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 2011Faculty Of Technology Conference

INTRODUCTIONINTRODUCTION

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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 coalCoal 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 Pressurized fluidized Bed combined cycle for electricical power generation using coal electricical power generation using coal (Sambo,2009)(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. 04/21/23 7

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 PRELIMINARY LABORATORY STUDIES STUDIES

Raw Material Acquisition and HandlingSample PreparationDetermination of Physical

Characteristics

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PROXIMATE ANALYSISPROXIMATE ANALYSIS

Volatile MatterMoisture ContentFixed Carbon Ash Content

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ULTIMATE ANALYSISULTIMATE ANALYSIS

Hydrogen ContentCarbon ContentNitrogen ContentSulfur Content Oxygen Content

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FUEL PREPARATIONFUEL PREPARATION

Sun DryingSeparationCrushing (using hammer mill) Sieving (using ISO standard sieve set)

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Summary of ASTM international standards used in Summary of ASTM international standards used in investigationinvestigation

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 forTotal Moisture in Coal

6 D3175 – 07 Standard Test Method forVolatile 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 2002)

Standard Test Methods for Carbon and 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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COMBUSTIONCOMBUSTION

04/21/23 17Fig. 1 Experimental Setup

COMBUSTIONCOMBUSTION

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DATA COLLECTIONDATA COLLECTIONTemperature- 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 EXPERIMENTAL CONDITIONSCONDITIONS

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Table 1: Summary of Experimental conditionsDesign parameters Material/value

Type of fuel/ feed size Lafia-Obi/(5-25mm)

Bed material / size (µm) Sandstone/ 350-500

Bed temperature (oC) 750-1200Static bed height (m) 0.1Fuel feed rate (kg/min) 0.2 & 0.3Bed diameter (mm) 150Fluidization Velocity(l/min) 350-2000Pressure drop across distributor plate (mmH2O) 43

Pressure drop across bed (mmH2O) 428.8

Distributor plate    No. of holes 311  Diameter of holes (mm) 1.5  Thickness (mm) 4

RESULTS

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Proximate & Ultimate Proximate & Ultimate AnalysisAnalysis

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Analysis ValueProximate Analysis    Moisture (wt %) 2.91  Volatile matter (% 1) 27.19  Ash (% db) 18.62  Fixed carbon (%) 51.28

  TOTAL100.00%

  Colour of ash Light greyUltimate Analysis (wt%1)    C 59.29  H 40.61  N 2.10  S 1.81Gross calorific (KJ kg-1) 23,721.40

Table 4.1: Analysis of Lafia-Obi Coal

Sudden temperature drops due to release of volatiles

Sudden temperature rise due to ignition of volatile

Fluctuations in temperature is coal particle size dependent

As the coal particle size increases, char combustion rate and in consequence, bed temperature decreases

Figure 4.2: Effects of coal particle size on temperature at a coal feed rate of 0.2kg/min

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Effect of on Lafia-Obi Coal Feed Size on Combustion Bed Temperature

Figure 4.4 Summary of the effects of coal particle size and feed rate on temperature04/21/23

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Effect of on Lafia-Obi Coal Feed Size on Combustion Bed Temperature cont.

Increased CO concentration with increased feed size

abrupt changes in CO concentration as a result of repeated char particle fragmentation,

Reduced CO concentration with increase in average bed temperature

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Figure 4.6: Effects of coal particle size on CO emission at a coal feed rate of 0.3kg/min

Effects of Coal Particle Size on CO Emission

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Effects of Coal Particle Size on CO Emission cont.Effects of Coal Particle Size on CO Emission cont.

Fig. 7 Effect of bed temperature on NOx emissions

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 concentration decreases with increasing particle size.

This investigation was carried out above the critical average diameter of the Lafia-Obi coal Jing et al., (2007),

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Figure 4.13 Effect of coal particle size on NOx emission at a coal feed rate of 0.3kg/min

Effects of Coal Particle Size on NOx Emission

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Fig. 9 Graph of flue gas temperature against NOx emissions

Effects of Coal Particle Size on NOx Emission cont.Effects of Coal Particle Size on NOx Emission cont.

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.

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.

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Effect of Temperature on CO emission

Figure 4.8: Effect of combustion bed temperature on CO emission at a coal feed rate of 0.2kg/min

Coal properties that affect NOX emissions include nitrogen content of the coal, coal reactivity, and mineral makeup of the coal.

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)

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Figure 4.10: Effect of temperature on NOX emission at a coal feed rate of 0.2kg/min

Effect of Temperature on NOx Emission

Effect of Fuel Feed Rate On Effect of Fuel Feed Rate On EmissionEmission

Figure 4.16: Effect of feed rate on NOx emission at a coal FED of 10mm

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

* 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

Source Emission (ppm)

NOx CO

Lafia-Obi Feed rate 10mm 15mm 20mm 25mm Mean 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

UK a 365.25* 870#

Chinab 350* -

World Bankc 350* 950*

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 Lafia-Obi coal has low moisture, high volatile matter and very high fixed carbon content. The volatile matter content, places Lafia-Obi 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

ThAnK ThAnK yOuyOu