The Chinese way: reducing emissions10 June 2015
Coal-fired plants remain the world’s primary means of electricity generation – but with fossil fuels, the emissions reduction issue must always be part of the conversation. While new and improved plants are on the way, maximising the efficiency of current-generation operations remains a crucial piece of the puzzle. James Lawson hears from the International Energy Agency’s Dr Keith Burnard about his recent visit to China, the world’s biggest electricity consumer, to gauge the success it’s had in keeping the old stock clean.
Coal is the world's principal fuel for electrical power generation. Emitting much more CO2 per unit of power produced than any other generation type, coal-fired plants also produce high levels of other harmful pollutants like sulphur dioxide (SO2), nitrogen oxides (NOX) and particulates.
The need to maximise efficiency, and so lower emissions, is clear.
Over the operational lifetime of a coal-fired unit, each percentage point increase in efficiency removes several millions of tons in total CO2 emissions. Unfortunately, many of the world's existing stock of coal-fired plants are relatively inefficient, subcritical - defined as having steam pressures below 221bar and temperatures lower than 550°C - units. At 33%, their average efficiency compares poorly with the 45% possible with the latest ultra-supercritical technology.
The world's largest electricity consumer, China, is on the front line of this efficiency drive. Relying on coal for 73% of its power, China is expected to commission more than 400GW of new coal generation by 2035, and its coal consumption will rise by 25% by 2035.
Investing in the latest, most efficient technology for its new plant, the country is working hard to improve the thermal efficiency and environmental performance of its existing units. China is also retiring the oldest, most inefficient plants as its new supercritical facilities go into operation.
"China has a lot of coal - it's going to continue to be the backbone of their power generation," says Dr Keith Burnard, head of the energy supply technology unit at the energy policy and technology division of the International Energy Association (IEA). "What they are doing with their existing plants can provide some good lessons for users elsewhere."
Under China's current regime, all operators have a duty to maintain their plant's design performance. By measuring the number of grams of coal equivalent (gce) required to produce 1kWh, it's possible to compare the efficiency of each plant, and so target the worst performers for examination and improvement.
The plants also need to be profitable or limit losses, so operators look for a reasonable payback period on new parts or maintenance through reduced fuel costs - usually within two years.
Aggregation of marginal gains
In 2013, Burnard and then colleague Julie Jiang travelled to China for a joint China-IEA study of the efficiencies possible through the upgrading and retrofitting of older coal-fired plants.
The IEA inspected two subcritical 300MW units. Operating at around 540°C and with net design thermal efficiencies of 39%, they were over ten years old and broadly representative of the sort of plants targeted for improvement. However, Burnard emphasises that no two plants will perform the same way, even if their original designs are identical. Their efficiency depends on how rigorous past maintenance has been and how they have been operated - their start-up procedures, load cycles and so on.
"Over time, plant efficiency tends to drop away," he says. "Heat transfer is not as good due to scale build-up, and there's wear and tear on turbine blades, pumps and other systems, so it requires continual maintenance and good operation to maintain the original design efficiency."
Both Chinese units had seen upgrades in recent years, with significant efficiency benefits. The general approach is to improve combustion performance and optimise boiler efficiency through refurbishment; to maximise energy conversion by optimising the steam cycle, as well as and overhauling and upgrading turbines; and to reduce auxiliary power consumption by upgrading various pumps, motors, drives and power supplies.
While increasing the size of the air heater surface in the boiler, optimising the condenser or refurbishing the steam turbine can offer major benefits, overall efficiency improvements are typically due to a host of smaller changes. These include reducing steam leaks, optimising air pre-heaters, cleaning boilers and furnaces to improve heat transfer, installing coal-drying equipment or soot blowers, and upgrading cooling systems.
Moving to supercritical or ultra-supercritical operation is another possible route for improving efficiency, but it is not a viable option for most older plants and certainly not for any under 400MW. It would require retrofitting new boiler systems and high-pressure turbines, along with piping and many other subsystems.
"It was an eye-opener to go along to these plants and find that the engineers very much see their job as maintaining and improving efficiency," says Burnard.
"The performances of the two units I visited were very good and close to their design efficiencies, though there was still room for improvement."
Burnard makes a broad estimate that, once complete, the work carried out to the plants would result in a 6-7% reduction in annual CO2 emissions. Most 300MW units in China have already been the subjects of similar upgrades.
Cogeneration, which captures otherwise-wasted thermal energy, offers substantially higher savings; equipment retrofitted to one unit saves an extra 46,000t of CO2 annually.
"Using the heat generated is only possible if they are near a large heat sink, like a nearby city or large industrial complex," notes Burnard. "They do more of that where they can, but it's not always possible."
As well as physical plant changes, improving operating procedures and maintenance practices can drive significant efficiency gains without the need for capital investment. Optimising operation also means plant equipment deteriorates more slowly.
"It's a relatively simple fix to improve operations," says Burnard. "The Chinese improve the control systems and optimisation packages where it is cost-effective to do so, and their new plant is generally state-of-the-art in that respect."
Better overall efficiency could be achieved by operating the most efficient Chinese plants at full capacity, but priority is more often given to those plants with the lowest cost of electricity rather than the most efficient ones.
Some generators can be tempted to lower costs by using cheaper, poorer-quality coal, thereby reducing efficiency and raising emissions. Running at part-load also reduces thermal efficiency: both the units inspected typically operated at significantly lower efficiency than those obtained in full-load performance tests.
"By and large, the authorities have tended to share out demand between all the plants rather than focusing on getting the maximum from the most efficient plant, which would be more effective," says Burnard. "They are gradually moving to a more market-based system."
Strict emissions standards targeting SO2 and NOX reduction that arrived in 2012 have meant further substantial upgrading. Plant operators must improve existing electrostatic precipitators and flue-gas desulphurisation systems, while NOX control will, in most cases, require selective catalytic reduction. Adding this emissions reduction equipment consumes energy and reduces overall efficiency, so the plant must be as efficient as possible to start with.
"You need the highest possible efficiency to cope with emissions equipment, and they don't want to add it to plants that are not already operating effectively," says Burnard.
China is planning to introduce some form of carbon tax within the next three years and has already started carrying out pilot programmes for emissions trading.
That gives the country the chance to take a lead in the application of carbon capture and storage (CCS) systems. China currently has the second-largest CCS programme in the world, after the US, but the technology has not, as yet, been commercially implemented.
"China is very active in developing CCS," says Burnard. "They are looking at it on a province-by-province basis and are planning pilots at the moment. As things stand, however, whether in China or elsewhere, the economics are not there for CCS at the moment."
Burnard also notes that, in common with other countries, China's new generating plants are not designed as being CCS-ready from the outset.
Retrofitting CCS is not a trivial exercise - there needs to be sufficient space at the plant to site the additional equipment, the steam cycle must be optimised and the plant has to be close to a CO2 sink.
"This is a major oversight - and not just in China," Burnard says. "We strongly recommend that all new coal-fired power units be designed as CCS-ready."
China: admirable of the fleet
Where older coal-fired plants are well serviced and operate close to their design efficiency, there may be little to learn from China; however, the Chinese experience is certainly applicable to South-East Asian nations like Thailand, Vietnam and Indonesia, as well as Australia, the European Union, India, Russia and the US. All of these have coal-fired fleets dominated by subcritical units, and non-OECD countries in particular will see electricity demand - and coal consumption - increase over the coming decade.
Though, in some cases, more-efficient ultra-supercritical new plants may be planned, there is often no active national programme to improve the performance of existing stock and neither is emissions legislation as stringent as that in China - which is proven to drive reduced CO2 emissions.
"The existing fleets of small, subcritical units in India and South-East Asia are neither particularly efficient nor well maintained," states Burnard. "They can look to China for a lead there."
In the meantime, China continues to improve its subcritical coal-fired plant performance. As well as the forthcoming carbon tax, the Chinese Government has also set an energy intensity reduction target of 16% during its 12th Five-Year Plan (2011-15), which provides another strong reason to raise efficiency.
Ultimately, this could mean individual power plants have their own performance targets.
"While its immediate goal is to reduce the ravages of air pollution, China is increasingly recognising that climate change is something that needs to be addressed," says Burnard. "They are looking at the role they should play in that and are bringing out policies to tackle it."