Dr. Harald Reissner, Austrian Energy & Environment, Austria
Neurath, a district of Grevenbroich in North Rhine-Westphalia, Germany, has a long tradition in terms of energy. After all, the first lignite deposit in the northern coalfield was discovered quite close by in1858. Nowadays, it is where the most high-tech lignite-fired power station blocks in the world are being built, the BOA generation (best optimized plant engineering). Lignite arrives at the plants, around the clock, from the opencast mines in Garzweiler and Hambach for baseload operation.
The two power plant blocks have a gross output of 1100 MW each and an efficiency of over 43 per cent. Austrian Energy and Environment (AEE) as the supplier of the flue gas desulphurization (FGD) plant is involved in this exceptional construction project. In addition to the top-class engineering being employed at Neurath in the field of boiler technology in order to boost the level of efficiency, BOA 2 & 3 are having limestone scrubbers built for them that are by far the world’s largest.
Table 1 lists the impressive specifications of the power plant. Its efficiency of more than 43 per cent compares with the approximate 31 per cent efficiency of existing plants still in operation. This means that annual carbon dioxide (CO2) emissions will be reduced by about six million tonnes when the existing plants have been taken out of service.
Downstream of the air preheater, the flue gases from the boiler plant will pass to the flue gas dedusting system directly. Electrostatic precipitators dedust the flue gas at an efficiency greater than 99.8 per cent. To overcome the total pressure loss of the flue gas in the plant as a whole after the dedusting, induced-draft fans feed the flue gas to the last heat-extraction point, a flue gas cooling system. Here, the temperature of the gas is reduced to the FGD plant inlet temperature of 125 °C, and the dissipated heat is fed to the condensate circuit of the boiler system.
The flue gas flowing into the AEE limestone scrubber (see artist’s impression in Figure 1) is cleaned by chemical reactions that take place in the spray tower to remove acid toxic gases sulphur dioxide (SO2), hydrogren chloride (HCl) and hydrogen fluoride (HF) so that the gas remains within specified limits. Separation of residual dust and aerosols is also carried out. All the limits conform to European Parliament and Council Directive 2001/80/EC of 23 October 2001 on the limitation of emissions of certain pollutants into the air from large combustion plants.
Figure 1: The scrubber at BOA 2 & 3 in Neurath. A portion of the cooling tower is visible top right
Throughout the entire FGD process, the limestone input material is turned into gypsum of a quality high enough to meet the demands of the plaster industry, which uses it to make plaster board and set control the admixture in cement production, etc.
The flue gas leaves the scrubber after flowing through a multi-stage demister at the top and is introduced to the cooling tower directly.
These requirements would not constitute any unusual challenge for the FGD process if there were not a for gas flow of 4.85 million m³/h having to be treated in a single limestone scrubber. This is the first time that such a large flow of flue gas is being desulphurized in a scrubber at BOA 2 & 3 and it places the highest demands on the experts at AEE with regard to the flow mechanics and hydraulic aspects of such apparatus.
Unprecedented scale of scrubbing
The cooling tower is four times taller than the scrubber, but with a diameter of 23.6 m and a total height of 46.2 m, the limestone scrubber is the largest ever to be built anywhere in the world.
With a total volume of 6200 m³, scrubber sump conditions are ideal for the oxidation and crystallization processes of gypsum formation. An optimum configuration of scrubber inlet and spray levels is the centrepiece of the scrubber and the key to ensuring the emission levels at the scrubber outlet remain as required.
The major challenge when designing large scrubbers – those with diameters greater than 18 m – is that the absorption zone proper of the scrubber changes from a cylindrical shape to a disk shape. If one compares, for example, the dimensions of the BOA 1 scrubber in Niederaussem, which has a diameter of 15.3 m and treats a flue gas flow of 2.3 million m³/h, the ratio of scrubber diameter to active absorption height, RAct, is 1.05. At BOA 2 & 3 in Neurath, RAct is 1.3.
The geometry of very large scrubbers exacerbates the problem of deflecting the virtually horizontal flue gas influx into the scrubber into a vertical, cylindrical upward flow. But it is not sufficient merely to deflect the flue gas flow by 90 °. Distribution over the entire scrubber cross-section must be uniform to avoid flow peaks, and hence concentration peaks.
These problems can only be reliably solved by using high-tech engineering tools such as computational fluid dynamics (CFD) simulations.
The risk of soiling and deposits means that it is not possible to install baffles or any other guiding vanes to influence the flow in a limestone scrubber. The only option is to optimize flow distribution by providing perfect inlet geometry and positioning the scrubber nozzles appropriately.
A CFD computation can simulate the velocity vectors of the gaseous phase. The computation indicates that the flue gas flow is distributed very homogeneously over the scrubber cross- section. After the last spray level, the gas flow is directed upwards vertically with sufficiently homogeneous distribution. That is the prerequisite to be able to reliably maintain the degrees of SO2 separation as defined in Table 1. Figure 2 illustrates the distribution of SO2 mass fraction over the height of the scrubber. Analysis of this simulation result shows that the integral degree of separation is greater than 95 per cent. In addition to integral maintenance of the degree of separation, the variance of SO2 outlet concentration has to be minimized to plus or minus 30 per cent to ensure the average figure. To make this separation performance possible, the scrubber is provided with four spray levels. Two are in dual-level form and two are single level. Nearly 500 double eccentric nozzles spray the huge recirculated flow over the four levels in the limestone scrubber.
Figure 2: Distribution of SO2 mass fraction over the height of the scrubber
AEE is the leading company worldwide in the computation of reliable results on SO2 fraction distribution in limestone scrubbers via CFD modelling. More than one decade of research and development projects and evaluation works with experimental data from different limestone scrubbers have lead to a powerful non-commercial code available for the computation of scrubber systems. This CFD modelling and the long-term experience of our engineering staff guarantees AEE’s technological advantage.
Novel approach to scrubber lining
The materials that have so far normally been used for scrubers of this size are steel line with either rubber or flake, stainless steel and standard steel with stainless steel plating. In the case of the scrubber for BOA 2 & 3, AEE has taken a new approach here too. The scrubber is of concrete and has a Bekaplast internal lining.
Bekaplast is a thermoplastic internal lining. Figure 3 shows how it is fixed to the concrete by mechanical means. Two individual plates are connected to one another via conical anchor studs by means of friction welding. The primary plate, the front plate in contact with media, has a thickness of 8 mm. The secondary plate, the rear one, has a thickness of 3 mm. Between the two plates is an inspection space with a clear width of about 4 mm, which is used for leak discharge.
Figure 3: How concrete and Bekaplast are fixed together mechanically
For the BOA project, the size of the plates for the cylindrical area is about 6.20 m by 1.45 m. The plates are fitted to the internal shell already installed. The connection between them is achieved with a specially made tear-off H-section strip. Attachment to the shell itself is easy to perform with assembly assistance. The Bekaplast plate fitted in this way is also cast in place during the concreting operation (Figure 4).
Figure 4: The shell of the BOA 2 scrubber in Neurath
Figure 4 (inset): the installation of the Bekaplast plates
The critical area of the scrubber inlet is made of stainless steel with alloy-59 to be able to control the temperatures occurring there, which can be up to 135 °C. Figure 5 shows a comparable inlet design for a different AEE scrubber made of concrete/ Bekaplast in Trbovlje, Slovenia. It is evident that the stainless steel inlet also protects the jacket of the scrubber in this inlet area by means of a stainless steel outer shroud that is 1 m wide. The spray levels in the BOA 2 & 3 scrubber are made of PP, and the supporting structure is made of stainless steel with alloy-59. Use of these materials is expected to reduce all maintenance activities to a minimum and thus ensure maximum levels of availability for base load operation at BOA 2 & 3 in Neurath.
Figure 5: Scrubber inlet made of concrete/Bekaplast at a plant in Trbovlje, Slovenia
Scrubbers close to full erection
Construction of the two scrubbers in Neurath is in full swing. Erection of the two concrete scrubbers will have been completed by spring 2008. In parallel, work on assembly of the steel structure and piping for the entire FGD plant, including all the auxiliary installations, is due to begin in autumn 2007. All the assembly work on the FGD plant of BOA 2 & 3 is to be completed by autumn 2008.
Commissioning of the plant will take place directly after completion of the assembly operations. The first flue gas is due to flow through the first scrubber in the summer of 2009. Handover of the two lines is scheduled for the end of 2009 (BOA 2) and spring 2010 (BOA 3).
Not only will the construction of BOA 2 & 3 see top-class engineering achievements in the field of power plant technology for the steam generator, but the FGD plant will have a new generation of scrubbers as far as size is concerned. The 4.85 million m³/h flow of flue gas being desulphurized in the two scrubbers will be a technological benchmark in terms of scrubber size for a long time to come. For scrubbers with such a large diameter, the use of CFD simulation is essential. In addition to the top-class construction achievements, AEE is also expecting the new material concept to minimize maintenance costs and thus bring about an improvement in plant availability.