close

Вход

Забыли?

вход по аккаунту

?

Emission Control During Co-Combustion of Coal and Biomass in a Fluidized Bed Combustor.

код для вставкиСкачать
Dev. Chem. Eng. Mineral Process., 7(5/6),pp.501-511,1999.
Emission Control During Co-Combustion of
Coal and Biomass in a Fluidized Bed
Combustor
Jinghong Wang, Hao Liu, Jidong Lu, Dechang Liu,
Xiaotao Xia and Liqiang Ma
National Laboratory of Coal Combustion, Huazhong University of
Science and Technology, Wuhun, Hubei, 430074, P.R. China
Co-combustion of coal and biomass in a bench-scale bubblingfluidued bed was
carried out to investigate the eflects of co-combustion on emissions of NO, N 2 0 and
SO,
The objective of these experiments was to jind a feasible method of
simultaneous NOdN20/SOx control. Several coals and coal chars were used and
mixed with wood chip and rice husk respectively. The influences of temperature, coal
properq andfiaction of biomass on emissions were investigated.
The experimental results revealed that co-combustion of coal and wood chip or
rice husk could reduce NOflzO/SOx emissions simultaneously compared to
combustion of coal alone. The effect of emission reduction depended on the coal
properv. The higher the fuel-N and the lower the volatile matter (VM) content of the
coal, the greater the emission reduction. Moreover, the higher the fiaction of
biomass (wood chip or rice husk), the lower the NOdN20/SOx emissions. In
addition, it wasfound that when co-combusting coal char and wood chip or rice husk,
NOx and N 2 0 as well as SOX could be reduced compared to combustion of coal char
alone. From the experiments, it was found that heterogeneous reactions played an
important role in N 2 0 reduction during co-combustion of coal and biomass.
Hence the process has been proved to be a feasible, economical and promising
techniquefor simultaneous N O ~ N ~ O / S O
control.
X
501
J. Wang et al.
Introduction
Biomass can be used as a renewable and clean fuel. At present, biomass in its various
forms contributes nearly 43 percent of the total energy required in developing
countries and 26 percent for some developed countries. For China, it is imperative to
seek a rational way to make full use of biomass energy. In recent years, fluidized bed
combustion has been developed as a clean coal technology. It has many advantages,
but it also has some shortcomings. For example, N 2 0 emission from fluidized bed
combustion (FBC)may be high as 200ppm [l]. Because of the N 2 0 effects on the
environment, much research has been focused on N20 formatioddestruction
mechanisms in FBC and control strategies [2-4]. In this paper, co-combustion of coal
and biomass as a method to control the N&/NzO/SOx emissions in fluidized bed
combustion was investigated. The objective of these experiments was to determine
whether co-combustion of coal and biomass can reduce the emissions of NOx, N 2 0
and SOXsimultaneously and the reasons for such reduction.
Experimental Details
The experiments were canied out in a bench-scale bubbling fluidized bed combustor
made of quartz tube. The schematic of the apparatus is shown in Figure 1.
LGas analysis system
N20: GC-ECD analyzer
NOx analyzer
SO, analyzer
O2 analyzer
1
Primary air
1.reactor 2.electric heater
3.fluidized bed feeding system
Figure 1. The schematic of bench-scale bubblingfluidized bed combustor.
502
Emission control during co-combustion of coal and biomass in FBC
The inner diameter of the quartz tube is 20mm and the height is 600mm. The
distributor consists of a quartz-perforated plate which supports the bed material with a
static bed height of 20mm. The quartz tube is heated by a electric heater and the bed
temperature can be controlled within the range fiom room temperature to 950°C. A
thermocouple immersed in the bed allows the temperature in the reactor to be
accurately controlled. Fuels are fed to the reactor through a fluidized bed feeding
system.
The proximate analyses and ultimate analyses of coalkhar and biomass used in the
experiments are listed in Table 1.
Table 1. Fuel properties.
Fuel
Proximate Analysis (%)
Volatile
Ultimate Analysis (%)
H
C
0
Moisture
Ash
N
GLCoal
1.08
16.90
10.86 72.98 3.46 3.342 1.117 1.061
SMCoal
3.34
4.04
30.11
CFCoal
4.9
31.3
16.3
83.7
5.4
8.6
1.3
1.0
GLChar
2.07
20.66
5.49
91.7
2.2
3.8
1.1
1.2
CFChar
3.05
39.91
12.18
90.0
2.1
5.3
1.5
1.1
WoodChip
6.49
3.86
81.13
43.55 5.61 40.13 0.21
0.15
RiceHusk
6.64
18.87
55'2
35.2
0.03
S
73.96 4.59 12.667 0.899 0.504
5.8 38.91 0.19
Results and Discussion
Figures 2 and 3 show the NOx emissions versus the bed temperature at different
blending ratios of biomass and coal. It can be seen that addition of biomass to coal
can reduce the emission of NOx. Moreover, the reduction tendency becomes more
obvious as the injected biomass increases.
The NOx emission is reduced by
8%-11% when the ratios of coal to wood chip are between 20:1 and 10:1, and by
17%-31% when the ratios are between 20:3 and 4:l. It can also be seen that by
adding wood chip to coal, the decrease of the NOx emission is nearly linear with the
change of temperature. With the temperature increasing, the reduction effect is
decreased. At a temperature is 920°C,results reveal that there is no significant effect
503
J. Wang et al.
on the reduction of NOx emission. It can also be seen that NOx reduction relates not
only to the hction of biomass and the bed temperature, but also to the fuel property.
The higher the fuel-N and the lower the volatile matter (VM) content of the coal, the
greater the emission reduction.
The decrease of NOx emission may be due to several of the following reasons:
Wood chip and rice husk contain a very high volatiles content and very small
amounts of fuel-N compared to coal. Adding wood chip or rice husk to coal
results in a larger release of volatiles and forms an oxygen-lean zone restricting
the NOx formation. Because of the lower amounts of fuel-N in wood chip and
rice husk, the conversion of fuel-N to NOx decreases.
The quick devolatilization of wood chip and rice husk results in the formation of
char with higher porosity and higher reactivity compared to coal [5]. This favors
the NOx decomposition.
The presence of high levels of hydrocarbon radicals originating from the
volatiles appears to give rise to a reduction in the amount of NOx [6].
A higher bed temperature reduces the char and CO concentrations in the
combustor, thus decreasing the heterogeneous reduction of NO on the char
surface. In addition, higher temperatures promote the oxidation of NCO to NO
via the reaction:
NCO + 0
+ NO + CO
...(1)
1i
4
E
.9
z
Y
C
Q
x
GL-coal:wood=20:1
~ ~ ~ ~ ~ : w o d = l O : l
GL-coal:wod=20:3
GL-coal:wood= 4:l
s
6z
600
700
800
Temperature
900
1000
:C
Figure 2. The emission of NO, during co-combustionof GL coal and wood chi$
504
Emission control during co-combustion of coal and biomass in FBC
.
...,...
. . . . ._ . . . . .. . . . . .
T
1
. ..
..
..
W CC)
Figure 3. The emission of NOX during co-combustion of SMcoaI and wood chip.
rn
o
o
x
GL-coal
GL-coal:wood=20:l
GL-~l:wood=lO:l
GL-coal:wood=PO:J
GL-coal:wood= 4:l
Temperature (‘C)
Figure 4. The emission of N20 during co-combustion of GL coal and wood chip.
505
J. Wang et al.
SM-coal:wood=lO:l
SM-coal:wood=20:3
SM-coal:wood=4:l
700
750
800 850 900
Temperature (‘C)
Figure 5. The emission of N 2 0 during co-combustion of SMcoaI and wood chip.
h
k
4 L a a l : R i c e Husk=ZO:l
4 L c o a l : R i c e Husk=lO:l
--.--GL-cd:Ria Husk=20:3
P
v
C
0
.-
)
.
2
52
s
3
Tenperatwe(&)
Figure 6. The emission of N 2 0 during co-combustion of GL coal and rice husk
506
Emission control during co-combustionof coal and biomass in FBC
g
h
220
n
-o-
.0
c
-A-
2 180
-v-
CF:RiceHusk=20:l
CF:RiceHusk=lO:l
CF:RiceHusk=20:3
160
140
3 120:
7b0
730
8bO
8bO
900
Temperature(oC)
Figure 7. The emission of N 2 0 during co-combustion of CF coal and rice husk
From Figures 4 to 7, it can be seen that the influence of adding wood chip and rice
husk on the NzO emission almost has the same tendency.
The NzO emission
reduction is due to:
Intensive release of biomass VM creates a reducing atmosphere, which affects
NzO formation.
The biomass contains a very high volatiles content and very small amounts of
fuel-N. These volatiles are generally released as NH;compounds rather than
HCN or other CN groups. Consequently, the reactions leading to the formation
of NzOare suppressed [6].
Radicals such as H and OH of the biomass VM reduced the NzO via the
reactions:
NzO + H + N2 + OH
...(2)
N20 + OH+Nz + HOz
...(3)
Biomass fuels contain relatively large amounts of calcium, potassium and
sodium, which have catalytic effects on NzO reduction.
Owing to the intensive release of VM, the residual char of biomass is porous
particles and the residual char particles have a significant reduction effect on
N20.
507
J. Wung et al.
Figure 8. The emission of SO2 during co-combustion of GL coal and wood chip.
r=-l
S~:unoCc.l&?
-0sMcod:em.3
--0-53vkud:&
41
-0-
Figure 9. The emission of SO2 during co-combustion of SMcoal and wood chip.
Figures 8 and 9 show that co-combustion of coal and biomass can reduce the SOx
emission not only due to biomass (wood chip, rice husk) containing a lower suIfbr
content in comparison to coal, but also due to calcium, potassium and sodium in the
biomass ash. SOz might be absorbed by CaO and MgO in the biomass ash to some
508
Emission control during co-combustion of coal and biomass in FBC
extent. Temperature is the most important factor affecting the sulfur retention. Cocombustion of coal and biomass creates a localised low-temperature region
contributing to a lower SOz emission. Biomass fuels forming a significantly larger
surface area are available for absorbing SO2. This indicates that the higher fraction of
biomass leads to lower SO2emission, as shown in Figures 8 and 9.
........................
T
W cc)
Figure 10. The emission of NOx during co-combustion of GL char and wood chip.
.................
Figure 11. The emission of NO, during co-combustion of CF char and wood chip.
509
J. Wang et al.
180,
1
.a
d
a
a
a
-
.1
.3
41
Figure 12. The emission of N 2 0 during co-combustion of GL char and wood chip.
Figure 13. The emission of N 2 0 during co-combustion of CF char and wood chip.
It is known that NOx and N20 are formed through homogeneous and
heterogeneous reactions in fluidized bed combustion. From the above results, it can
be seen that using a mixture of biomass and coal can cause a reduction of NOx and
N20 emissions. In order to acquire a deeper understanding of the influence on the
510
Emission control during co-combustionof coal and biomass in FBC
NOx and N 2 0 formation when adding biomass, experiments were conducted using the
mixed coal char and biomass. The char contains very high porosity and very little
volatiles. When comparing Figures 10 to 13 with Figures 2 and 4, it can be seen that
during co-combustion of char and biomass, the reduction of NOx and N20 emissions
is higher than for co-combustion of coal and biomass. This indicates that adding
biomass mainly influences the heterogeneous formation of NOx and N20.
Conclusions
Co-combustion of coal and biomass may provide a method of simultaneous control of
NOx/N201SOx.The reducing effect on the NOx/N20/SOx emission increases with the
fraction of biomass and relates to the bed temperature and fuel property. During cocombustion of coal and biomass, the heterogeneous reactions play an important role
in the formation of NOx and N 2 0 as seen by comparing the results of char- and coalbiomass mixtures.
References
1. Hein, K., and Raible, L., EPAAFP European Workshop on the Emission of Nitmus Oxide from Fossil
Fuel Combustion, Rueil-Malmaison, France, 77-78,1988.
2. Gustavsson, L., and Lecher, B., I Ith International Conference on Fluidized Bed Combustion, ASME,
New York,677-685, 1991.
3. Lecher, L., and Gustavsson, L., J. Inst. Energy, 64:I 76-182,I991.
4. Wojtowicz, M. A., Pels, J. R., and Moulijin, J. A., Fuel, 73(9): 1416-1422,1994.
5. Sjostrom, K.,Bjornbom, E., Zanzi, R., Brage, C., Lundgren, M., Gahnemi Zadeh, B.,Qizhuang, Y.,The
Pyrolysis Step in Combustion of Biomass and Coal, APAS Clean Coal Technology Programme 19921994,Vol 11: Co-combustion of BiomasdSewage Sludge and Coals.
6. Jukka, L., Formation of NH3 and HCN in slow-heating-rate inert pyrolysis of peat, coal and bark, Fuel,
74(9), 1363-1368,1995.
511
Документ
Категория
Без категории
Просмотров
2
Размер файла
365 Кб
Теги
coal, biomasa, fluidized, bed, combustors, emissions, combustion, control
1/--страниц
Пожаловаться на содержимое документа