Q+A-How do emissions from LNG and coal compare?

Tue May 10, 2011 4:26am EDT

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May 10 (Reuters) - Demand for liquefied natural gas (LNG) and coal is soaring to meet the energy needs of booming economies in Asia, particularly China and India. And with that demand comes growing greenhouse gas emissions.

The LNG sector is set to grow dramatically over the next two decades, with about A$200 billion in investments planned for Australia alone.

LNG is cleaner than coal in terms of carbon emissions and toxic pollutants but extraction, processing and shipping the gas in liquid form is an emissions-intensive exercise.

Following are details comparing the two fuels.

HOW CLEAN IS LNG?

Burning LNG in power plants produces roughly 40 percent fewer greenhouse gas emissions compared with black coal. This is based on a series of studies that compared the total lifecycle emissions of both fuels based on extraction, production, shipping and burning in power plants overseas.

LNG also has a higher energy value, meaning it generates more electricity per tonne than black coal.

Studies have compared Australian LNG and black coal exported and burned in Chinese power plants to test LNG's advantage in emissions, particularly when displacing coal.

The results vary but for LNG produced in Australia, a higher portion of the emissions are from the LNG production process than digging up and shipping black coal.

For example, Australia produces about 20 million tonnes of LNG a year and 500 million tonnes of black coal and a smaller portion of brown coal. Total emissions from the nation's LNG production total about 8 million tonnes versus about 27 million tonnes for the entire coal mining industry.

A 2008 report by consultancy WorleyParsons for Woodside Petroleum found that LNG generated 7.1 megawatts/hour per tonne versus 3 MW/hr for black coal when both fuels were burned in high-efficiency power plants.

Overall, the emissions intensity for LNG burned in a combined-cycle gas turbine plant was 0.44 tonnes and 0.72 for black coal burned in a power plant with an ultra-super critical boiler, rising to 1.02 tonnes for a sub-critical plant.

Still, LNG's overall advantage is heavily dependent on the CO2 content of the gas field and the efficiency of the coal-fired power plant it is being compared with. The advantage is greatest if the CO2 gas field content is low, say two percent, and if the power plant uses sub-critical boiler design.

In China, where much of Australia's gas will be burnt, the government is pressing for new power plants to be much more efficient.

Many new LNG fields, particularly in Australia, have much higher CO2 contents or have to pipe from extensive coal-seam networks. For a related factbox, see: [ID:L3E7FS0HJ]

The planned 8.4 mtpa Ichthys project in the Northern Territory, a venture between Inpex and Total , will extract raw gas from two fields, with average CO2 content of 8 and 17 percent respectively, compared to 2 percent for Woodside's Pluto LNG project due to start production this year.

Inpex says the project will produce 7 million tonnes of greenhouse gases a year and just over a third will come from the CO2 stripped from the raw gas.

WHAT ARE THE DIFFERENCES DURING PRODUCTION?

Coal mines can produce large amounts of greenhouses gases, CO2 and methane, which is a much more powerful greenhouse gas than CO2. But the amounts vary greatly and can be hard to measure. Other sources of emissions are from earth-moving equipment and electricity generation.

Overall, though, Australia's large coal mining industry has a lower emissions profile during production than LNG.

For example, for the year ending June 2010, Rio Tinto Coal Australia reported total emissions of 3.44 million tonnes based on coal production of 47.5 million tonnes.

By comparison, Inpex and Total's Ichthys project will produce its 7 million tonnes of emissions from an 8.4 million tonne per year LNG project.

LNG PRODUCTION: MAIN SOURCES OF EMISSIONS?

For LNG to be liquefied, the CO2 has to be stripped out and is usually vented into the atmosphere. The higher the CO2 content, the greater the energy needed to remove the CO2 and the greater the carbon footprint of the project.

Generally, for low-CO2 content gas fields, the vast majority of emissions are from pumping the natural gas to the processing plant and then chilling it to minus 161 degrees Celsius.

But for fields with a high CO2 content, the equation changes substantially.

ARE COAL-SEAM GAS PROJECTS CLEANER?

No. While coal-seam gas has a low CO2 content, usually about 2 percent, these projects have much larger fuel combustion emissions from running compressors that extract and pipe the gas to the LNG plant. These projects usually involve thousands of wells and many hundreds of kilometres of piping.

The emissions intensity of these investments tend to be near the top end for LNG projects.

WHAT CAN BE DONE TO REDUCE LNG EMISSIONS?

Carbon capture and storage, long-term tree plantations or buying carbon credits from clean-energy projects elsewhere can offset some or all of the emissions. Higher carbon pricing in Australia could encourage investments in these schemes to offset emissions.

Chevron's A$37 billion Gorgon project in Western Australia is planning to inject 3.3 million tonnes, or 40 percent of its CO2 emissions, into deep rock formations and is the only Australian project currently to do this. It will cost the company A$2 billion.

LNG projects are also turning to more efficient gas turbines to run compressors and to generate power as well as capturing more of the waste heat from the production process, and more of the LNG that evaporates. (Reporting by David Fogarty; Editing by Simon Webb)

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