The benefits of supercritical technology have previously only been able to be enjoyed by those with access to high quality coal. Now Mitsui Babcock has developed a solution that makes it an option for all.

David Hough, Mitsui Babcock, UK

China is currently experiencing an unprecedented expansion of its electricity supply industry. This expansion has pulverized coal fired power generation at its base and has more recently adopted supercritical steam conditions as the norm, with the resulting increase in cycle efficiency and associated reduction in coal consumption and gaseous emissions being powerful incentives.

This technology shift is also occurring in other countries and is expected to continue in the near future as security of supply and the replacement of ageing fleets become priorities in the developed world, although so-called developing countries like China are clearly at the forefront today. Roughly 230 supercritical units rated at 600 MW or larger have been ordered in China in the last few years, with a further 170 units anticipated before the year 2010.

This huge expansion is underpinned by China’s equally impressive coal resources. Coal is available across the length and breadth of the country and there is an enormous range of quality available. There is also a considerable strain being placed on China’s coal mining industry as more and more coal is required. Often, the better quality coal is preferentially supplied for steel making purposes resulting in the steam raising industry having an increasingly uncertain supply.

The net result is that the coal quality supplied to power stations is getting poorer and poorer, with known difficult coals often the only reliable source. Lean coals – those with just 10 to 20 per cent dry ash free volatiles – are increasingly being looked to as steam raising coals but they have in the past presented a number of combustion challenges. Such challenges have been met by Mitsui Babcock during the design of the 2 x 600 MW supercritical units at the Wangqu power plant, paving the way for the more extensive use of lean coal for power generation purposes.

Lean coal challenge

The boilers at Wangqu will be among the first supercritical units to operate using China’s low volatile lean coals.

Outside the combustion zone, the boiler is in all respects a conventional, two-gas pass type, supercritical boiler, using a divided rear pass arrangement with biasing dampers for reheat control. The combustion zone is special not because of its size or arrangement but because of its components – advanced low NOx burners together with high velocity or boosted overfire air (BOFA) – and because of its performance on such a difficult fuel.

Lean coals have presented a challenge for wall-fired boiler manufacturers for decades. In China itself, domestic boiler suppliers have for many years fired these coals using licensed, corner fired technology with mixed success. Bituminous coals historically fit with wall-fired and corner fired technologies, and anthracites with downshot technology, but there has been for a long time a grey area in the middle where there has not been an obvious choice. Downshot furnaces are usually refractory lined and extremely flexible in that they can handle almost any coal very effectively, but the technology is not easily applied in a supercritical form, though it is now available using a low mass flux vertical tube furnace.

For wall-fired and corner-fired manufacturers, truly competitive performance on lean coals has been a step too far for a long time. The headline issue is flame stability, and whether or not operational flexibility via effective turndown ratios can be achieved with these coals in anything other than a downshot furnace. The other issue is NOx, since the high temperatures involved in traditional downshot combustion and the coal difficulty are not consistent with the lowest NOx levels.

Chinese NOx legislation recognizes coal quality as a real issue and sets limits accordingly. For lean coal, the NOx limit for new plant is 650 mg/Nm3, a difficult target for downshot firing but a realistic target for wall firing, and in recent years much progress has been made by wall-fired manufacturers.

Burner development

At Renfrew, in Scotland, Mitsui Babcock has the largest single burner test facility in the world. The issues of scale-up in combustion and burner development are among the most difficult to overcome. This piece of research technology has been in use since the late 1980s, with a rebuild and enlargement in 1996. It was used to develop and test the original low NOx axial swirl burner (LNASB), which was installed in Drax Unit 6 in 1989 and halved NOx emissions both there and elsewhere throughout the UK. To date there are over 2500 of those burners operating or being installed in plant around the world, and since that first important step the test facility has been used to develop new burner ideas and evaluate performance on new coals. The trend in recent years has been towards fewer, larger burners and mills, so that new burners and new fuels are assessed at 70 MWt, a procedure which requires hundreds of tonnes of coal at a time over a two to three day test period.

Figure 1. The arrangement of a standard burner
Click here to enlarge image

In the mid-1990s, Mitsui Babcock developed its Advanced LNASB using its Renfrew test facility. This burner achieved greater NOx control via the use of an additional air stream, which achieved greater separation of the coal from the bulk of the (tertiary) air supply, and provided enhanced flame stability. A 25-30 per cent improvement on NOx compared with the standard burner was seen on the test facility without any noticeable deterioration in combustion efficiency. On plant, at 300 MW scale, overall reductions in excess of 70 per cent were realized. The visible stability improvement and the NOx potential made this burner an obvious candidate for lean coal applications. See Figure 1 for a general arrangement.

Differing ash content

Coals from the UK were tested first, at 16-20 per cent dry ash free volatiles, and on the back of immediate success, coals were imported for testing from China, specifically those intended for supply at Wangqu. These coals were slightly lower in volatiles than the UK coals and also higher in ash so that the useful volatile content was actually lower still. At 14-15 per cent dry ash free volatiles, and 15-16 per cent ash, the useful volatile content was just 13-14 per cent on an ASTM dry mineral matter basis, which would result in a classification of semi-anthracite, not bituminous coal at all.

The full-scale burner testing first took place in late 2002 in preparation for the Wangqu tender, with further confirmatory tests conducted the following year. NOx levels were below 600 mg/Nm3, burnout was as expected rather worse than with the earlier coals but in line with expectations, and turndown to 50 per cent burner load was possible without any loss of flame stability.

A test facility figure of 600 mg/Nm3 translates into a full-scale furnace figure somewhat in excess of the 650 mg/Nm3 target, so the moderate use of overfire air is necessary to deliver the environmental requirements. Further consideration of the coal difficulty in this respect led to the incorporation of boosted or high velocity overfire air. The effectiveness of high velocity overfire air in reducing unburned losses has been repeatedly demonstrated on bituminous coals during the last few years, from the earliest experiments at Longannet as part of the gas reburn project in the late 1990s through a number of subsequent retrofits where combustion efficiency was an important component in the pursuit of low NOx.

Figure 2. A comparison of the results from burning various types of coal
Click here to enlarge image

Normal OFA actually penetrates into the furnace very little and combustion efficiency is limited as a result. Boosted OFA penetrates much further and achieves greater mixing between unburned coal and the final admission of air, improving burnout as a result. On retrofit, this is particularly useful where residence time is limited and NOx reductions difficult to achieve via traditional means.

On a new boiler like Wangqu, where residence times are much larger, high velocity OFA will deliver a much improved efficiency, with the additional mixing being used to offset the coal difficulty. Ports are positioned around the furnace and CFD is used to position them most effectively. Mill performance is critical too, of course, with dynamic classifiers in place to ensure no compromise on fineness. As a result, anticipated boiler efficiency is in excess of 94 per cent LHV with just a one per cent loss on combustion efficiency, something which on its own would be hard to beat using a downshot design and which in this instance is coupled with a NOx guarantee at 650 mg/Nm3 and a turndown guarantee at 40 per cent BMCR.

Leaner and meaner

Since the proving of the Wangqu coals, coals with lower and lower volatile contents have been fired in the test facility. British, low ash coals with just 9-10 per cent dry ash free volatiles (8-9 per cent dry mineral matter free) have been successfully fired with a burner turndown capability of 70 per cent, but firing a 6-7 per cent volatiles, high ash coal from Vietnam (a region famous for its coal difficulty and a prominent downshot market at the moment) has resulted in a flame on the edge of stability.

Clearly, the burner in its current form is reaching the limit of its capability as it stands, though it should be remembered that the burner in question is the standardized test burner, not a purpose-designed burner, so it may be possible to achieve further improvements in performance. In particular, it is thought that a burner design at different primary air quantities and higher velocities may provide the solution. Future work to develop in this direction is planned.

The pronounced stability of the burner has enabled the Wangqu furnace design to proceed without consideration of refractory. Earlier wall-fired attempts to fire these coals have included the use of burner zone refractory and it is fair to say that it has been a considerable nuisance as operators have been forced to reduce the coverage in an effort to control the slagging difficulty originally experienced.

The combination of advanced low NOx burners and high velocity or boosted overfire air will be used for the first time at Wangqu to maximize NOx control and combustion efficiency at the same time. Bringing the two technologies together in this way, matching a tried and tested overfire air capability with a newly proven burner, offers an overall performance not previously thought possible on such difficult coals. Advanced burner development will continue in an attempt to demonstrate that all types of coal can be handled in a wall-fired arrangement.