Miki Adderley, Alstom Power, Sweden
The Pego power station, one of only two coal power stations in Portugal is located approximately 150 km from Lisbon. The plant is owned by Tejo Energia, an independent power producer (IPP) owned by European energy companies International Power (50 per cent), Endesa (39 per cent) and Energias de Portugal (EDP) (11 per cent). it was built between 1988 and 1995 to increase domestic electricity generating capacity to meet the higher consumption in the 1990s and to satisfy the need to diversify energy sources.
The retrofit of a new flue gas cleaning system means the Pego power plant can meet both current and future emmissio targets
Pego power plant was originally equipped with only electrostatic precipitators (ESPs) to collect flue gas particulate from between the boiler and the stack. As the European Union (EU) introduced the Large Combustion Plant Directive (LCPD) however, it became necessary for the station to meet the new emission limits that were being directly implemented by the Portuguese Environmental Agency.
The EU LCPD was subsequently transposed to national legislation and required the new emission limits to be achieved by 2008. In 2006, Alstom was awarded the contract to supply emission reduction systems to Pego. The scope included supply of new equipment designed to reduce emissions of dust, nitrogen oxides (NOx) and sulphur dioxide (SO2) to well below the emission levels required by the LCPD.
The incoming emission limits determined by the LCPD for Pego power station were (at dry, six per cent O2) by January 2008: 400 mg/Nm3 of SO2, 500 mg/Nm3 of NOx and 50 mg/Nm3 of particulates, while by January 2016 the limits are 400 mg/Nm3 of SO2, 200 mg/Nm3 of NOx and 50 mg/Nm3 of particulates.
The Portuguese environmental authorities decided to impose the following emissions limit values for the power station thereby ensuring that the design of the new flue gas cleaning system had to have the capability to operate not only below the incoming LCPD emission limit values, but also those to be introduced in 2016. This meant SO2 emission specifications of 200 mg/Nm3 from the existing 2000 mg/Nm3, NOx emissions of 200 mg/Nm3 from the 800 mg/Nm3 and particulate emissions of 50 mg/Nm3 from the 100 mg/Nm3.
Other technical requirements specified by Pego also included the need to maintain current coal specification, current ash characteristics and current plant autonomy and existing dynamic parameters, reduce or eliminate waste disposal and minimize thermal efficiency losses.
To achieve the specified emission limit values and meet the technical requirements, the Alstom solution included: a high dust selective catalytic reduction (SCR) system, primarily to reduce NOx, a limestone gypsum wet flue gas desulphurization (WFGD) scrubber and plant, primarily to reduce SO2 and a new ESP energization upgrades to the existing ESP to increase particulate collection efficiency but at a lower power consumption.
Implementation of SCR
The abatement of NOx production using primary measures (such as low NOx burners, over-fire air, etc) would not have been sufficient to reduce the emissions at the Pego plant to the required emission levels of 200 mg/Nm3 by 2016. Therefore SCR was selected, which is an established environmental control system for secondary measure high removal efficiency (over 75 per cent) of NOx from flue gas emissions.
Schematic of Pego’s flue gas cleaning system prior to retrofit
The major scope of the SCR supply included one SCR reactor per boiler train (two reactors in total) including an ammonia (NH3) storage and injection system for the distribution of NH3 at the inlet to the reactor, an acoustic cleaning system for removal of dust build-up in the catalysts, an ash extraction/conveying system for removal of the fallout dust at the catalyst outlet, supporting steel structures and access to the SCR reactor positioned adjacent to the boiler economizers and concrete foundations for the reactor support structure.
The SCR reactor consists of three catalyst layers in total, however initially only two catalyst layers were installed, which are capable of achieving the specified NOx emission limit. The third layer of catalyst will be installed at a later date when the activation levels of the first two catalysts have decreased over time.
There are many challenges involved in the construction of SCR reactors. Because the installation of an SCR unit is, in almost every case, a retrofit, it means that there is inevitably congestion in the limited space available to position the unit. In this sense the Pego retrofit was no different. The Pego SCR reactor had to be situated 35 metres above the ground in order to position the connecting ductwork from the economizers to the SCR in the most efficient arrangement.
The optimum solution for further particulate emission reduction was to modify the current ESPs through the installation of high frequency power supplies (SIRs) developed by Alstom. After the ESP upgrade, particulate emissions obtained were well below the guaranteed limit. As an additional advantage SIRs have higher power conversion efficiency than conventional power supplies, leading to lower power losses.
Wet Flue Gas Desulphurization
After passing through the main ESPs, the flue gases enter a wet WFGD scrubber, also known as an absorber, in which the SO2 is removed through a chemical reaction with the limestone. The process results in at least 90 per cent of the total SO2 being removed from the gas, and produces gypsum as a by-product.
The major scope of the wet limestone/gypsum WFGD system supply included a three-level absorber per boiler train (two absorbers in total), including three recirculation pumps per absorber (one pump per level), a gypsum processing plant and gypsum silo (including transport equipment), a limestone delivery and preparation plant, a wastewater treatment system and both civil and structural works.
Under normal operation, the absorber only needs to run two of the absorber’s spray levels to achieve the SO2 output emission limits. The third level allows for redundancy or the option of additional SO2 reduction. The uppermost spray level provides the highest SO2 removal efficiency, however it is the lower two levels that are normally taken into regular operation because running the two lower level recirculation pumps offers lower total energy consumption.
FLow Model Studies
Preliminary studies were conducted in the form of both computational fluid dynamics (CFD) modelling and physical scale model testing for the SCR. Only CFD studies were done for the WFGD scrubber. These studies and testing formed the basis of the final sizing and design of the SCR and WFGD components.
Schematic of Pego’s flue gas cleaning system following retrofit
CFD modeling of the Pego SCR system was used for duct and ammonia injection grid (AIG) optimization, flow visualization, reducing the pressure drop in ducting and improving flow and NH3 distribution upstream of the AIG and SCR reactor. The design was then later verified in the physical model.
The CFD model for the Pego SCR extended from the economizer through the first catalyst layer. All the ducts, mixers, dampers, AIG and guide vanes were modeled as in the plant, while the economizer, catalyst inlet flow rectifier and catalyst layers were modeled as porous media.
The geometry of the CFD model was partially built in Catia V5 and modified in CFX-build and Ansys Design Modeler. Then, CFX10 code was used to solve the equation system of continuity, momentum, the shear stress transport model, the energy equation, together with the transport equations to predict the velocity field, turbulence level, temperature, NH3 and NOx concentration in the system.
The optimized flow distribution through the catalyst combined with the correct mixing of the NH3 and the flue gas ensures high-efficiency removal and allows for a small amount of NH3 slippage within the guaranteed maximum slippage level of <2 ppmv.
Additional to the CFD modeling the physical model was also used to minimize the NH3 slip and ash build-up in duct and the SCR reactor. The model was built in a 1:10 scale from steel metal sheet, with sufficient external stiffening to resist the suction. It also had plexiglas windows for inspection of the internals and visualization of the flow.
The complete system from the economizer outlet to the air preheater inlet connection was modeled, including the economizer bypass duct and connected mixing system, joining with the economizer outlet duct. This model enabled Alstom to optimize the bypass connection for the optimal functioning of the SCR during boiler load variation and increasing NOx and temperature mixing in the system. A CFD modelling of the WFGD absorber was used to define gas and liquid distribution inside the absorber to ensure optimized SO2 removal with the lowest possible power consumption.
Despite being a proven technology, SCR is always a challenging implementation because they are a retrofit to an existing boiler installation, which means that every SCR project is unique from a structural and layout point of view. In preparation for the design of the Pego SCR installation, a laser scan study was completed to ensure the validity of the existing building as built drawings and model the design of the SCR unit to be installed.
By combining the laser scan modelling with the computer model of the SCR reactor unit, an investigation was made to identify and avoid any potential spacing clashes. This saved potential problems on-site during preassembly and ensured that the outage time could be kept to a minimum.
Fan replacement most cost & operationally efficient
Retrofitting a power plant with SCR and WFGD changes the process parameters. One of these parameters is the pressure drop. This is an important parameter as it imposes a significant change on fan loading. To accommodate the additional pressure drop that the SCR and WFGD introduce to the system, there were three main options available: add a new booster fan to the existing ID fan capacity (per boiler line), retrofit the ID-fan or change the ID-fan
In the case of Pego, it was decided that changing the ID fans from the original centrifugal fans to new axial ones was the preferred solution, based on cost and operational efficiency The disadvantages of replacing the ID fans were that the civil work and foundations had to be replaced and remade. The motor power centre, cabling and ducts also had to be changed to accommodate the new fans.
Even after discovering that the original sizing of the foundations did not match the existing fan design, the operation was completed successfully. The total replacement was executed within 50 days, from the dismantling of the existing fans to the operation of the new one (for both ID fans per unit). From an operational point of view; the implementation of the new ID fans was a complete success with the new fan designed for the new conditions, demonstrating an increased efficiency and reduced power consumption.
Acid attack in Stack
Implementing the wet scrubber meant a change to the thermo-chemical conditions in the stack. According to the new input data, it was calculated that the ‘zero pressure point’ in the stack would move down approximately 100 metres from the top of the stack, causing concern over a higher risk of corrosion from the increased condensation on the internal stack surface.
Under these new conditions, if no remedy device were implemented, it would mean that protective cladding would be required at least for the first 100 metres from the top of the stack. This solution was not feasible with the expected available outage period of only 60 days.
Therefore to improve the natural draft and consequently shift the zero pressure point to the highest possible level, the solution proposed was to cut the top 3.5 metres of the protruding steel liner of the stack and substitute it with the same height of a diverging element. Its internal diameter varied from 5.1 metres to 5.25 metres, and was joined to a flange fixed to the steel liner. The net result was a reduction in the internal surface area that requires acid protection.
The flue gas cleaning system was designed with 100 per cent redundancy. This means that there are either two parallel trains for all major components of the retrofitted system or that there is a bypass in place that allows continuous operation without the need to take the system offline.
The following values were measured during the performance tests: SO2 at stack <185 mg/Nm3; NOx at stack <150 mg/Nm3; particulate at stack <15 mg/Nm3; ammonia slip at SCR reactor outlet <1.1 mg/Nm3. Gypsum quality was as follows: 97 per cent purity as calcium sulphate; moisture < nine per cent and chlorides <100 ppm.
The commissioning of the new flue gas cleaning system at Pego power plant was completed in December 2008. The new installation is consistently performing to the new emission limit standards and has demonstrated its capacity to achieve even lower outlet emission than the targets emissions.
The new flue gas cleaning system delivered by Alstom meets the Pego technical requirements and operating requirements, including minimizing thermal efficiency losses and increasing the efficiency of power consumption. Pego power station is now able to fulfil the stringent new local regulation limits at present and in 2016. Currently both SO2 and NOx limits are set at 200mg/Nm3 (at dry, six per cent O2), while at the same time producing saleable commercial grade gypsum as a by-product.