Profile of the Soma 2 x 255 MWe circulating fluidised bed power plant.
Driven by their very low cost relative to premium coals, low rank coals and lignites are increasingly being used for power production in a number of countries, eg, India, China, Indonesia, Australia, South Africa, Mozambique and Turkey, the focus of the present article.
Since fuel cost makes up about 85%- 90% of the total operating cost of a large power plant, the economic benefits of using low quality fuels are significant. Until recently, low quality coals were mainly used domestically and were not part of the international coal market. This is because the lower energy content means their economic benefits are quickly eroded by higher transport costs.
But today, we are seeing more low quality coals coming into the global coal market, driven by steep price discounts against a tight market for premium coals.
This is good news for power generators using CFB (circulating fluidised bed) technology. Due to the CFB’s fuel flexibility, plant owners can use the full range of discount coals, buying fuels for maximum economic benefit while avoiding the high priced premium coals.
Lignite and brown coal are also being recognised as valuable and secure domestic energy resources in a number of countries with large indigenous reserves (eg, Poland and Germany).
Circulating fluidised bed has emerged as a leading combustion technology for utility-scale solid-fuel-fired power plants, especially when multi-fuel capabilities are required (for example co-firing of biomass and waste) and/or fuel is of low quality, eg, lignites and brown coals.
There has been a long history of low reliability when trying to use conventional PC technology to fire these low grade fuels, but with modern CFBs, the technology to cleanly and reliably convert such fuels to electricity is readily available.
Among the many advantages of CFBs over conventional PC boilers when it comes to firing lignites are the following: no ash slagging, minimising boiler fouling and corrosion; avoidance of the high costs and maintenance issues associated with milling lignites; downsizing or even elimination of back-end FGD and deNOx equipment; and more compact boiler with reduced overall plant footprint.
The Amec Foster Wheeler CFB fleet now consists of over 450 units, including both subcritical and once-through supercritical boilers. The largest operating Amec Foster Wheeler CFB boiler is Tauron’s 460 MWe Lagisza power plant in Poland (Figure 1) – the world’s first supercritical CFB plant – which has already celebrated five years of successful commercial operation, while a 4 x 550 MW CFB plant is under construction in Korea (Figure 2). CFB technology is currently available up to a size range of 600-800 MWe for operation on a wide range of solid fuels.
Lignite to fuel Turkey’s growth
The most recent utility-size Amec Foster Wheeler CFB boiler project with lignite as the main fuel is the 2 x 255 MWe Soma plant in Turkey.
Currently, Turkey is amongst the world’s fastest growing economies, with a correspondingly rapid growth in energy demand. Having over 10 Gt of proven domestic reserves, lignite is considered a feasible alternative to imported coal for large-scale electricity production.
In the case of Soma, Amec Foster Wheeler was awarded a contract by Harbin Electric International Co Ltd (HEI) for the design and supply of two CFB boiler islands and flue gas scrubbers for HIDRO-GEN Energy Import, Export, Distribution and Trading Inc (a subsidiary of the Kolin Group of Companies). HEI is acting as EPC contractor for HIDRO- GEN’s Soma power plant, to be built close to lignite mines in the west of Turkey, 135 km north of Izmir (see Figure 3).
Amec Foster Wheeler’s scope includes design and supply of the two 255 MWe (gross) boilers and auxiliary equipment for the boiler islands, flue gas cleaning systems with CFB-scrubbers, and technical advisory services during erection and commissioning.
The Soma CFB boilers (the largest awarded to date in Turkey) are designed to utilise local lignite reserves located near the town of Soma. The design fuel data are: S, 0.95%; N, 0.53%; moisture, 23.3%; ash, 42.9%; LHV, 6.77 MJ/kg.
The Soma project envisages using fuel of unprecedently low quality at the largest scale yet for a lignite fuelled CFB plant and can be considered to open a window of opportunity for other countries seeking to unlock the value of their own secure domestic low grade energy resources.
The Soma CFB units (Figure 4), each rated at 545 MWt, are of the natural circulation drum boiler type with reheat. The steam conditions are as follows: SH flow, 198.5 kg/s; SH pressure, 173 bar; SH temperature, 565°C; RH flow, 173 kg/s; RH pressure, 53 bar; RH temperature, 565°C; feedwater temperature, 262°C. Design emissions levels (adjusted to dry flue gas @ 6%O2) are as follows (at the stack): NOx, SO2 and CO, <200 mg/Nm3; particulates, <30 mg/Nm3.
The CFB boiler design incorporates solids separators built from steam cooled panels integrated with the combustion chamber. The final superheating stages are INTREX heat exchangers located in special enclosures at the bottom of the furnace (adjacent to the main combustion chamber).
Local lignite from the Soma area is supplied to the boiler silos by the plant owner’s conveying system.
Each boiler has five daily silos located in the front wall. Gravimetric type conveyors feed fuel to the front wall and rear wall of the furnace.
The combustion of high ash Turkish lignite, containing 35-50 % ash, results in relatively high bottom ash flow. The bottom ash removal system is designed accordingly, with each Soma CFB boiler equipped with six drum coolers. The drum coolers drop bottom ash into redundant drag chain conveyors. It is then taken by elevators to the intermediate bottom ash silo (see Figure 5). The capacity of the drum coolers is such that the maximum bottom ash flow rate can be sustained with one out of six coolers out of service.
Low combustion temperature and staged combustion in the CFB result in inherently low NOx emissions without the need for downstream treatment, while the bulk of sulphur removal is done in-furnace, mainly by limestone injection. There are twelve limestone feeding points located in the lower furnace. These are served by pneumatic conveying lines and designed to ensure that limestone is evenly distributed to the furnace front and rear walls.
Flue gases leaving the furnace are then passed through a scrubber, which also employs a circulating fluidised bed (ie, there are two circulating fluidised beds in each boiler unit, one for combustion and one for flue gas scrubbing).
In the CFB-scrubber (CFBS) SO2 emissions are reduced to the permit level by injection of hydrated lime (Ca(OH)2). Some of the unreacted CaO in the boiler fly ash is also utilised for SO2 capture in the CFBS, which helps reduce the hydrated lime consumption (see below).
As well as SO2, the CFBS also removes a wide array of other pollutants from the flue gases and is a viable way of achieving cost- effective multi-pollutant control.
As shown in Figure 6, boiler flue gas enters at the bottom of the CFBS up-flow absorber vessel. The gas mixes with the hydrated lime and water injected into the absorber, as well as recirculated solids from the downstream fabric filter.
Multiple flue gas venturis, as shown in Figure 7, provide the required fluidising gas dispersion and adequate suspension of the solids across the full diameter of the CFBS absorber vessel. The multi-venturi design allows a wide capacity range while minimising scale-up risk. Water injection nozzles provide an atomised spray cloud of water droplets enhancing heat and mass transfer rates over the large surface area of solids churning within the confines of the vessel walls. Residence time for gases entering the tall and narrow reflux CFB absorber can be as high as five seconds providing excellent capture efficiency for a range of flue gas pollutants, while maintaining a small absorber footprint. CFB absorber maintenance is minimal as the vessel is self-cleaning. Water spray nozzles can be replaced, if necessary, while the unit is on-line. The absorber is fabricated from carbon steel, avoiding expensive liners or alloy metals.
The largest CFB-scrubber in operation to date is that at Basin Electric’s 440 MWe Dry Fork pulverised coal station (Figure 8), which, due to its 4430 ft site elevation, has a flue gas volumetric flow rate equivalent to a 520 MWe plant at sea level. In addition there are six CFB boilers in the USA employing CFB- scrubbers for flue gas cleaning, while globally
Figure 8. The largest CFB-scrubber in operation to date is that at Basin Electric’s 440 MWe Dry Fork PC station in the USA
Amec Foster Wheeler has supplied over 70 CFB-scrubbers for various applications.
An advantage of CFBS over spray drier technology (SDA) is that the amount of sorbent injected and the absorber temperature can be controlled independently, the latter by water spraying. In an SDA system, sorbent is injected in slurry form, which is constrained by absorber temperature control requirements (water content) and maximum solids (sorbent content). In contrast, the CFBS allows much higher levels of sorbent to be used, while the absorber exit temperature can be controlled independently by water sprays.
There are two identical flue gas cleaning lines at Soma, each consisting of a CFB- scrubber plus baghouse with solids recirculation.
A multi-compartment baghouse is located downstream of the absorber vessel for high efficiency capture and recirculation of the solid particles. Most of the ash and FGD products separated by the fabric filter bags are recirculated back to the absorber by means of non-mechanical air slide conveyors. Bed inventory in the absorber is maintained at set point by controlling the ash flow rate of air slides. The excess ash from the baghouse hoppers is discharged to the ash storage silo.
The hydrated lime used in the CFB scrubbing process can be purchased directly from suppliers. However, for high sulphur fuel applications requiring larger quantities of reagent, or in locations where hydrated lime suppliers are not available, owners can purchase less costly quicklime (CaO) and hydrate it on site, with a dry lime hydration system located near the CFB absorber vessel. This is the approach taken at the Soma power plant.
As shown in Figure 9, lime and low pressure steam are injected into the hydration reactor, where the calcium oxide is converted to calcium hydroxide. The hydrated lime produced in the hydrator is separated from the hydrator exhaust in a downstream cyclone and then collected in a hopper. From the hopper, the hydrated lime can be sent directly to the CFB- scrubber absorber or to a hydrated lime storage silo.
The dry lime hydration system does not require a dedicated fabric filter to handle the cyclone overflow as this stream is sent directly to the CFB-scrubber.
As already noted, in a CFB boiler, limestone injection is used for in-furnace capture of SO2 from flue gas during combustion. Overdosing of limestone is required, which leaves part of the CaO unreacted in ash streams. During recent years, CFB-scrubber technology has been developed with the aim of making use of this unreacted CaO, helping to reduce the sulphur sorbent costs. Due to the reduced sorbent usage the quantity of FGD products is reduced and, consequently, ash disposal costs are lower.
Part of the CaO can be utilised in the scrubber, where it is hydrated by the water sprays in the absorber. Fly ash can also be conditioned separately in an external hydrator and the activated ash injected into the scrubber. This requires either pre-separation of fly ash upstream of the scrubber for hydration or recirculation of fly ash from the particulate filter via the hydrator.
The CFB-scrubbers at the Soma power plant are equipped with two lime hydrators for conditioning the fly ash, which is partially separated from CFB boiler flue gas in an electrostatic precipitator (ESP) located upstream of the scrubber.
Hydrated lime from offsite suppliers is only required during filter bag conditioning and start up.
From the hydrator the ash is fed directly to the CFB-scrubber. By utilising the fly ash in the integrated hydrator/CFB-scrubber process, the required SO2 control is fully achieved by injecting limestone (which is relatively cheap) into the furnace. Limestone consumption is minimised by optimising the SO2 capture between the CFB furnace and FGD system.
The flue gas cleaning system of the Soma plant is summarised in Figures 10 and 11.
Construction costs are minimised as the major FGD system components can be pre- assembled on the ground and lifted into place during system erection, while at the same time high pollutant removal efficiencies are achieved, up to 99% for SO2, SO3, HCl and HF. Further, the absorber/fabric filter arrangement is highly adaptable to sorbent injection for removal of heavy metals including mercury.
The contract for the Soma plant boilers was awarded to Amec Foster Wheeler by power plant EPC contractor Harbin Electric International, according to the project schedule, in January 2014. Start of erection is scheduled for January 2016 and commercial operation in January 2018. This includes eight months delay due to customer permitting issues, resulting in a change of site location. The boiler design has been adjusted to the new site conditions.
Authors: Kalle Nuortimo, Aki Pyykkönen & Reijo Kuivalainen, Amec Foster Wheeler Energia Oy, Varkaus, Finland
(Originally published in MPS September 2015)