How upgraded chimney design can enhance coal-fired power station efficiency and flexibility1 September 2015
These are challenging times for coal and lignite fired power plants, requiring them to explore all possibilities for increased efficiency, flexibility and optimisation. Rethinking the flue gas exit path, in particular the deployment of borosilicate linings, can prove beneficial, as three case studies, briefly summarised below, demonstrate.
Coal-fired power stations are facing escalating competitive pressure. In the EU, coal's share of the energy mix has dropped from 27% in 1990 to 17% in 2013, whereas renewables' share has tripled from around 4% to 12% over the same period.1 All the EU countries have committed to greater use of renewables by 2020. In the US, although coal's use as a power source is projected to increase over the next 25 years, the overall percentage in the energy mix will decline as the amount of renewable energy increases.2 To compound this situation, the cost of producing renewable energy is also forecast to drop.3
All this puts greater pressure on coal- fired power plant operators to drive greater efficiencies, flexibility and optimisation. One way to address these issues is to focus on rethinking the flue gas exit path, which impacts energy consumption as well as emissions.
Borosilicate chimney linings are becoming increasingly popular as a response to these challenges. Many power stations the world over are already seeing the benefits of this versatile lining system, known commercially as PennguardTM, deriving from its ability to facilitate warm start-ups, reduce and protect against condensate, and improve energy efficiency by working in tandem with innovative flue gas reheat systems, or helping to eliminate conventional reheaters altogether.
Case study 1: West Burton - reducing potential environmental impacts of frequent starts and stops
West Burton is a 4x500 MW power station in the eastern part of central England and is owned by EDF. As a merchant power plant in a liberalised market, the West Burton units are subject to a flexible operating regime. During winter, the units operate in baseload mode. At other times of the year, especially in summer, they often operate on a 'two-shifting' schedule, which calls for daily starts and stops.
During every start-up, the chimney is relatively cool. In the initial phases of operation, flue gas volumes are still small and while the internal flue surfaces are heating up, there is a strong tendency for acid condensate formation within the chimney. In addition, the cool chimney lowers the temperature of the exit plume. As a result, plume dispersion during start- up may be significantly worse than during steady state operation.
Both of these phenomena can affect the environmental performance of a power station: the liquid condensate can be ejected from a chimney as acidic droplets or particles; and a cool exit plume has a higher risk of grounding near the station, causing increased ground level concentrations of flue gas components.
To minimise these effects, EDF took the decision to line West Burton's two 200 m tall chimneys, each containing two 6 m diameter steel flues, with PennguardTM borosilicate linings. With these highly insulating linings on the insides of the chimney the flue internal surfaces heat up very quickly during start-up (see Figure 1). Due to this fast thermal response, condensate formation is severely limited and the exit plume reaches its full operational temperature within minutes.
Case study 2: Amercentrale 9 - eliminating flue gas reheat and conversion of chimney to a lined wet stack
Essent, a member of the German utility group RWE, operates Amercentrale 8 and 9 power stations in the south west of the Netherlands. Commissioned in 1994, Amercentrale 9 has a 640 MW supercritical coal-fired boiler and it produces a further 350 MW of district heating. In recent years it has co-fired biomass, and this now accounts for 35% of the total fuel input.
Amercentrale 9 has used a flue gas desulphurisation (FGD) system from the outset, and this included a steam reheater downstream of the FGD system, which increased the temperature of the flue gas going into the chimney from around 45°C to around 60°C.
After some 20 years of operation, the steam reheater of Amercentrale 9 had come to the end of its useful life. Essent then considered the options of replacing the reheater, or converting the chimney to a 'wet stack'. It was important to consider not only the initial investment needed for this, but also any impact on running costs.
An in-depth study by Essent and its parent company RWE Group found that there was little difference in the necessary capital investment between the two options. However, converting to 'wet stack' operation with a PennguardTM lining would deliver a significant advantage, being inherently more energy- and cost-efficient, as there is no reheater to operate. In addition, lining the brick flue with PennguardTM (see Figure 2) was achievable within an already scheduled 31-day outage. The project was executed under the supervision of RWE Technology International GmbH - Project Development and Execution.
Case study 3: Rovinari - FGD with wet booster fans and lined concrete chimneys for maximum efficiency
The Rovinari lignite-fired power station near Targu Jiu, Romania, has four 330 MW units constructed between 1976 and 1979, which are considered to be the most efficient low-cost electricity generators in the country.
The locally mined lignite used by Rovinari has a sulphur content of between 0.5% and 1.35%. Romania's membership of the European Union required it to reduce its sulphur emissions to EU standards.
The owner of Rovinari opted to construct wet limestone FGD plants - a significant design decision being to place the required booster fans downstream of the FGD absorbers. Here, in the low temperature, dense flue gas, the fan energy consumption is dramatically reduced. As a conservative estimate, the energy use of each booster fan, at nominal load and 2 000 000 Nm3/h gas flow rate, is reduced from 11.6 MW to 9.5 MW.
The supplier of the wet booster fans developed a special design to prepare them for operation in low temperature, acid- condensing flue gas. This involved the use of special corrosion-resistant materials as well as a water spray system that would prevent any build-up of gypsum on the fan blades.
The flue gas from the Rovinari FGD scrubbers is at 61°C and water saturated. It also contains up to 50 mg/Nm3 of fine droplets, even after passing through the mist eliminators.
To evaluate the thermal effects of the booster fan, the evaporation of the 50 mg/ Nm3 liquid load had to be considered. In addition, the wet booster fans have a two- minute wash cycle running 12 times per hour, during which water is injected at a rate of 400 litres per hour. The evaporation and flow of this washing water downstream of the fan also had to be considered.
Based on a set of thermodynamic calculations, Alden Research Laboratory found that, at nominal load, the temperature rise in the wet booster fans should be sufficient to: (a) evaporate the 50 mg/Nm3 of droplets in the gas flow; (b) evaporate the washing water from the wash cycle; while at the same time (c) still raising the gas flow temperature 1.95° above its water dewpoint.
In order to keep the flue gas temperature in the chimneys as high as possible, it was decided to use PenngardTM lined chimneys for this project.
The PennguardTM Block, a closed cell borosilicate glass block, is the main component of the PennguardTM lining system. It has a thermal conductivity of 0.087 W/m°K at a mean temperature of 38°C. As a result PennguardTM lined chimneys are well insulated and the temperature losses of a flue gas stream flowing from the bottom to the top of these chimneys will be low.
The Alden Research Laboratory study for the Rovinari project showed that the combination of wet booster fans and PennguardTM lined chimneys results in minimised condensate formation within the FGD wet stacks.
The chimney concept used at Rovinari power station (see Figure 3) is what Hadek calls the "New Chimney Design", a PennguardTM lined concrete chimney that offers lower construction costs than more traditional chimneys that contain internal flues.
The basic construction is a concrete shell protected by a PennguardTM lining applied directly to its inside concrete surface. This avoids the need for a separate internal flue, allowing a slender structure.
This design concept is estimated to offer an overall construction cost saving of around 20% compared to any other, similarly sized chimney design containing a separate internal flue.
In fact, actual project studies have established that two chimneys built in accordance with the New Chimney Design would cost less than a single, twin flue chimney designed for the same gas flows.
A plant inspection was performed at Rovinari in April 2012 and WFGD system operating data were obtained. The Rovinari plant experienced one of the coldest winters on record in 2011- 2012 with temperatures as low as -22°C, however no stack liquid discharge was observed. This leads to the conclusion that combining the temperature rise across a wet booster with the insulating properties of a PennguardTM lined stack is an effective means of minimising and/or eliminating the potential for stack liquid discharge. From a thermal and liquid collection standpoint, the system is operating very well and effectively results in what might be called a 'dry wet stack'.
PennguardTM is a registered trademark of Ergon Asphalt & Emulsions, Inc
Author: Albert de Kreij, Hadek Protective Systems, The Netherlands
(Originally published in MPS September 2015)