(542a) Basic Co-Firing Characteristics of Ammonia with Pulverized Coal in a Single Burner Test Furnace

Ammonia is expected as a potential fuel to
substitute fossil fuels, because it does not discharge carbon dioxide and is
easily handled by liquefaction. There are several ways for the direct use of
ammonia as a fuel; for example, use in fuel cells and combustion devices. One
of the possible application is the combustion use in thermal power plants. In
particular, co-firing of ammonia in coal-fired power plants seems to have a
relatively great advantage on the suppression of greenhouse gases, because coal
is one of the main emission source of carbon dioxide. On the other hand, it is
concerned that concentration of nitrogen oxides (NO_x), which is one
of the typical atmospheric pollutant, in the flue gas would considerably
increase due to the oxidation of ammonia. To utilize ammonia as a co-firing
fuel in existing pulverized coal-fired power plant, without causing additional
costs for the modification of the denitration
equipment, it is important to develop a combustion technology that can suppress
the NO_x concentration in the flue gas. Co-firing characteristics of
pulverized coal and ammonia, however, had not been evaluated except in the case
of very low co-firing rate for the purpose of denitration
in the pulverized coal flame. In this study, basic co-firing characteristics of
pulverized coal and ammonia were investigated using a bench-scale single burner
test furnace.

A schematic diagram of the single burner
test furnace is shown in Fig. 1. The rated thermal input of the furnace is 760
kW, which corresponds to approximately 100 kg/h of coal for common bituminous
coals. The total air ratio (the inverse of the equivalence ratio) is 1.24, in
which the oxygen concentration at the furnace exit is 4%. In addition, two-stage
combustion is employed to reduce NO_x emission by creating a reducing
environment, and the staged air was fed from the ports located at a distance of
3.0 m from the burner. The rate of staged air is 30% of the total air flow.
Ammonia was fed into the furnace in two different ways; one is the injection
into the center of the pulverized coal burner, and the other is separate
injection through the side port of the furnace wall.

Fig. 1. Schematic of the combustion test
furnace.

First, we examined the effects of ammonia
co-firing rate and injection method on NO_x concentration in the flue
gas. Fig. 2 shows the effect of ammonia co-firing rate on NO_x concentration
in the flue gas in the case of ammonia injection into the burner. There was no
significant change in NO_x concentration in the case of 5 and 10%
co-firing rate compared to the case of single coal combustion. However, as
increasing co-firing rate to 15 and 20%, NO_x concentration also
increased, and the concentration at co-firing rate of 20% was approximately 20%
higher than that at single coal combustion. This indicates that fuel NO_x
was produced in the flame by the oxidation of ammonia, or combustion of
pulverized coal was aggravated, and then it resulted in the increase of NO_x
emission. On the other hand, it was found that NO_x concentration in
the flue gas at 20% ammonia co-firing depended on the position of ammonia
injection into the furnace. Fig. 3 shows the changes of NO_x
concentration against different locations of ammonia injection, where ammonia
co-firing rate is constant at 20%. When ammonia was injected through the side
port which is located at 1.0 m apart from the burner, NO_x concentration
decreased compared to the case of ammonia injection into the burner, and was
almost equal to that for single coal combustion. When ammonia was injected
through the port located at 1.4, 1.8 or 2.2 m apart from the burner, however,
NO_x concentration became higher as the injection position was
shifted to downstream, and the NO_x concentration exceeded that for
single coal combustion. The reason that the NO_x concentration did
not increase when ammonia was injected through the port at 1.0 m from the
burner is expected the denitration effect of ammonia
on NO_x generated in pulverized coal flame. Therefore, by injecting
ammonia into the region of low-O₂ and high-NO_x
concentration, it is expected that not only the generation of fuel-NO_x
is effectively suppressed, but also ammonia can work as a denitration
agent against the existing NO_x in the flame.

Fig. 2. Effect of ammonia co-firing rate on
NO_x concentration in the flue gas when ammonia was injected into the
pulverized coal burner.

Fig. 3. Effect of the location of ammonia
injection (distance from the burner) on NO_x concentration in the
flue gas.

The influence of ammonia co-firing on
unburned carbon concentration in the fly ash and concentration of ammonia and
nitrous oxide (N₂O) in the flue gas, where N₂O is one of
the strongest greenhouse gases, was also evaluated. When ammonia was injected
into the center of the burner with the co-firing rate of 20%, unburned carbon
concentration in the fly ash increased by approximately 20%. In this case,
flame temperature near the burner was lower compared to the case of single coal
combustion, and it presumably affected the increase of unburned carbon
concentration in the fly ash. Concentration of unburned ammonia and N₂O
in the flue gas was also a little higher in this case, but these increases were
less than a few ppm and 1 ppm, respectively. Since ammonia is also used in the denitration equipment with selective catalytic reduction
process, increase of unburned ammonia by a few ppm would not cause significant
problem on the actual coal-fired power plants. Also, increase of N₂O
by a few ppm is almost negligibly low to work as a greenhouse gas, because
emission of carbon dioxide is decreased by almost 20% in the 20% ammonia
co-firing condition. In the case ammonia was injected through the side port at
0.6 or 1.0 m from the burner, unburned carbon in the fly ash, unburned ammonia
and N₂O in the flue gas did not increase compared to the case of
single coal combustion; therefore it is considered that the significant problem
would not occur, either in this case. As a result, we could have demonstrated
the possibility of ammonia co-firing up to 20% in the coal-fired thermal power
plants.

This research was supported by a program of
Energy Carrier in Strategic Innovation Promotion Program (SIP), led by Japan
Science and Technology Agency (JST). We appreciate the support from all
concerned.