Dry Sulfur Dioxide (SO2) Control Systems

One technology for reducing sulfur dioxide (SO2) emissions from combustion sources that does not generate any liquid sidestreams is dry flue gas desulfurization (FGD). This technology is prevalent in treating acid gas emissions from waste incinerators. In dry FGD, the flue gas containing SO2 is contacted with an alkaline material to produce a dry waste product for disposal. This technology includes the following:

• Injection of an alkaline slurry in a spray dryer with collection of dry particles in a fabric filter or electrostatic precipitator (ESP)
• Dry injection of alkaline material into the flue-gas stream with collection of dry particles in a fabric filter or ESP
• Addition of alkaline material to the fuel prior to or during combustion
These technologies are capable of SO2 and hydrogen chloride (HCl) emission reduction ranging from 60 to 90% and 70 to 90+% respectively depending on which system is used. Typical reagents used with these technologies include lime, limestone (only in furnace injection), sodium carbonate, sodium bicarbonate, and nahcolite. These technologies have been used on boilers burning low sulfur coal (usually less than 2%), municipal waste incinerators, and hazardous waste incinerators and are attractive alternatives to wet scrubbing technology, particularly in the arid western U.S.

Spray Dryer with a Fabric Filter or ESP
One type of dry FGD installation is a spray dryer (sometimes referred to as a dry scrubber)
and can be used on utility boilers and waste incinerators. Alkaline material is injected into
a spray dryer with dry particle collection in a fabric filter or ESP. Spray dryers have been
used in the chemical, food processing, and mineral preparation industries over the past 40
years. Spray dryers are vessels where hot flue gases are contacted with a finely atomized
wet alkaline spray. The high temperatures of the flue gas, 250 to 400°F (121 to 204°C),
evaporate the water from the wet alkaline sprays, leaving a dry powdered product. The dry
product is collected in a fabric filter or ESP (Figure 1).

Figure 1. Spray dryer absorber and baghouse system

Flue gas enters the top of the spray dryer and is swirled by a fixed vane ring to cause intimate
contact with the slurry spray (Figure 2). The slurry is atomized into extremely fine
droplets by rotary atomizers or spray nozzles. The turbulent mixing of the flue gas with
the fine droplets results in rapid SO2 absorption and evaporation of the moisture. A small
portion of the hot flue gas may be added to the spray-dryer-discharge duct to maintain the
temperature of the gas above the dew point. Reheat prevents condensation and corrosion
in the duct. Reheat also prevents bags in the fabric filter from becoming plugged or caked
with moist particles.

Figure 2. Spray dryer

Sodium carbonate solutions and lime slurries are the most common absorbents used. A
sodium carbonate solution will generally achieve a higher level of SO2 removal than lime
slurries (EPA 1980). When sodium carbonate is used, SO2 removal efficiencies are
approximately 75 to 90%, lime removal efficiencies are 70 to 85% (EPA 1980). However,
vendors of dry scrubbing systems claim that their units are capable of achieving 90% SO2
reduction using a lime slurry in a spray dryer. Lime is very popular for two reasons: (1) it
is less expensive than sodium carbonate and (2) sodium carbonate and SO2 form sodium
sulfite and sodium sulfate, which are very soluble causing leaching problems when landfilled.

Some of the evaporated alkaline spray will fall into the bottom of the spray dryer. In coalfired
units where appreciable quantities of HCl do not exist, this material can be recycled.
In municipal and hazardous waste incinerators, this spray dryer product is not recycled
due to the presence of calcium chloride. Calcium chloride is formed when HCl in the flue
gas reacts with calcium hydroxide (lime slurry). Calcium chloride is very hygroscopic and
can plug bags, hoppers and conveyors if the material is not kept dry and the exhaust gas
stream conveying this material is not kept well above the dew point. The majority of the
spray reacts with SO2 in the flue gas to form powdered sulfates and sulfites. These particles,
along with fly ash in the flue gas, are then collected in a fabric filter or ESP. Fabric filters have an advantage because unreacted alkaline material collected on the bags can
react with any remaining SO2 in the flue gas. Some process developers have reported SO2
removal on bag surfaces on the order of 10% (Kaplan and Felsvang 1979). However, since
bags are sensitive to wetting, a 35 to 50°F (2.5 to 10°C) margin above the saturation temperature
of the flue gas must be maintained in coal-fired installations (EPA 1980). With
waste incineration facilities this margin must be increased to around 100°F (38°C) due to
the presence of calcium chloride. ESPs have the advantage of not being as sensitive to
moisture as fabric filters. However, SO2 removal is not quite as efficient when using ESPs.

In a spray dryer, finely atomized alkaline droplets are contacted with flue gas, which is at
air preheater outlet temperatures of 250 to 400°F (121 to 204°C). The flue gas is humidified
to within 50 to 100°F (28 to 56°C) of its saturation temperature by the moisture evaporating
from the alkaline slurry. Reaction of SO2 with the alkaline material proceeds both
during and following the drying process. However, sodium-based sorbents are more reactive
in the dry state than calcium-based sorbents are. Since the flue gas temperature and
humidity are set by air preheater outlet conditions, the amount of moisture that can be
evaporated into the flue gas is also set. This means that the amount of alkaline slurry that
can be evaporated in the dryer is limited by flue gas conditions. Alkaline slurry sprayed
into the dryer must be carefully controlled to avoid moisture in the flue gas from condensing
in the ducting, particulate emission control equipment, or the stack.

Many spray dryer systems have been installed on industrial and utility boilers. Some are
listed in Table . Permit reviewers should review the EPA BACT Clearinghouse for additional information on spray dryers and baghouse systems. Spray dryers will be particularly useful in meeting New Source Performance Standards (NSPS) for utility boilers burning low sulfur coal that
require only 70% SO2 scrubbing in addition to achieving the requirements of the acid rain
provisions included in Title IV of the 1990 Clean Air Act Amendments.

Table . Commercial spray dryer FGD systems using a baghouse or an ESP

Dry Injection

In dry injection systems, a dry alkaline material is injected into a flue gas stream. This is
accomplished by pneumatically injecting the dry sorbent into a flue gas duct, or by precoating
or continuously feeding sorbent onto a fabric filter surface. Most dry injection systems
use pneumatic injection of dry alkaline material in the boiler furnace area or in the
duct that precedes the ESP or baghouse. Sodium-based sorbents are used more frequently
than lime for coal-fired installations but hydrated lime is prevalent in waste burning incinerators.
Many dry injection systems have used nahcolite, a naturally occurring mineral
which is 80% sodium bicarbonate found in large reserves in Colorado. Sodium carbonate
(soda ash) is also used but is not as reactive as sodium bicarbonate (EPA 1980). The major
problem of using nahcolite is that it is not presently being mined on a commercial scale.
Large investments must be made before it will be mined commercially. Other natural
minerals such as raw trona have been tested; trona contains sodium bicarbonate and
sodium carbonate.