A new chapter in flue gas cleaning technology has opened in South Korea with the successful performance test of a flue gas desulphurization plant based on circulating dry scrubber technology, write Sung Yurl Kim, Yonghee Kang, Claudia Bautsch and Christian Moser
Gunjang Energy’s Gunsan city plant
Two years ago, South Korean electricity company Gunjang Energy decided to install a new unit, called Seagull, at its existing power plant at Gunsan city.
The circulating fluidized bed (CFB) boiler size was 275 MW and it needed to fulfil strict emission requirements which came into force in January 2015. Therefore, a suitable flue gas desulphurization (FGD) technology to meet this limit became mandatory. Gunjang Energy wanted to install the most economical FGD technology for these demands, and therefore selected circulating dry scrubber (CDS) technology. The contract was awarded to Hamon Korea in June 2014 and commercial operation started on 18 May 2016.
The basic principle underlying a CDS is the removal of gaseous components and a downstream filter for dust removal. The flue gas from the upstream boiler flows through the CDS and then a filter, and is released into the atmosphere via induced draft fans and stack.
The main component for the removal of waste gases is the CDS with the high solid concentration situated inside it. These solids contain the dust brought in by the raw gas, a metered quantity of hydrated lime as the absorbent, and over 90 per cent of FGD product, recirculated from the downstream arranged filter. Figure 1 shows the arrangement of the whole CDS system.
The FGD is located downstream of the air preheater in the power plant arrangement. The CDS system itself consists of a CDS absorber, a low pressure fabric filter with eight chambers for dedusting downstream of the CDS absorber, the connecting ductwork including the recirculation duct, the ID fans downstream of the fabric filter and the connection to the stack. All equipment has been installed in a compact and space-saving way.
The Seagull plant with absorber, filter, solid handling system, ID fans and ductwork
The CDS system is designed for boiler operation from 35 to 100 per cent. This wide variation of load cases is a challenge for every kind of FGD technology. The CDS system provides stable operation conditions for all load cases due to the opportunity of gas recirculation.
The most important thing for proper operation of the CDS plant is a uniform flue gas distribution to the CDS absorber. Downstream of the venturi section, the recirculated solids are inserted in the gas stream. Both solids and water, which is injected upstream of the venturi section, influence the flue gas flow through the whole CDS absorber.
The interaction of three phases (gas, solids, water) and the thermodynamic behaviour can only be simulated using specific software with in-house developed add-ons. During the design and engineering phase of the CDS system, three main parts of the CDS technology were investigated in detail: gas flow and absorber design; solid handling system; and water humidification system.
|Figure 1. Arrangement of Seagull CDS plant|
Development and final check of the design for the six venturi CDS system were done with help of high sophisticated computational fluid dynamics (CFD), performed on a high performance computing (HPC) cluster with 260 cores. Due to this high calculating capacity, it is possible to describe the CDS process in a very detailed way in one single model. This is mandatory to determine all relevant physical effects and their interdependencies.
The description and modelling of a three-phase simulation consisting of two continuous phases (gas and solids) and one particle phase (water droplets) needs a complex simulation approach with two Eulerian and one Lagrangian phase.
The specifications of all material properties are temperature- dependent and the system is fully coupled between all phases. In addition, it allows the engineer to evaluate the performance (temperature profile, velocity distribution, solid distribution, water evaporation, etc) at any point in the process and how the performance changes with alterations such as turning vanes, water lance insertion depth, number of solid injection points, number of venturi, etc).With this approach, the ducting, all installed guiding vanes and other process-relevant internals of the absorber are optimized and finally defined via CFD simulation considering all relevant load cases.
To achieve a homogeneous gas velocity distribution downstream of the venturi section, the miter bow is equipped with two optimized ladder-grid guide vanes. These guiding vanes are a Hamon design that is validated in several CFD simulations as well as in physical flow model tests.
The wide range of flows/loads also influences the solid handling system. The solid handling system covers the total transport of solids in the CDS plant. It includes the silos for the absorbent and for the dry product as well as the through hopper underneath the fabric filter which stores the solids to be recirculated.
The air slides, which connect the through hopper and the CDS absorber, also belong to the solid handling system.
Additionally, the range of volume flow influences the absorbent feeding system. Due to the range in loads the absorbent feeding system has to operate in a range of 40 kg/h up to 2000 kg/h.
A further design challenge is the wide range in water volume flow for humidification nozzles. Here, the right nozzle type, nozzle position and number of nozzles have to be chosen.
The water humidification system is used to inject water into the CDS absorber. It is located in the round section of the CDS. The water itself is necessary for different reasons. It is used to cool the flue gas flow down to 10-15 K above water dew point. This increases the absorption and leads to less lime consumption because the stoichiometric ratio is decreased.
|Figure 2. Gas flow through ducts, absorber and filter|
The most important reason for injecting water is to start and enhance the absorption of the sulphur dioxide (SO2) from the flue gas.
The commissioning started with cold commissioning from November to December 2015 and was followed by hot commissioning from January to April 2016. The CDS desulphurization system has successfully been in operation since May. During commissioning and performance testing of the new unit it became clear that the CDS system is able to operate safely at higher than full load with lower absorbent consumption and higher SO2 removal than guarantee values.
The performance test was successfully passed on 1 June, with a flue gas flow of 1,530,000 Am³/h. The SO2 absorption efficiency was almost 99 per cent and the dust removal efficiency was over 99 per cent.
Sung Yurl Kim is Executive Director of Air Quality Systems and Yonghee Kang is Senior Process Manager of Air Quality Systems at Hamon Korea. Claudia Bautsch is Specialist for CFD and Plant Optimization and Christian Moser is Managing Director at Hamon Enviroserv, Germany. www.hamon.com