POWER April 2012 - 41

BIOMASS POWER
in particulate control devices. Oxidized
mercury is water-soluble and will readily
be adsorbed using activated carbon technologies.
Elemental mercury is a very
stable molecule and difficult to remove
from flue gas. Typically, elemental mercury
must be forcibly oxidized in order
to promote its capture; otherwise, it most
stone, hydrated lime, or sodium-based sorbents
is most economical and can achieve
the required HCl control. Limestone (calcium
carbonate) requires a substantial
amount of heat to reduce hydrogen chloride
into calcium chloride. For this reason,
milled limestone is typically injected into
the furnace for maximum removal effiThe
control of mercury and dioxin/furan
emissions is most effective when using a
powdered activated carbon injection system.
likely will pass through the boiler and air
quality control systems and be released
into the environment.
Sparse testing for dioxin/furan emissions
of biomass sources has been conducted;
however, the results of available
data show that emissions for all species of
dioxins and furans are generally insignificant.
Because of the low emission rates,
it is impractical to destroy dioxin/furan
emissions by means of thermal oxidation.
The more realistic removal technique is
sorbent capture and removal by a particulate
control device. This is another reason
why dioxin/furan emissions for biomass
facilities are best controlled by using PAC
systems. A PAC system will absorb the dioxins
and furans, which will then be collected
in the particulate control device.
The capital cost of a PAC system is estimated
to be in the $21 to $42/kW range.
Additionally, the average cost for activated
carbon is in the range of $1,000 to $1,500/
ton of sorbent. For industrial-size plants,
this adds a significant capital cost plus a
high operational cost.
Hydrogen chloride (HCl) emissions will
vary greatly, depending on the source of
the biomass fuel. Recent data suggests that
clean woody biomass can achieve uncontrolled
HCl emissions in the 0.004 to 0.006
lbm/million Btu range. Although these
emissions rates are low, the promulgated
regulation requires additional reduction to
maintain compliance. If a facility requires
control of sulfur dioxide (SO2
) emissions,
then control of HCl should not be an issue.
This is because sorbents used to control
SO2
HCl over SO2
emissions will be a byproduct of SO2
emissions.
have a greater affinity to react with
. Therefore, reduced HCl
control.
Biomass fuels, however, are naturally
low in sulfur and do not always require
control of SO2
In the absence of SO2
April 2012 | POWER
controls, a dry
sorbent injection (DSI) system using limeSodium
bicarbonate
2NaHCO3
Calcination
Sodium carbonate
Na2CO3
+
Carbon Dioxide
CO2
+
Water
H2O
ciency. Hydrated lime (calcium hydroxide)
requires less heat for the removal of
HCl and is typically injected into the flue
gas downstream of the furnace. Figure 1
shows the complete reaction for hydrogen
chloride mitigation using limestone and
hydrated lime.
Sodium sorbent injection (trona and
sodium bicarbonate) has also proven effective
for reducing hydrogen chloride
emissions. This class of sorbents requires
a moderate amount of heat (650F to 900F
at the injection point) to effectively remove
hydrogen chloride. Injection points
will vary from upstream of the economizer
to downstream of the air heater. Figure 2
shows the full reaction, including the calcination
of the sodium sorbents.
Unfortunately, some wood fuels and
other biomass fuels contain levels of chlorine
that can be an order of magnitude
higher than in clean, woody biomass. In
these cases, sodium-based sorbents may be
the only solution for controlling HCl emissions
to regulatory limits. A DSI system
for control of HCl will add an additional
$28 to $63/kW of capital cost to a project.
Sorbent prices will also vary greatly,
depending on sorbent type and location of
the facility, but will contribute to a significant
increase in operating cost.
Fortunately, with release of the new
proposed Boiler MACT for Major Sources,
all of these limitations have been removed.
If the new proposed regulation is
1. Reducing HCl with lime. Limestone or hydrated lime can be injected into the furnace
to reduce HCl emissions from the burning of biomass. Source: KBR Power & Industrial
HCI removal with limestone
Calcium carbonate
(limestone)
CaCO3
+
Hydrogen chloride
2HCI
Calcium chloride
CaCI2
+
Water
H2O
+
Carbon Dioxide
CO2
HCI removal with hydrated lime
Calcium hydroxide
(Hydrated lime)
Ca(OH)2
+
Hydrogen chloride
2HCI
Calcium chloride
CaCI2
+
Water
2H2O
2. Reducing HCl with sodium sorbents. HCl can also be removed from stack
emissions of a biomass-fired furnace by injecting the sorbents trona and sodium bicarbonate.
Source: KBR Power & Industrial
Sodium sorbents
Trona
2* (NaHCO3 * Na2CO3 * 2H2O)
Calcination
Sodium carbonate
3Na2CO3
+
Carbon Dioxide
CO2
+
Water
5H2O
HCI acid mitigation
Sodium carbonate
Na2CO3
+
Hydrogen chloride
2HCI
www.powermag.com
Carbon Dioxide
CO2
+
Sodium chloride
2NaCI
41
http://www.powermag.com

POWER April 2012

Table of Contents for the Digital Edition of POWER April 2012

Contents
POWER April 2012 - Cover1
POWER April 2012 - Cover2
POWER April 2012 - Contents
POWER April 2012 - 2
POWER April 2012 - 3
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