Why CFB scrubbing has stepped out in front14 October 2013
Power plant emissions limits are becoming ever more stringent and regulators are showing increasing interest in expanding the scope of regulation, eg to metals. This regulatory context, combined with concerns about water scarcity, is driving increased interest in dry FGD options, in particular circulating fluidised bed based technologies, which are now proven at large scale - over 520 MWe at Basin Electric's Dry Fork plant - and at a wide range of sulphur levels. CFB is now increasingly favoured for retrofit projects in the USA, for example.
By Robert Giglio
Emission limits for "conventional" pollutants, notably SOx, NOx and particulate matter, continue to tighten for power plants and industrial facilities around the world. For new pulverised-coal-fired power plants above 500 MWt, Europe's new IED (Industrial Emissions Directive) has for example reduced both SOx and NOx emission limits by 50 mg/Nm3 and particulate limits by 10 mg/Nm3 compared to the prior LCPD (Large Combustion Plant Directive) - the new limits being 150 mg/Nm3 for SOx and NOx and 10 mg/Nm3 for particulates. Last year China lowered its SOx, NOx and particulates limits for power plants to levels even less than those in Europe's IED.
In the USA the CSAPR (Cross State Air Pollution Rule), while vacated in August 2012, is likely to be re-proposed and eventually adopted mandating further SO2 reductions on owners of coal fired power plants who will continue to evaluate the merits of adding back-end air quality control systems (AQCS) versus shutting down units that would otherwise be out of compliance. This has triggered a country wide debate on how low states should go with their SOx, NOx, and particulate emission limits, causing concerns about power reliability due to coal plant retirements.
And now there is a new cloud on the horizon with regulation expanding to substances never before regulated. The US is leading in this field with metals, acid gases and organic compounds being added to the list of pollutants that must be controlled under the new MATS (Mercury and Air Toxics Standards) rules. Europe is not too far behind with its IED requiring a BAT (best available technology) standard for these additional substances as well. These potential pollutants have always been regulated in certain applications, eg, waste-to-energy plants and incinerators, but now regulators are increasingly considering them for all boilers including large utility coal units.
The trend is clear: environmental concerns are continuing to drive down limits for an expanding range of pollutants.
Benefits of CFB scrubbing
In the past, due to its good track record at large scale and proven ability to capture sulphur dioxide over a wide range of fuel sulphur levels, wet flue gas desulphurisation (WFGD) scrubbing technology has been the most popular choice for removing sulphur from boiler flue gases in large power plants and industrial facilities.
WFGD technology has low operating costs as it uses limestone, which is cheap, as the reagent and can produce gypsum for sale to wallboard manufacturers. However, on the downside, WFGDs are expensive to build, use the most water, occupy the largest amount of real estate and can keep a full crew busy maintaining their large numbers of pumps, pipes, valves and vessels. But more important, due to the chemistry and process, WFGDs are only marginal for capturing metals including mercury and acid gases such as SO3, HCl, or HF.
Now, with US regulations requiring capture of mercury, acid gases, dioxins and furans, in addition to SO2 and particulates, dry FGD technologies are becoming more popular due to their ability to capture this expanded set of pollutants. Dry FGD technologies include simple injection of a sorbent into the boiler flue gas (direct sorbent injection or DSI), the more established spray dryer absorber (SDA) system, which sprays a fine dry mist of lime into the flue gas, and the relatively new concept of employing circulating fluidised bed (CFB) technology, with boiler ash and lime circulated through an absorber reactor and typically a fabric filter.
The different dry FGD technologies have their pros and cons, but for many mid to large scale power facilities, CFB scrubbers are growing in popularity. This is reflected in the pipeline of US retrofit scrubber projects where more and more projects are opting for CFB technology over other options.
Dry scrubbing technology has much lower capital cost and uses less water than wet FGD technology but in the past has generally only been selected for projects where the boiler size was not too large and the fuel sulphur level was not too high. Today, however, CFB scrubber technology has broken through these limitations, with single unit designs up to 700 MWe backed by operating references on coal power plants of over 500 MWe and on fuels with sulphur levels above 4%. CFB scrubber technology has now stepped out in front of other technologies due to the combination of five key advantages:
- high multi-pollutant capture capability;
- low installed cost;
- low water use;
- capability for deployment at large scale; and
- flexibility to handle wide range of fuel sulphur levels.
As the comparison table below shows, CFB scrubbers also offer other benefits including low maintenance cost, compact footprint, and the flexibility to use low quality lime and water. A particular process advantage of a CFB scrubber is that, unlike dry SDA technology, the amount of lime injection is not limited by the flue gas temperature, allowing significantly improved acid gas scrubbing performance.
Cost effective and reliable
The CFB scrubbing technology offered by Foster Wheeler, through its acquisition of Graf-Wulff (completed last year), provides a way of achieving multi-pollutant control in a cost effective manner. The technology employs advanced CFB scrubbing, which efficiently and economically captures a wide array of pollutants, such as SOx, PM, acid gases, and organic compounds while using minimal water.
As shown in Figure 1, boiler flue gas enters at the bottom of the CFB scrubber's up-flow absorber vessel. The gas mixes with hydrated lime and water injected into the absorber, as well as recirculated solids from the downstream fabric filter. The turbulator wall surface of the absorber causes high turbulent mixing of the flue gas, solids and water to achieve a high capture efficiency of the vapour phase acid gases and metals contained within the flue gas.
The CFB scrubbing technology incorporates a number of built-in features to maximise reliability. The absorber vessel is a self-cleaning upflow reactor with a cloud of water droplets spreading over a large surface area of solids churning in a 75 ft high section within the confines of the vessel walls.
Water injection nozzles, located on the perimeter of the absorber above the introduction points for the recirculated and sorbent solids, provide an atomised spray cloud of water droplets. These nozzles must be removed periodically for replacement of components subject to wear. However, the entire perimeter of the CFB absorber vessel is used to locate the water nozzles thus additional nozzle locations are typically available to allow installation of a spare nozzle prior to removing an operating nozzle for inspection or maintenance.
One or more multi-compartment fabric filter baghouses are located above the absorber vessel to allow recirculation of particulate solids. The multi-compartment baghouse lends itself to on-line replacement of filter bags with one compartment off-line. Separate compartments are each lockable on the flue gas side for maintenance purposes thus it is possible to shut down one compartment for maintenance while running the remaining compartments with 100% boiler flue gas flow. The baghouse hoppers serve as temporary storage bins for the large portion of the material that is fed into the solids recycling system. This is accomplished by means of a control valve via maintenance-free air-slides back into the absorber.
World's largest CFB scrubber
In June 2011, a 520 MWe coal power plant at Basin Electric's Dry Fork station (Figure 2) went on-line in Gillette, Wyoming, USA (due to its 4430 ft site elevation, the actual net plant output of the Dry Fork power plant is rated at 420 MWe). Behind its pulverised coal boiler sits the largest single absorber dry scrubber operating in the world today.
Since it has gone on-line, the CFB scrubber has demonstrated a very high, 98%, availability while meeting all the strict emission requirements set by the US EPA and the state of Wyoming. The emission regulations are designed to directly or indirectly limit a broad array of compounds designated as pollutants such as SO2, SO3, HCl, H2SO4, HF, PM10, PM2.5, mercury and other heavy metals.
During the project planning phase, Basin Electric hired Sargent and Lundy to evaluate and recommend the FGD technology based on the criteria of achieving these strict emission limits while delivering the best economics and reliability. After months of study and evaluation, Sargent and Lundy recommended the Graf-Wulff CFB scrubber technology ultimately selected by Basin Electric.
Since it went into operation, the CFB scrubber at the Basin Electric plant has exceeded its design performance, reducing SOx by 95-98%, to levels below 0.06 lb/MMBtu (50-60 mg/m³). It also passed a 30 day mercury removal compliance test by meeting the permitted emission limit of 20 lb/TWh (2.35 µg/m³) while demonstrating a mercury removal rate in excess of 95%.
In addition, the CFB scrubber provided other key benefits to the Basin Electric Dry Fork project such as reduced scrubber water requirement by 30% and real estate by 80% relative to WFGD technology. Furthermore, the scrubber ash can be put to good use, filling and stabilising a nearby open pit coal mine.