POWER May 2010 - 114

EMISSIONS
Dry Injection of Trona for SO3 Control
In 2006 and 2007, POWER ran a three-part series on the formation of SO3, O&M
issues caused by SO3, and sorbent injection control for SO3 control. Three
years later, many plants still struggle with their SO3 mitigation systems or
remain undecided on which mitigation path to follow. This article explores
the advantages of dry sorbent injection technology.
By Dr. Yougen Kong, PE and Michael D. Wood, Solvay Chemicals Inc.
T
he formation mechanisms and high costs
of controlling SO3
emissions have been
POWER articles on " SO3
thoroughly discussed in a series of three
's impacts on Plant
O&M " (Part I in October 2006, Part II in February
2007, and Part III in April 2007). In that
series, the impact of SO3
, in sufficient concentrations,
was shown to penalize plant heat
rate, increase plant operation and maintenance
(O&M) costs for back-end equipment through
the formation of weak acids that are very corrosive,
and increase fouling of air heaters and
selective catalytic reduction (SCR) catalysts
due to the reaction of SO3
with ammonia. In
the three years since those articles were published
we still find many utilities struggling to
implement SO3
mitigation measures.
One of the key conclusions presented in the
final article in the series was that injection of
a carefully selected finely atomized sorbent,
such as trona, is very effective in mitigating the
formation of SO3
technology for SO3
. Dry sorbent injection (DSI)
control is gaining popularity
in the utility industry due to its low capital
cost, small installation footprint, ease of operation,
and flexibility to adapt to fuel changes.
Trona is a naturally occurring mineral that
is produced in Green River, Wyoming (Figure
1). Interestingly, milling of trona is not
necessary, as it is already produced as a fine
powder. Although milling trona can increase
its SO3
removal efficiency, the cost to do so
must be balanced against the added cost of
equipment and maintenance.
DSI Principles
In a DSI system, a fine sorbent powder, such as
trona (Na2
lime (Ca(OH)2
Air
CO3 * NaHCO3 * 2H2O) or hydrated
), is injected into the flue gas duct
to remove SO3. Trona is calcined in the hot flue
gas (>275F) to form porous sodium carbonate
(Na2
3Na2CO3(s) + 5H2
CO3), as shown in the following equation:
2(Na2CO3 * NaHCO3 * 2H2O)(s) + heat →
O (gas) + CO2
(gas)
The release of water vapor and CO2
in the
calcination process creates numerous micropores
inside the sorbent, a phenomenon called
the " popcorn " effect, that produces a surface
area five to 20 times the original surface area.
114
Trona/air
The specific surface area of calcined trona is
approximately 10 m2
(Figure 2).
/g. This relatively high
surface area has one significant advantage: It
enables fast reactions between sodium carbonate
and SO3
Trona can be injected at almost any location
in the gas stream as long as the flue gas
temperature is above 275F (Figure 3). It is our
experience that natural mechanically refined
fine trona reactivity to sulfur oxides is improved
as the injection temperature is raised,
unlike chemically refined milled sodium biwww.powermag.com
Upstream
of the SCR (Location 1)
■ Injecting trona at this location can remove
most SO3
the formation of NH4
HSO4
bisulfite inside the catalyst and consequently
lower the minimum operation temperature.
POWER | May 2010
ahead of the SCR to eliminate
or aluminum
carbonate, which has a practical upper operating
limit of about 800F. The typical reference
location options are shown in Figure 3. Nevertheless,
each location has its own advantages
and disadvantages, as explained below.
3. Many injection options. There are many options for injecting trona into the hot gas
stream. Each has advantages and disadvantages. The specific equipment at your plant will determine
the best option. Source: Solvay Chemicals Inc.
Economizer
Boiler
1
SCR
Trona
silo
2
Air
heater
Wet FGD
3
ESP
4
1. Raw trona. A microscopic view of raw
trona. Courtesy: Solvay Chemicals Inc.
2. Calcined trona. Note the micropores
formed in the trona after heating it to
temperatures above 275F. Courtesy: Solvay
Chemicals Inc.

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POWER May 2010

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