MJ/kg), air-dry wood contains about 16 MJ/kg, and kiln dried wood contains about 19 to 20 MJ/kg.





Wet coal

Burning wet coal has much the same disadvantages as burning wet or green wood. And the energy loss is not only in the burning, transporting all that worse-than-useless water also has a cost. Coal can contain 30% or more water, that means that a thousand-tonne train load of coal could contain 300 tonnes or more of water. If it was possible to substantially reduce the water content of the coal at the mine a huge amount of energy would be saved and a huge amount of greenhouse gas would no longer have to be released into the atmosphere.




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Composition of Leigh Creek coal

Analysis provided by Rod Brunker of NRG Flinders
As fired at Port Augusta power stations...

ConstituentPercentage
Moisture26
Ash21
Carbon37.5
Hydrogen2.3
Sulphur0.25
(Readers may notice that the above components add up to 87.05%.)

"HHV" 14.2MJ/kg
(Presumably HHV is the amount of energy released on burning.)

Analysis extracted from a PIRSA document, "Coal Resources in South Australia"
ConstituentPercentage
Moisture31
Ash13
Carbon35
Volatile matter21
(Total 100%)

Heat value 15.2MJ/kg
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Other coals

Brown coal (lignite), such as that mined at Yallourn in Victoria (Australia) has a moisture content of 60 to 70%. See Australian Institute of Energy.




Removing water from coal

There are methods under trial for removing some of the water from coal before it is burned. Sustainable energy can be used to dry the coal.

Where locally produced sustainable energy might not be available, the power required for the drying of coal could be taken from the grid when electricity is abundant - that is, when sustainable electricity generation methods are puting more electricity into the grid than is being consumed. See sustainable electricity.






Notes

The operator of the Leigh Creek coal mine is NRG Flinders.

Frank Nizynski of the Australian Coal Association kindly provided me with a link to a useful document on Leigh Creek coal. This document stated that Lobe B Leigh Creek coal is classified as sub-bituminous black coal and typically has a water content of 31% and a carbon content of 35%. (It also contains 13% ash, 21% volatile matter, 0.5% sulfur, 0.3% chlorine and 4% sodium [in the ash].) It has a calorific value around 3 600 kcal/kg.

Lignite can contain 50% or more water. Sub-bituminous coal can contain 30% water.

The cost of transporting a 300 tonnes of water with every thousand tonnes of (wet) coal the 250km from the mine to the power station (in the case of coal mined at Leigh Creek and burned at Port Augusta) must be very high, but I would think that it would be insignificant compared to the energy loss resulting from burning wet coal.






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Solar assisted coal

Macquarie Generation, the operator of a coal-fired power station in the Hunter Valley, is trialing a method of using solar energy to pre-heat boiler feed-water. The pilot plant will have a thermal capacity of 1.2MW and if tests produce acceptable results capacity will be increased to 40MW.

The collector is being called a Compact Linear Fresnel Reflector (CLFR) by the developers; it uses flat mirrors to collect the Sun's heat. More about this can be read at Solar Access News.

Note that even if the 40MW scale-up is built and works as planned, the saving in carbon dioxide emissions to the 2000MW power station will be small in relative terms. Also, it will, of course, only work while the Sun shines. But still, every little helps.






The HRL Group of companies is working on reducing the moisture content of brown coal, thus improving energy output from power stations in relation to the quantity of carbon dioxide released. They call the process, Integrated Drying Gasification Combines Cycle or IDGCC. Unfortunately, the page provides minimal useful information about the process.

Cleanpower give a little information about a coal-drying process called Mechanical Thermal Expression. (This Net page uses the smallest possible standard print, they seem to want to make people work hard to read it.)

A study looking into a process called Mechanical and thermal expression, that can reduce the water content of Loy Yang (Yallourn) coal from 60% to 30%, is discused in an Australian Institute of Energy document.

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Index

Home
On this page...
Coal gasification
Coal liquefaction
Composition of Leigh Creek coal
Greener coal
Introduction
Links
Notes
Solar assisted coal
Top
Water in coal