Modern cities are ever more hungry for power, but increasingly prefer to keep their power plants out of sight and out of mind. Whilst the smogs suffered by London in the 1950s or many Chinese cities today are extreme examples of the effects of old coal power plants placed close to city centres, even modern plants are rarely found in urban environments.
However, situated on Tokyo Bay close to the heart of Yokohama, Isogo is no ordinary coal plant. Combining high efficiency power generation with a pioneering technology that restricts key pollutants to almost negligible levels, the plant has come to be regarded as a showcase for cleaner coal power, and a potential blueprint for a more acceptable form of coal power generation for the 21st century.
The plant actually consists of two 600 MW generation units completed in 2002 and 2009, built by owners JPower to replace an existing pair dating from the 1960s. Such is the demand for power in this heavily populated part of Japan, a major challenge for the upgrading project was the need to keep the original plant intact and operational throughout construction of the first new unit. The Tokyo Bay area is also dotted with gas-fired power plants which naturally have minimal impact on air quality, so the decision to retain a coal-fired plant in the region is somewhat indicative of Japan’s wider energy policy. With practically no fossil fuel resources of its own, Japan is obliged to import both coal and natural gas to meet its power demand, and has long regarded coal power as an essential means of avoiding an over-reliance on costly gas imports and keeping energy prices as low as possible.
This dependence on coal power and a desire to get the most ‘bang for buck’ from its coal imports has helped spur Japan to lead innovation in coal power since the early 1990s, and the country now boasts the highest efficiency fleet of coal plants in the world.
Much of this research has concentrated on developing new steels and other metals able to withstand hotter and higher pressure steam, known as ‘ultra-supercritical’ conditions, which allow more of the coal’s energy to be converted to electric power. Now adopted in many other countries, the units at Isogo still represent two of the finer examples of this technology, with the newer unit achieving over 45% power generation efficiency. In comparison to the world average for coal power of 35%, this represents almost a 20% reduction in CO2 emissions, highlighting the enormous greenhouse gas reductions possible if the Japanese approach to coal were more widespread.
However, for the city authority of Yokohama, a more pressing concern was to dramatically cut the emissions of the harmful air pollutants associated with coal combustion: the oxides of nitrogen and sulphur often known as SOx and NOx for short. To meet this challenge, JPower chose to implement two separate devices for cleaning the contaminants from the dirty flue gas produced by the coal boilers. Firstly, the relatively well-established selective catalytic reduction technology is used to remove to the majority of the NOx; reacting it with ammonia over a catalyst at high temperatures.
In order to eliminate as much SOx as possible, as well as cleaning up the remaining NOx, the plant turned to a much more novel solution known as the regenerative activated coke technology or ‘ReACT’, which uses a high-surface area carbon material to absorb the pollutants from the gas. The carbon is then heated to remove a stream SO2 which is used to produce sulphuric acid as a valuable by-product, and in the process regenerating a clean carbon for reuse. In comparison to the tanks of water and limestone usually employed for sulphur removal in coal plants, this process also vastly reduces water consumption and has the added benefit of removing mercury – another harmful pollutant which is only now becoming subject to legislation for coal plants in the US and Europe.
These technologies combine to successfully meet the strict concentration limits of 13 parts per million (ppm) for NOx and 10 ppm for SO2 imposed by the city authority, and frequently maintains them both well below 10 ppm. These levels are equivalent to those emitted by gas-fired plants, and can be compared to limits around 10 times higher which are typically thought practical for coal plants in the US or Europe.
Isogo’s elegantly curved, 200 metre tall smokestack may simply appear a fittingly futuristic tribute to these cutting-edge technologies, but it was also built to the meet the brief of a city almost as concerned with the visual impact of the plant as with its emissions. The tower’s parabolic shape exposes a much narrower face towards the city, whilst the whole plant is decked out in pastel marine tones intended to soften its appearance from the sea.
Within the grounds of the plant, around 20% of the space is devoted to grass and trees, and the plant is also involved with local tree planting schemes. Perhaps most interestingly, the noticeable height difference between the two boiler buildings was specially designed in order to help disperse emissions from a neighbouring gas power plant.
Although a few other coal plants in Denmark, Germany, and China claim slightly higher efficiencies than Isogo, the Japanese plant remains unique in the world for its remarkably low emissions of non-greenhouse gas pollutants.
Only recently, Isogo’s pioneering ReACT system has seen adoption by a US coal plant, whilst optimisation of more established pollutant control technologies in China is proving capable of similar results.
It is clear that coal power should no longer be considered as a necessarily ‘dirtier’ fossil fuel than natural gas, and can continue to play a role as a lower cost source of power – even in highly urbanised environments if necessary. With deployment of coal power gathering pace in India and much of the developing world, it is imperative that more plants follow Isogo’s example.
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