Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Posts Tagged ‘gas turbines

Nuclear-heated gas turbine power systems

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There is an interesting little thread over at the The Oil Drum discussing gas turbines for nuclear energy systems, and the engineering challenges associated with the same, for example the pros and cons of different gases which may be used.

Given that Brayton-cycle powerplants are certain to play an important part of the development of modern nuclear energy technology, (irrespective of whether you’re interested in LFTRs or PBMRs or any one of a dozen different types of reactor technology, they can all usefully be combined with gas turbine energy conversion) it’s a very interesting area to keep abreast of.

There’s also another, longer, more heated recent thread debating nuclear energy in general on TOD, which started off as a post on seawater uranium, which has degenerated a little bit into a familiar Sisyphean argument about nuclear power, but it’s there all the same, if you want to have a look.

Written by Luke Weston

October 10, 2008 at 3:18 pm

More on the Garnaut report.

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The next thing that gets on my nerves about the Garnaut report is its repeated emphasis – and optimism – on what are described as “near-zero emissions coal technologies”.

The success of near-zero emissions coal technologies will mean that new fossil fuel plant will continue to be coal, while Australia continues to gain as an exporter from the ongoing high global gas prices.”

“For Australia, the importance of reducing emissions from coal combustion is of large national importance.”

If this industry is to have a long-term future in a low-emissions economy, then it will have to be transformed to near-zero emissions, from source to end use, by the middle of this century. A range of technical, environmental and economic challenges must be addressed effectively to achieve this objective, in a time frame consistent with a global agreement on climate change and Australia’s own domestic commitment.

There is seemingly precious little discussion about the maturity of such technologies, how scalable they are, how far they are away from practical widespread use, the extent to which they actually mitigate carbon dioxide emissions, and how much these technologies will cost, and their economic competitveness, even under a GHG emissions trading scheme.

At its simplest, the challenge is to develop technologies that allow coal combustion with zero, or near-zero, carbon dioxide emissions while maintaining its relative competitive position as a fuel.

Those same forces of high capital costs, high world gas prices and relatively strong export coal prices will strongly favour retrofitted (post-combustion capture) coal plants with captive coal supplies and low-emissions profiles and ultimately, near zero emissions plants involving integrated coal drying and gasification technology.

This notion of “zero-emissions” or “near-zero-emissions” from coal is simply ridiculous and unfounded.

The IPCC Special Report on Carbon Capture and Storage reports that:

“Available technology captures about 85–95% of the CO2 processed in a capture plant. A power plant equipped with a CCS system (with access to geological or ocean storage) would need roughly 10–40% more energy than a plant of equivalent output without CCS, of which most is for capture and compression. For secure storage, the net result is that a power plant with CCS could reduce CO2 emissions to the atmosphere by approximately 80–90% compared to a plant without CCS.”

There’s an interesting paper here which provides a realistic analysis of the implementation of retrofitted CO2 capture for an existing pulverised coal power station.

This paper examines the retrofit of a 400 MWe pulverized-coal fired plant emitting 368 t/h of CO2 to enable CO2 capture while maintaining 400 MWe output, where 90% of the CO2 emitted from the coal plant is captured. To do so, the extra energy required for capturing CO2 was supplied by natural gas-fired gas turbines.

I’ve quoted the key points from the paper below. I’ve applied some minor editing and collation, but most of the material is straight from the paper cited.

Even when CO2 emitted from the gas turbines is not captured, the overall process still have some impact in reducing CO2 emissions, corresponding to an overall reduction in carbon dioxide output to the atmosphere of between 60-70%, with atmospheric CO2 emissions of between 110 and 138 t/h, compared to 368 t/h for the original coal combustion plant, at the cost of a natural gas requirement of about 1350 GJ/h.

When CO2 from the gas turbines is captured, the reduction in CO2 emission is between 68 and 77%, or CO2 emission to the atmosphere of 86-113 t/h. However, a considerably large amount of natural gas is required (around 3140 GJ/h), which is certainly not reasonable.

The total amount of natural gas required is between 3100 and 3300 GJ/h. To put this number into perspective, this amount of natural gas would generate about 460 MWe when used in a combined-cycle gas turbine plant without CO2 capture. The relative emissions are between 23% and 32% of the original coal power plant, which has a significant impact on the reduction of CO2 emissions.

In addition to the cost analysis, the future work will investigate other possibilities of integrating gas turbines into the coal power plant. The configuration presented in this paper represents one extreme, where all the electricity is generated from coal and the auxiliary power is generated from the gas turbine and natural gas boilers. Another extreme is to consider the classical view of producing all the auxiliary power from the coal plant, at the expense of decreasing the net electrical output. To compensate for the lost electrical output, a combined-cycle gas turbine with CO2 capture will be added. Combinations between these two extremes will also be investigated.

For a nameplate capacity of 400 MW, and a capacity factor which we may assume to be, say, 85%, carbon dioxide emissions of between 86 and 138 t/h correspond to emissions of between 253 to 406 grams of carbon dioxide per kWh generated.

The results from this study certainly paint a less optimistic assessment of the carbon dioxide emissions intensity of “clean coal” than the 80-90% reductions estimated by the IPCC.

That’s not clean coal. Much like the use of fossil fuel methane, it’s a bit cleaner, but it certainly isn’t clean.

Indeed, such levels of carbon dioxide emissions intensity are not much better than existing high efficiency combined-cycle natural gas fired gas turbine power plants. Given that the carbon dioxide emissions aren’t grossly worse, and given that gas turbine plants are already established, mature technology which is likely to remain far more economically competitive with this expensive, immature, unproven future technology, it is easy to envision that natural gas will be the main focus of the fossil fuel combustion energy industry over coming years, as opposed to the development of CCS technology, even where emissions trading is introduced.

The exception to this, of course, is where there is a vested interest in keeping the existing coal-fired power plants, and enormous coal-mining infrastructure, even if it is a less than sensible choice on many different levels. Natural gas turbines are of course, along with nuclear power, by far one of the biggest “threats” to the coal mining and coal-combustion electricity generation industry.

Additionally, that’s only considering greenhouse gas emissions from combustion at the power plant – without any consideration of whole-of-life-cycle analysis of coal mining and natural gas production – the enormous scale on which coal is mined, along with the fugitive emissions associated with the production and handling of natural gas, and so forth.

There is a large body of literature and knowledge of the whole-of-life-cycle analysis of energy intensity, environmental impact and greenhouse gas emissions associated with, for example, nuclear power, solar photovoltaics or wind power.

However, since fossil fuel combustion energy generation has such a comparatively fantastically high level of carbon dioxide emissions in the combustion process itself, I feel that sometimes it’s easy for some of us to forget that it’s only sensible to apply the same metrics across the life cycle for fossil fuels, just as people insist on analysis the impact of the entire life cycles for nuclear energy or solar energy, for example.

What is desperately needed is further research into the life cycle analysis of coal or other fossil-fuel combustion based energy generation systems when they are combined with proposed carbon capture and storage technologies. This is clearly important if a fair comparison is to be made between the economic and environmental feasibility of fossil-combustion CCS and alternatives such as nuclear power, wind, hydro or so forth.

Personally, I believe that as such analyses are performed, the extreme skepticism with which many of us view the comparative practicality of these “low-emissions” fossil fuel combustion technologies will begin to be vindicated.