NID proves attractive for flue gas desulphurisation where size matters

Alstom’s NID system is a dry FGD technology, which aims for maximum simplicity and compactness (see panel, right, and MPS August 1997). The first installation was in Poland at the 2x120 MWe Lasiska site, which was commissioned in 1996, with supporting laboratory work at Växjö. Since then there have been a number of important developments of the technology, notably combination with an ESP, use of integrated lime hydration and use of ash from a CFB as the reagent. The technology also looks promising for the retrofit area, where the fact that the combined full-scale plant for the removal of flyash and SO2 can be fitted into the space occupied by existing electrostatic precipitators is of particular interest.

So far there have been ten commercial installations of NID, but the trend towards liberalisation has tended to slow the pace of FGD adoption in recent years. However there are promising niches for future development, in particular small CHP plants in eastern Europe. The small footprint makes NID a good fit for smaller plants, where its cost advantages become more pronounced compared with larger plants. There is also a tendency for a tightening of emissions standards as applied to plants in the smaller size range.

Another important future potential market could be in the developing countries, where shifting to natural gas from coal may not always be economically feasible but where high sulphur coal is an abundant energy resource (eg India and China).

NID+ESP

Normally a fabric filter is used with NID, but in some cases an electrostatic precipitator (ESP) may be the preferred dust collector. This combination has been subject to an R&D effort, including aerodynamic model studies as well as testing in a pilot plant.

Testing reveals that the SO2 removal efficiency drops a few percentage points, but the combination may still be of particular interest, especially when it comes to retrofit situations, where an existing ESP may be re-used. Extra collecting surface may be needed in some cases, but the resistivity of the dust is drastically reduced during NID operation, so an existing ESP may do surprisingly well if NID is installed.

In case of an ESP, the NID installation has to be configured somewhat differently from that for a fabric filter. The reactor is made an integral part of the ESP inlet nozzle. This means that recycle ash is transported back to the inlet side of the ESP, and that the ash then passes through the mixer before being dispersed in the reactor duct.

Such an ESP-NID combination has been installed at a new 280 MWt coal fired boiler in Quzhou, Zheijang province, China, with operation due later this year. The gas flow is 300 400 Nm3/h and the expected sulphur removal is 85 per cent.

Integrated lime hydration

Both lime (CaO) and dry hydrated lime (Ca(OH)2) may be used directly as the reagent. Although hydrated lime is the active species in the absorption process, there are good reasons to use lime rather than dry hydrated lime, namely cost and ease of transport (due to the density difference).

The simplest way of utilising lime rather than dry hydrated lime is to feed lime as CaO directly to the process. Experience shows, however, that this mode of operation often leads to a reduced utilisation of the alkalinity of the reagent, for reasons that are not entirely clear.

It is therefore preferable to hydrate the CaO first. Conventional lime hydrators are not suitable for this purpose so the NID Integrated Lime Hydrator was developed.

Lime and water are fed into the hydrator, which comprises two stages. In the first stage water and lime are mixed together. In this stage the “induction period” of the hydration reaction takes place. This period normally lasts only 1 - 2 minutes; until the heat evolving reaction starts.

From the first stage, the wetted lime overflows into the second stage, where the material undergoes a drastic physical transformation; the somewhat lumpy material from the first stage literally falls apart into a very fine powdery material: the hydrated lime. The second stage has a larger volume than the first one and it is designed with respect to the reactivity of the lime quality to be used for a particular situation.

The design of the Integrated Hydrator is open and the flow direction is perpendicular to the mixer shafts of the respective stages. The lime is fed to an intake box from where it overflows by gravity into the hydrator, passes its two stages, to finally overflow into the NID mixer.

The unique design of the hydrator allows free passage of material from inlet to outlet; the design also prevents excessive mechanical stresses on the equipment as a consequence of the flow direction.

Due to this, the hydrator can be operated at lower temperatures than conventional dry hydrators, which in turn allows for the production of a hydrated lime with a very high surface area. Test results show that hydrated lime of a BET surface in excess of 30 m2/g can be produced, a value to be compared with that of commercial dry hydrated lime, which is limited to a BET surface area of 15 - 17 m2.

The NID Integrated Hydrator has so far been installed at five plants (including EWAG, see later).

Use of ash from CFBs

It has long been suggested that alkaline ashes could be used as make up for FGD systems. This is potentially a very interesting approach to lower the operational costs of desulphurisation. Ashes may be alkaline as a result of the actual composition of the ash of the coal. The ash may, however, also have an artificially high level of alkalinity, for example as a result of limestone injection into the boiler.

In the boiler the limestone is subject to two parallel reactions: sulphation and calcination. Generally, one would expect a material like an ash from a CFB boiler to be rather inactive for a post combustion process.

The sulphation reaction is thought to limit the utilisation of the reagent by blocking the pores of the calcined limestone with a surface layer of calcium sulphate. In the boiler the aim is to utilise the limestone to its maximum; thus yielding an ash of a high sulphate content.

CFB boilers are often operated around 850°C. This combustion temperature more or less coincides with the optimum temperature for the sulphation reaction of the limestone. Since a CFB ash is a highly sulphated material it would then have an inactive surface layer when reacted in a post combustion process.

These preconditions would seem to argue against the use of CFB ash as a reagent in an FGD system based on lime. Pilot tests indicate, however, that it is indeed possible to use the CFB ash for further absorption of SO2 in a NID system.

Combining a NID system and a CFB boiler is particularly interesting for high removal efficiencies, especially when firing high sulphur fuels.

By shifting some of the sulphur uptake to the NID system, the limestone flow to the CFB boiler can be reduced. The potential reduction is greater as the demand on the total removal rate is increased. Studies indicate that the limestone flow in some cases could be reduced by as much as 30 per cent; if the total required sulphur reduction exceeds 97 per cent, it may well be possible to reduce the limestone feed to the CFB even further.

The advantages of combining the CFB and a NID system are considerable. Over 95 per cent combined sulphur removal can be realised even for high sulphur fuels, at the same time reducing limestone consumption. Since less limestone is used, the quantity of ash to be disposed of is likewise reduced. The CFB boiler efficiency is also increased, since the endothermic decomposition reaction of the limestone to form lime is reduced by the reduced limestone feed.

The first commercial step towards a combined CFB + NID system is being taken in an installation at the Formosa Petrochemical Corporation’s refinery, Mai-Liao, Taiwan, where petroleum coke is produced. Construction work was due to start here in mid November, with operation expected in late 2001.

As this is the first installation of a NID after a CFB, the required removal in the NID is limited to a modest 75 per cent removal as a “polishing” step. This will limit the emission of SO2 to the atmosphere from the CFBs to a maximum of 50 ppm.

The pet coke will be combusted in two 150 MW circulating fluidised bed (CFB) boilers. The order for the boiler plant has been awarded to Formosa Heavy Industry (FHI), which in turn has subcontracted the CFBs to Alstom Power. A separate contract was awarded to Alstom Power for the delivery of NID systems to these boilers.

The petroleum coke may have a very high content of sulphur, up to 6-7 per cent. Petroleum coke alone, or mixed with residual oil, will be used as fuel. As stringent demands are put on the SO2 emissions, it was necessary to combine sulphur removal by limestone addition to the CFB with a tail end desulphurisation system, hence the use of NID.

While the plant is being started up, in 2001, optimisation efforts will be undertaken to lower the limestone feed to the CFBs in order to minimise the use of limestone.

Recent refurbishment projects

So much for technological enhancements of NID. Two recent projects serve to illustrate applications of NID in the refurbishment context.

EWAG project

The EWAG district heating power station in Nuremberg comprises three coal-fired boilers, each 106 MWt. The flue gases from these boilers, 3 x 160 000 Nm3/h, are first passed through electrostatic precipitators and are then taken to individual FGD systems.

In 1999, the conventional dry FGD system on boiler 3 was rebuilt as a NID system. The decision to do so was based on a study of the overall economics, especially in terms of ease of operation and simplicity compared with the old plant.

The bottom part of the old spray absorber was cut off and the flue gas duct was redirected into two NID-reactors. The existing fabric filter was changed into a two-compartment design, to fit the inlet conditions after the NID reactors. The lower part of the fabric filter hopper was likewise cut off and a fluidised bottom was installed.

The transport of recycled material is via an extension of the fabric filter hopper, from where the recycled material is fed to the mixer. Lime is pneumatically transported to two small intermediate silos, from where lime is fed to the integrated lime hydrators.

After start-up, in October 1999, some minor modifications were made to the plant. The construction work is finished and all three NID trains are in sucessful operation. Although not fully tested yet, early indications from the plant instrumentation suggest required performance criteria are being met.

Fifoots Point

AES Fifoots Point Ltd has awarded a consortium of Alstom and General Electric International a contract for the refurbishment of the 3x120 MW power plant at Fifoots Point. This is situated at the mouth of the river Usk, near Newport.

The project, led by GEI, aims to upgrade the existing boilers, turbines, generators and auxiliaries to produce electricity cost effectively while meeting today’s stricter environmental standards.

The refurbished plant has been equipped with Alstom low NOx burners and a NID system will take care of the flue gas desulphurisation.

The NID system is being installed in exactly the same area where the old electrostatic precipitators were located. In this respect, the situation is the same as at the first NID installation, in Lasiska, Poland.

The three boilers are each followed by a twin train of NID reactors, and the dust collectors are new high ratio fabric filters of the Optipulse design. Just like Lasiska, there is no precollector for fly ash prior to the NID system. The total gas flow is 450 000 Nm3/h and the removal efficiency as requested by the operating permit means reducing the sulphur emissions by 80 per cent.

The NID plant has been started up.