INVESTIGATION OF
THE BEHAVIOUR OF LAFIA-OBI COAL
IN A FLUIDIZED BED COMBUSTION
CHAMBER
O.T. POPOOLA and A. A. ASERE
Department of Mechanical Engineering,
Faculty of Technology,
Obafemi Awolowo University,
Ile-Ife, Nigeria.
OAUTekCONF 2011
Faculty Of Technology Conference
INTRODUCTION
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General Background

There is a large deposit of coal in the middle belt and eastern part
of the Nigeria which remains underexploited.
The coal reserves in Nigeria are estimated to be in excess of
2.5 billion tonnes.
• Available data show that Nigerian coals are mainly sub-bituminous
steam except for the Lafia-Obi bituminous coal
•


Coal is a major source of energy. Coal has played this important
role for centuries – not only providing electricity, but also as an
essential fuel for steel and cement production, and other industrial
activities.
The value of coal is partially offset by the environmental impacts of
coal combustion.
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Geological Map of Nigeria indicating Lafia Obi coal area
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General Background cont.

Fluidized Bed Combustion is a Clean Coal
Technologies that:





effectively and inexpensively combust low-grade coals
with high content of moisture (≤60%), ash (≤ 70%) and
Sulphur (≤ 10%),
achieve high combustion efficiency (>99%);
achieve boiler flexibility with type and quality of coal;
provide effective environmental protection from SO2,
NOx and solid particles (SO2 < 400mg/m3, N0x< 200
mg/m3, solid particles < 50 mg/m3);
achieve a wide range of load turndown ratio (20-100%);
and
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Pressurized fluidized Bed combined cycle for electricical
power generation using coal (Sambo,2009)
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 The
objectives of this study are to :

investigate the effect of Lafia-Obi coal particle
sizes on combustion bed temperature;

determine the effects of the coal particle sizes
and combustion temperature on emission
characteristics; and

evaluate the coal ash content at various
operating conditions.
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
This study covers the combustion of Lafia-Obi
coal in a fluidized bed with the view to generate
electricity.

Emphasis will be on variation of coal
combustion feed size at varying coal
combustion feed rate and

The resultant effects on combustion bed
temperature, NOx , CO and rate of ash
generation will be measured and analyzed.
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METHODOLOGY
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PRELIMINARY LABORATORY
STUDIES
Raw Material Acquisition and Handling
 Sample Preparation
 Determination of Physical Characteristics

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PROXIMATE ANALYSIS
Volatile Matter
 Moisture Content
 Fixed Carbon
 Ash Content

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ULTIMATE ANALYSIS

Hydrogen Content

Carbon Content

Nitrogen Content

Sulfur Content

Oxygen Content
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FUEL PREPARATION
 Sun
Drying
 Separation
 Crushing (using hammer mill)
 Sieving (using ISO standard sieve set)
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14
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Summary of ASTM international standards used in investigation
S/N Designation
Title
1
D5192 – 09
Standard Practice for Collection of Coal Samples
2
D2013/D2013M – 09
Standard Practice for Preparing Coal Samples for
Analysis
4
D3172 – 07a
Standard Practice for Proximate Analysis of Coal and
Coke
5
D3302/D3302M – 10
Standard Test Method for
Total Moisture in Coal
6
D3175 – 07
Standard Test Method for
Volatile Matter in the Analysis Sample of Coal and
Coke
7
D3174 – 04
Standard Test Method for Ash in the Analysis Sample
of Coal and Coke from Coal
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Summary of ASTM international standards used in investigation
cont.
8
D5373 – 08
Standard Test Methods for Instrumental
Determination of Carbon, Hydrogen, and
Nitrogen in Laboratory Samples of Coal
9
D 3179 – 02
Standard Test Methods for Nitrogen in the
Analysis Sample of Coal and Coke
10
D 3178 – 89 (Reapproved Standard Test Methods for Carbon and
2002)
Hydrogen in the Analysis Sample of Coal and
Coke
11
Designation: D3177 – 02
(Reapproved 2007)
Standard Test Methods for Total Sulfur in the
Analysis Sample of Coal and Coke
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COMBUSTION
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Fig. 1 Experimental Setup 17
COMBUSTION
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DATA COLLECTION






Temperature- 6 Chromel-Alumel Thermocouples and 6-channel
digital readout were used to measure the variations in the
combustion bed and freeboard temperatures.
The temperature along the height of the atmospheric reactor was
taken at distances of 2, 10, 40, 60 and 80 cm above the gas
distributor
The composition of the flue gas at the various conditions was
determined using a gas analyzer.
The ash and fragmented coal particles content of the extracts after
quenching at different operation conditions were tested and
classified.
A U-tube manometer was used to measure the pressure drop
through the bed.
Air was supplied by a 4hp centrifugal blower and the air flow rate
was measured by means of a rotameter.
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EXPERIMENTAL CONDITIONS
Table 1: Summary of Experimental conditions
Design parameters
Material/value
Type of fuel/ feed size
Lafia-Obi/(5-25mm)
Bed material / size (µm)
Sandstone/ 350-500
Bed temperature (oC)
Static bed height (m)
Fuel feed rate (kg/min)
Bed diameter (mm)
Fluidization Velocity(l/min)
750-1200
0.1
0.2 & 0.3
150
350-2000
Pressure drop across distributor plate (mmH2O) 43
Pressure drop across bed (mmH2O)
428.8
Distributor plate
No. of holes
Diameter of holes (mm)
Thickness (mm)
311
1.5
4
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RESULTS
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Proximate & Ultimate Analysis
Table 4.1:
Analysis of Lafia-Obi Coal
Analysis
Value
Proximate Analysis
Moisture (wt %)
2.91
Volatile matter (% 1)
27.19
Ash (% db)
18.62
Fixed carbon (%)
51.28
TOTAL
100.00%
Colour of ash
Light grey
Ultimate Analysis (wt%1)
C
59.29
H
40.61
N
2.10
S
1.81
Gross calorific (KJ kg-1)
23,721.40
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Effect of on Lafia-Obi Coal Feed Size on Combustion
Bed Temperature
1050
1000
Sudden temperature drops
due to release of volatiles
950
900
Sudden temperature rise
due to ignition of volatile
Temperature (C)
850
800
10mm
15mm
20mm
25mm
Linear (10mm)
Linear (15mm)
Linear (20mm)
Linear (25mm)
750
700
650
600
0
5
10
15
20
25
Time (s)
Figure 4.2: Effects of coal particle size on
temperature at a coal feed rate of 0.2kg/min
Fluctuations in temperature
is coal particle size
dependent
As the coal particle size
increases, char combustion
rate and in consequence,
bed temperature decreases
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1200
Effect of on Lafia-Obi Coal Feed Size on Combustion Bed Temperature
cont.
0.2kg/min
0.3kg/min
1000
Temperatrure (OC)
800
600
400
200
0
2
4
6
8 10 12 14 16 18 20 2
4
6
8 10 12 14 16 18 20 2
15mm
4
6
8 10 12 14 16 18 20 2
20mm
4
6
8 10 12 14 16 18 20
25mm
Time(s)
Figure 4.4 Summary of the effects of coal particle size and feed rate on temperature24
Effects of Coal Particle Size on CO
Emission
23000
10mm
15mm
20mm
21000
25mm
Increased CO concentration
with increased feed size
Temperature(OC )
19000
17000
abrupt changes in CO
concentration
as a result of repeated char
particle fragmentation,
15000
13000
11000
9000
0
5
10
Time (s)
15
20
Reduced CO concentration
with increase in average bed
temperature
Figure 4.6: Effects of coal particle size on CO
emission at a coal feed rate of 0.3kg/min
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Effects of Coal Particle Size on CO Emission cont.
25000
0.2kg/min
0.3kg/min
CO Emission (ppm)
20000
15000
10000
5000
0
2
4
6
8 10 12 14 16 18 20 2
4
6
8 10 12 14 16 18 20 2
15mm
Time (s)
4
6
8 10 12 14 16 18 20 2
Fig. 7 Effect of bed temperature on NOx emissions
20mm
4
6
8 10 12 14 16 18 20
25mm
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Effects of Coal Particle Size on NOx
Emission
800
700
NOX formation increased with
increasing carbon conversion,
attributed to a decrease in
NOX reduction in the pores of
char particles as they shrank.
Tullin, et al., (1993)
Nox Emission (ppm)
600
500
400
NOX concentration decreases
with increasing particle size.
300
This investigation was carried
out above the critical average
diameter of the Lafia-Obi coal
Jing et al., (2007),
10mm
200
15mm
20mm
100
25mm
0
0
5
10 Time
(s)15
20
25
Figure 4.13 Effect of coal particle size on NOx
emission at a coal feed rate of 0.3kg/min
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Effects of Coal Particle Size on NOx Emission cont.
800
700
0.2kg/min
0.3kg/min
600
NOx Emission (ppm)
500
400
300
200
100
0
2
4
6
8 10 12 14 16 18 20 2
4
6
8 10 12 14 16 18 20 2
15mm
4
6
8 10 12 14 16 18 20 2
20mm
4
6
8 10 12 14 16 18 20
25mm
Time (s)
Fig. 9 Graph of flue gas temperature against NOx emissions
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Effect of Temperature on CO emission
26000
10mm
15mm
24000
20mm
25mm
Linear (10mm)
CO emission (ppm)
22000
Linear (15mm)
Linear (20mm)
Theoretically, as the
bed temperature increase,
there is an increase in the rate
of char combustion and an
increase in the CO oxidation
rate; this situation causes lower
CO concentration in the flue
gases.
Linear (25mm)
20000
18000
16000
14000
12000
700
750
800
850
900
Temperature (OC)
950
1000
1050
The level of the
scatter for the lines is an
indication of the level of
variation
exhibited
by
spontaneously varying rates of
reaction which has been
attributed
to
the
fragmentation of the coal
articles in the fluidized bed
combustor.
Figure 4.8: Effect of combustion bed temperature on
CO emission at a coal feed rate of 0.2kg/min
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Effect of Temperature on NOx Emission
550
Coal properties that affect
NOX emissions include
nitrogen content of the
coal, coal reactivity, and
mineral makeup of the
coal.
500
NOx Emission (ppm)
450
400
10mm
350
20mm
15mm
300
25mm
Linear (10mm)
Linear (20mm)
250
Linear (15mm)
Linear (25mm)
Because this experimental
procedure occurred at a
temperature lower than
10000C, the NOx measured
during this experiment
will be thermal NOx.
Lohuis et al., (1992) and
Kilpinen & Hupa (1991)
200
700
750
800
850
900
Temperature (OC)
950
1000
Figure 4.10: Effect of temperature on NOX
emission at a coal feed rate of 0.2kg/min
1050
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Effect of Fuel Feed Rate On Emission
800
18000
0.2kg/min
0.3kg/min
0.2kg/min
0.3kg/min
16000
700
14000
600
NOx Emission (ppm)
12000
CO Emission (ppm)
500
400
300
10000
8000
6000
200
4000
100
2000
0
2
4
6
8
10
12
Time (s)
14
16
18
20
Figure 4.16: Effect of feed rate on
NOx emission at a coal FED of 10mm
0
2
4
6
8
Time (s)
10
12
14
16
18
20
Figure 4.17: Effect of feed rate on CO
emission at a coal FED of 10mm
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Table 4.6: Comparison of emission levels of Lafia-Obi coal with some international standards
Source
Emission (ppm)
Lafia-Obi Feed rate 10mm
15mm
NOx
20mm
25mm
Mean
CO
10mm 15mm 20mm 25mm Mean
0.2kg/s
455.35
376.69
323.35
277.35 358.185
14,620 17,400 19,300 21,560 18,220
0.3kg/s
626.68
496.02
364.02
316.69 450.853
13,080 16,620 17,040 19,140 16,470
365.25*
870#
Chinab
350*
-
World Bankc
350*
950*
UK a
* Emission Standard for New Coal Power Plants Power with Plant Size < 300 MW
#
Emission Standard for all industrial Plant combusting coal in the United Kingdom
Source
a: Notification of the United Kingdom Ministry of Industry No. 2, B.E. 2536 (1993), issued under Factory Act B.E. 2535 (1992),
dated July 20, B.E. 2536 (1993), published in the Royal Government Gazette, Vol.109, Part 108, dated October 16, B.E. 2536
(1993) London.
b: Notification of the Chinese Ministry of Science, Technology and Environment published in the Government Gazette, Vol.113
Part 9 Page 220, dated January 30, 2539 (1996) and Notification of the Ministry of Science, Technology and Environment
published in the Government Gazette, Vol.113 Part 9 Page 220, dated January 30, 2539 (1996)
c: World Bank Group, 1988
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CONCLUSION
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The bench-scale fluidized bed combustor
methods Modified in this program has
provided relevant information for assessing
the behaviour of Lafia-Obi coal in Fluidized
Bed Combustion.
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The studies conducted revealed that LafiaObi coal has low moisture, high volatile
matter and very high fixed carbon content.
The volatile matter content, places LafiaObi in the medium-volatile bituminous rank.
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 This
study has provided useful empirical
projection of emissions of greenhouse
gases from FBC combustion of Lafia-Obi
Coal
 This data obtained is useful in application
of fluidized bed combustion for energy
production using Lafia-Obi Coal
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ThAnK
yOu