Application of Circulating Fluidized Bed Scrubbing Technology for

 Application of Circulating Fluidized Bed Scrubbing Technology for Multi‐Pollutant Removal Horst Hack Foster Wheeler North America Corp. Robert Giglio Foster Wheeler Global Power Group Rolf Graf Foster Wheeler Energie GmbH Presented at The 38th International Technical Conference on Clean Coal & Fuel Systems Clearwater, Florida, USA June 2 to 6, 2013 Application of Circulating Fluidized Bed Scrubbing Technology
For Multi-Pollutant Removal
Horst Hacka (horst_hack@fwc.com), Robert Giglioa (robert_giglio@fwc.com), Rolf Grafb (rolf.graf@fwde.fwc.com) a
Foster Wheeler North America Corp., Perryville Corporate Park, Hampton, N.J., 08827, USA b
Foster Wheeler Energie GmbH, Am Zollstock 1, 61381 Friedrichsdorf, Germany Abstract
Emissions reduction of heavy metals including mercury, arsenic, chromium, nickel and acid gases such
as hydrochloric (HCl) and hydrofluoric (HF) acids is now within the primary focus of the nation’s coal fired
power plants by virtue of the EPA Mercury and Air Toxics Standards (MATS) rule enacted in December
2011. The EPA’s 2011 Cross State Air Pollution Rule (CSAPR), while vacated in August 2012, will likely
be re-proposed and adopted, further mandating 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.
Circulating Fluidized Bed (CFB) Scrubbing technology, offered by Foster Wheeler, offers a viable
pathway for addressing multi-pollutant control in a cost effective manner. Combining lime hydration and
storage equipment, a circulating fluidized bed upflow reactor/absorber, and downstream fabric filter or
ESP, all CFB scrubber system equipment can be installed in one building, or outdoors. Construction
costs can be reduced as the major system components can be pre-assembled on the ground and lifted
into place during system erection.
The Foster Wheeler multi-pollutant CFB scrubber is a flexible and economical technology capable of
removing a wide array of pollutants such as SOx, particulate matter, metals, acid gases and organic
compounds from flue gases from almost any combustion and industrial process. It does all this while
using the least amount of water, a vital resource.
Results for a CFB scrubber installation at a coal-fired utility boiler in the United States will be discussed.
A New Direction for Environmental Regulations
Emissions limits for conventional pollutants emitted from power plants—particularly SO2, NOx, and
particulate matter—continue to tighten around the globe. In the U.S., the Cross-State Air Pollution Rule,
although vacated in August 2012, will likely be reworked and eventually promulgated, mandating
additional SO2 reductions. Consequently, plant owners must continue to evaluate the costs and benefits
of adding a back-end air quality control system (AQCS) against shutting down noncompliant units. The
timing of rule changes makes this a very difficult evaluation.
The North American Electric Reliability Corp., responsible for the bulk power system reliability, has
spoken often about the impact of increasingly restrictive emissions regulations on the reliability of the U.S.
power delivery infrastructure. In effect, the debate centers on balancing lower emissions limits with the
effect of excessive coal plant retirements on system reliability.
This discussion is not limited to the United States. Europe’s new Industrial Emission Directive (IED) has
tightened both SOx and NOx emission limits by 50 mg/Nm3 and particulate limits by 10 mg/Nm3
compared to the prior large combustion plant directive. Recently, China lowered its SOx, NOx, and
particulate limits for power plants to levels even lower than those in Europe’s IED.
New constituents are also being added to the U.S. regulatory framework, such as metals, acid gases, and
organic compounds included under Mercury and Air Toxics Standards rules. Europe is not far behind,
with the IED now requiring a best available technology standard for these constituents as well. These
constituents have always been regulated for certain waste fuel applications, such as waste-to-energy
plants and incinerators, but now regulators have set limits for all boilers, including large utility coal boilers.
Plant owners are being asked to make billion-dollar AQCS purchase decisions when the regulations are
in flux and court oversight is uncertain. In this era of regulatory ambiguity, selecting the most flexible
AQCS design is surely the prudent decision.
A Better Way to Clean Flue Gases
In the past, due to its proven large scale and ability to capture a high percentage of SO2 over a wide
range of fuel sulfur levels, wet flue gas desulfurization (WFGD) scrubbing technology was the most
popular choice for removing sulfur from boiler flue gases in large power plants and industrial facilities.
WFGD technology has a low operating cost, because it utilizes low-cost limestone as the reagent and can
produce gypsum for sale to wallboard manufacturers. However, on the downside, a WFGD system is
expensive to build, uses the most water, occupies the largest amount of real estate, and can keep a full
crew busy maintaining its large number of pumps, pipes, valves, and vessels. But more importantly, due
to its chemistry and process, a WFGD system is only marginal for capturing metals, including mercury, or
acid gases such as SO3, HCl, or HF.
Now with U.S. regulations requiring capture of mercury, acid gases, dioxins, and furans, in addition to
SO2 and particulates, other FGD technologies are becoming more popular due to their ability to capture
this expanded set of pollutants. There are different types of technologies, ranging from simple injection of
a sorbent into the boiler flue gas (direct sorbent injection) to the more established spray dryer absorber
(SDA) technology (which sprays a fine dry mist of lime into the flue gas), to newer circulating fluidized bed
(CFB) technology, which circulates the boiler ash and lime between an absorber reactor and fabric filter.
These different FGD technologies have their pros and cons, but for many power and industrial facilities,
CFB scrubbers are growing in popularity. This is most evident in the pipeline of U.S. retrofit scrubber
projects, where more and more projects are selecting CFB technology.
In the past, these alternative scrubbing technologies were typically chosen over wet FGD technology for
their much lower capital cost and water usage, provided that the boiler size was not too large and the fuel
sulfur level was not too high. Today, 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 sulfur levels above 4%. CFB scrubber technology has now stepped out in front of
other technologies due to five key advantages:
•
•
•
•
•
High multi-pollutant capture capability
Low installed cost
Low water use
Low maintenance cost
Wide fuel sulfur flexibility
CFB scrubbers also offer other benefits, including compact footprint, and the flexibility to use low-quality
lime and water. One key process advantage of a CFB scrubber, unlike SDA technology, is that the flue
gas temperature does not limit the amount of lime injection. This feature allows a significant increase in
acid gas scrubbing performance.
The multi-pollutant CFB scrubber is a flexible and economical technology capable of removing a wide
array of pollutants from flue gases of nearly any combustion or industrial process. 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 high capture efficiency of the vapor phase acid gases and metals contained
within the flue gas.
Figure 1. Advantages of circulating fluidized bed (CFB) scrubbers
Figure 2. Schematic of CFB lime hydration system
Major Components
CFB Absorber
Multiple flue gas venturies, shown in Figure 3, provide the required fluidizing gas dispersion and adequate
suspension of the solids across the full diameter of the absorber vessel. The multi-venturi design allows
a wide capacity range while minimizing scale-up risk.
Water injection nozzles provide an atomized spray cloud of water droplets enhancing heat and mass
transfer rates over the large surface area of solids churning some 75 feet 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 multiple gas pollutants, while maintaining a small
absorber footprint.
The 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.
Figure 3. Venturi flue gas inlet at bottom of absorber
Fabric Filter
A multi-compartment baghouse is located downstream of the absorber vessel for high efficiency capture
and recirculation of the solid particles. In certain installations, such as the Basin Electric Dry Fork Unit 1
example described below, there are separate compartments, each lockable on the flue gas side for
maintenance purposes, making it possible to shut down one compartment for maintenance while running
the remaining compartments with 100% boiler flue gas flow.
Solids from the absorber entering the baghouse are completely dry, given the small amount of water
added, and the long flue gas and solids residence time in the absorber. Low gas velocity and the bafflefree design result in pressure drops several times lower than conventional baghouses. Thus the
baghouse itself is free of wetted solids and the housing remains very clean. The penthouse area is shown
in Figure 4.
The baghouse is equipped with a Pulse Jet cleaning system which delivers intermittent compressed air
bursts to the separate compartments based on either baghouse differential pressure or flow rate.
Optimized pulse pressure and frequency across filter sections ensures efficient ash collection, dust
capture and long bag life.
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 air-slides back into
the CFB absorber. A small percentage of the scrubber by-product is continuously discharged from the
insulated filter hoppers by means of a control valve and material transport system to the product silo for
further utilization.
Figure 4. Baghouse penthouse above pulse-jet fabric filters
Dry Lime Hydration System
Hydrated lime [Ca(OH)2] used in the CFB scrubbing process can be purchased directly from suppliers.
However, for high sulfur fuel applications requiring larger quantities of reagent, or in locations where
hydrated lime suppliers are limited, owners can purchase less costly quicklime (CaO) and hydrate it on
site.
A dry lime hydration system can be located near the CFB absorber vessel. As shown in Figure 2, lime
and low pressure steam are injected into the hydration reactor for conversion to calcium hydroxide.
Hydrated lime product from the hydrator is separated from the hydrator exhaust vapors in a downstream
cyclone and then collected in an ash hopper. From the product hopper, the hydrated lime can be sent
directly to the CFB 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. The hydration system is low maintenance with no
rotating equipment except for a screw conveyor to meter lime to the hydrator. The hydration system has
25% turndown capability for following the boiler load.
High Reliability by Design
CFB scrubbing technology incorporates a number of built-in features to maximize reliability. The absorber
vessel is a self-cleaning up-flow reactor. Water injection nozzles, located on the perimeter of the absorber
above the introduction points for the recirculated and sorbent solids, provide an atomized spray cloud of
water droplets. These nozzles must be removed periodically for replacement of wear components.
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.
The multi-compartment baghouse lends itself to online replacement of filter bags with one compartment
off-line. Separate compartments, such as those installed at Basin Electric Dry fork station, are each
lockable on the flue gas side for maintenance purposes, making it 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 material that is fed into
the solids-recycling system reducing equipment and improving overall system reliability and cost.
The Largest CFB Scrubber in the World
In June 2011, a 440-MWe rated coal power plant at Basin Electric’s Dry Fork station (Figure 5) went
online in Gillette, Wyoming. (Due to its 4,430-foot site elevation, the Dry Fork plant has a gas volumetric
flow rate equivalent to a 520 MWe plant at sea level.) Behind its pulverized coal boiler sits the largest
CFB scrubber operating in the world today.
Figure 5. The world’s largest CFB scrubber is found at Basin Electric Dry Fork Unit 1.
Courtesy: Basin Electric Co-Op and Wyoming Municipal Power Agency
During the project planning phase, Basin Electric hired Sargent and Lundy to evaluate and recommend a
FGD technology based on the criteria of achieving strict emission limits while delivering the best
economics and reliability. After months of study and evaluation, Sargent and Lundy recommended the
CFB scrubber technology ultimately selected by Basin Electric.
Since it has gone online, the CFB scrubber has demonstrated a very high, 98% availability while meeting
all the strict emission requirements set by the U.S. Environmental Protection Agency 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.
The CFB scrubber has exceeded its design performance, reducing SOx by 95% to 98%, to levels below
0.06 lb/MMBtu (50 to 60 mg/nm³). It also passed a 30-day mercury removal compliance test by meeting
the permitted emission limit of 20 lb/TWh (2.35µg/m3) while demonstrating a mercury removal rate of up
to 70% without activated carbon injection.
The CFB scrubber provided other key benefits to the Basin Electric Dry Fork project such as reducing the
scrubber’s water requirement by 30% and real estate by 80% compared to WFGD technology. In
addition, the scrubber ash is being used to fill and stabilize a nearby open pit coal mine
CFB Scrubber Start-up and Commissioning Experience
The first tests under full operation at the Basin Electric Dry Fork project were successfully demonstrated
in June 2011. During startup, only two major CFB scrubber adjustments were made. One adjustment
focused on optimizing the fluid dynamics, and the other adjustment addressed response time within the
CFB absorber for the final boiler outlet conditions.
Stable operation was maintained down to at a flue gas flow turndown of 33% of full flow, and the CFB
scrubber system maintained emissions below all contracted and permitted levels, with more than 98%
SO2 removal, opacity <1%, less than 3 ppm PM emissions and up to 70% mercury removal (without
injection of activated carbon). Flue gas properties and emission reduction levels are presented in Table
1.
Table 1. Flue gas properties and emission reductions (Design Data)
Flue Gas Flow
Temperature
SO2
SO3
HCl
Dust
Units
Inlet
Outlet
ACFM
m3/h
⁰F
⁰C
lb/MMBtu
mg/m3
ppmv
mg/m3
ppmv
mg/m3
ppmv
lb/MMBtu
mg/m3
1,792,000
3,045,000
294
146
0.66-1.79
800-2200
280-770
25-42
8-14
8-15
5-9
3-5
4,000-6,000
1,550,000
2,630,000
155-175
70-80
0.06
60-75
20-25
1-2
0.3-0.6
4-6
2-3
0.012
14-20
Typical
Efficiency
~85-99%
~99%
~60-85%
~99%+
Conclusion
The patented Foster Wheeler multi-pollutant CFB scrubber is a flexible and economical technology
capable of removing a wide array of pollutants such as SOx, particulate matter, metals, acid gases and
organic compounds from flue gases from almost any combustion and industrial process. It does all this
while using the least amount of water, a vital resource.
The CFB scrubber maintains advantages over other technologies in these five key areas:
• High multi-pollutant capture capability
• Low installed cost
• Low water use
• Low maintenance cost
• Wide fuel sulfur flexibility
The technology has been applied widely in power plants, steel mills, refineries, waste-to-energy plants,
combined heat and power plants, and plants in many other industries. It has been demonstrated over a
range of flue gas flow rates from small industrial boilers to large coal power plants with capacities over
500 MWe.