Cost pressures drive flue gas treatment into new and fertile regions20 November 2000
In the fiercely competitive power market sometimes the most cost effective way to remove SO2 is to convert it into something saleable – fertiliser, or stabilising landfill for example
The liberalisation of the power market world wide and the tendency to IPP projects on a project finance basis have led to severe price pressure on power production in all parts of the world. If power production is not to be subsidised, for political reasons, it follows that the whole power production industry has to undergo further changes in terms of investment and modes of operation in the near future.
The major portion of power plant operation costs is influenced by the price of the fuel utilised. There is, besides the optimisation and simplification of processes and equipment, a strong tendency towards the use of cheaper fuels, for example vis-breaker, petcoke, orimulsion, open field lignite, imported coal or fuel oil No 6. Other than imported coal all these fuels have in common a high sulphur content and a high concentration of impurities or trace metals. This tendency towards utilising cheaper fuels has had a big impact on the flue gas cleaning market.
As using low cost fuels with a high sulphur content also implies a high consumption on the reagent side and a corresponding production of by-products for these fuels the technology involves somewhat higher investment costs but since there is a real return on capital employed on the product side this is an excellent solution.
Suppliers of flue gas cleaning systems have to meet market demands of low investment costs, low operation costs and the utilisation of low costs fuels with alternative technologies such as ammonia water scrubbing, sea water scrubbing sometimes in combination with wet electrostatic precipitators and special bag-houses. They have to design to market which means meeting the customers expectations regarding all relevant costs; and they have to be able to supply a package solution for each individual flue gas cleaning problem.
Using cheaper fuels very often goes along with high ash content as well as high sulphur content. This immediately creates a demand for optimisation of the existing electrostatic precipitators, which can be found as the most common de-dusting device in power stations world wide. In many cases these problems can be handled very efficiently by applying either advanced microprocessor controller technology or, in more demanding cases, with flue gas conditioning systems or in some cases ESP-to-fabric filter conversions.
The second notable tendency in the de-dusting market is the increasing demand for bag-house technologies, which, compared to electrostatic precipitators, can tolerate more difficult fuels and open up, due to the removal mechanism in a filter cake, further possibilities with respect to removing trace metals or very fine particles. Besides the very well-known reverse air and pulse jet filter types (Figure 1) LLB offers a low pressure pulse jet fabric filter technology with benefits regarding corrosion resistance, pressurised air consumption and space requirement. This technology has been used in combination with our circulating fluidised dry scrubbing system installed at the Setuza industrial heating and power plant in the Czech Republic. (see Table 1). This is a CFB-FGD plant cleaning the flue gases of four boilers. It can handle a gas flow rate of 290 000 m3/h using a single nozzle absorber and low pressure pulse jet fabric filter for de-dusting downstream of the CFB filter. Comparing the Usti (Setuza) unit with the very similar Plzen unit showed, interestingly, that in terms of clean gas desulphurisation and clean gas concentrations it made no difference, with a CFB-FGD system, whether there was a fabric filter or an electrostatic filter installed downstream, even at desulphurisation efficiencies of more than 95 per cent, or, more importantly, clean gas concentrations of less than 300-400 mg SO2/m3. The CFB absorber is able to achieve the desulphurisation efficiency without the additional desulphurisation capacity of the downstream fabric filter.
The desulphurisation product which is mainly calcium sulphite and sulphate, and calcium carbonate, free hydrated lime and some fly ash is mixed with fly ash and water to produce a stabilising compound used in open lignite mines as a landfill. Alternatively the product can be used directly as a sulphur fertiliser, an option that has been tested for three years and proved successful.
The impurities and trace metals in the low cost fuels listed above have also led to an increasing demand for the application of wet electrostatic precipitators. This technology has been used for example in the refinery at Leuna where the wet ESP was installed upstream from the limestone-based FGD unit first of all to remove the impurities from the flue gas and secondly to make sure that a marketable gypsum could be produced. The firing of vis-breaker in Leuna has, as expected, proven the economics.
Low investment FGD technologies
The demand for low investment FGD technologies has been met by the development of seawater scrubbing technology with forced insitu oxidation. Such a system is now in operation in Paiton, Indonesia. The project started in 1996 and was brought to test-ready status in the autumn of 1999. It consists of a system of three absorber sprays together with two absorber sump level sprays feeding a total of 34 300 m3/h to the absorber array.
Another low investment technology is the combination of a semi-dry circulating fluidised bed scrubber with a low pressure pulse jet filter. Such a system is installed at Setuza.
When dealing with lime and limestone FGDs it is important to follow the target for further optimisation and simplification in order to keep investment and operation costs as low as possible. LLB has called this programme AIDA which stands for Advanced Innovative Designed Absorbers. The absorber, as the so-called "heart of the system", is based on several patents held by LLB; for example, the sump dividers that provide the advantage of a dual-loop scrubber in a single-loop open spray tower and the impulse suspending systems with substantial advantages over other technologies involving sump agitating impellers. On the material-of-construction side this concept can benefit from a number of material concepts such as rubber lined, flake lined, FRP or high alloy corrosion protection.
Alloy 59 absorbers are currently in use in Lippendorf (Figure 4 and photograph on p33), a 2x930 MWe lignite burning combined heat and power station with an 850 MWe/230 MWh desulphurisation plant (unslaked lime and gypsum) using 300 000 tonne/y of unslaked lime and 1 000 000 tonne/y gypsum; performance is characterised by flue gas concentrations of 10 000 mg/Nm3 SO (maximum) corresponding to 1 750 000 Nm3/h moist gas volume flow; with clean gas at 400 mg/Nm3 (maximum 5 per cent).
This plant went into operation at the end of 1999 and has to date performed as expected.
Wet ammonia process
In 1998, LLB bought the former Walter Process from Thyssen Krupp Uhde (where the process was hibernating) and improved the design (Figure 5) by maintaining the chemistry but changing the apparatus concept into a two-stage spray tower adding a wet electrostatic precipitator on the clean gas side for aerosol removal. As this technology produces a fertiliser - ammonium sulphate - at the rate of four tonne of product per one ton of reagent, it creates a substantial revenue on the product side.
LLB have established a co-operation agreement with Foster Wheeler Energy International for the utilisation of petcoke in circulating fluidised bed boilers with the ammonia water process for the FGD; an installation (Figure 6) of a semi-dry scrubber at Jackson Electricity A’s Northside power station already exists; the two companies are now putting forward a proposal for a 300 MW project using a wet ammonia scrubbing/CFB combination.
For LLB de-nitrification means selective catalytic reduction (SCR) technology. Due to heavy price pressure the design has to be simplified to succeed in international competition. For this reason, the number of spare layers had to be reduced sometimes even down to zero and design for the reactor had to be simplified and standardised. In recent years there has also been tremendous price pressure on the catalyst market and currently catalyst suppliers' hope, that with a high demand for their product in the United States, they will achieve higher prices in the near future. LLB is following up the existing market for this technology in Taiwan. The developing market in Korea as well as an enormous market in the United States will probably last for the coming three to four years. In the United States LLB has formed a technology joint venture with Wheelabrator Air Pollution Control which has already been working on a 660 MW SCR installation.