the coking properties of coal at elevated pressures. - Argonne ...
the coking properties of coal at elevated pressures. - Argonne ... the coking properties of coal at elevated pressures. - Argonne ...
BED AGGLOMERATES FORMED BY ATMOSPHERIC FLUIDIZED BED COMBUSTION OF A NORTH DAKOTA LIGNITE Steven A. Benson, Frank R. Karner, and Gerald M. Goblirsch Grand Forks Energy Technology Center U.S. Department of Energy Grand Forks, North Dakota 58202 David W. Brekke Department of Geology University of North Dakota Grand Forks, North Dakota 58202 INTRODUCTION The goal of atmospheric fluidized bed combustion (AFBC) research at the Grand Forks Energy Technology Center is to provide a data base for design, opera- tion, process control, and emission control requirements for low-rank coals. The application of the AFBC process has the potential to solve some of the problems associated with conventional combustion. These problems are .ash fouling on heat exchange surfaces, the expense and reliability of SO2 control devices such as scrubbers, and the system sensitivity to fuel variables (moisture, Na20 concentra- tion, etc.). These problems can be reduced by the AFBC process for low-rank coals be- cause the alkaline characteristics of the ash or sorbent added directly to the com- bustion zone provides the sulfur retention, which would eliminate or reduce the need for post combustion SO2 controls. The temperatures in the combustion zone are at the right levels to provide maximum reaction of SO2 and alkali to form solid alkali sulfate waste. One problem which the fluidized bed combustion of low-rank coals seems to exhibit is a tendency toward the formation of agglomerates of the material which are used to make up the bed. Agglomerates in this case are defined as a cluster of individual bed material particles held together by a substance not yet well understood, and manifest in many differing forms. The understanding of the mechanism of formation of these agglomerates is vital to their control, and there- fore the full utilization of low-rank coal in AFBC. Once formed these agglomerates will tend to decrease heat transfer, and fluidi- zation quality resulting in poor combustion efficiency and loss of control of bed oper- ational parameters (i.e., excess air, temperature, etc.). In severe cases the forma- tion of agglomerates can lead to a forced premature shutdown of the system. While the addition of limestone, or calcium bearing materials into the fluid bed, or the forming of the bed itself by limestone particles has shown a tendency to in- hibit the formation of agglomerates, agglomerates of a severe nature have been ob- served in a bed of limestone alone, or limestone and sand particle mixtures while burning a high sodium coal for an extended period of time. In general with a high sodium coal the agglomeration of limestone bed material is dependent only on the length of run time, if the run is long enough agglomera- tion in a limestone bed will occur, and can be as devastating as those which occur with a silica bed. 174
GFETC constructed a 0.2 square meter experimental AFBC. A detailed description of the unit is given by Goblirsch and others (1). ated over a wide range of conditions as listed below: The combustor can be oper- Average bed temperature -- 700 to 982OC Superficial gas velocity -- 0.9-2.7 m/sec Excess air -- 10 to 50% Ash reinjection (% Of primary cyclone catch) -- 0 to 100%. The nominal coal feed rate is 80 kg/hr at 1.8 m/sec superficial gas velocity and 20% excess air. This paper discusses the performance of quartz or limestone as a bed material during the combusting of high sodium North Dakota lignite. The lignite is from the Beulah mine of Mercer County, North Dakota. The composite coal and coal ash anal- ysis is summarized in Table 1. The lignite was partially dried before this series of tests; its as-mined moisture content was 3630, and its heating valve 15,000 J/g. TABLE 1. TYPICAL COAL AND COAL ASH ANALYSIS OF HIGH Na BEULAH LIGNITE Ultimate Analysis , As Fired Coal Ash Analysis, % of Ash Carbon Hydrogen Nitrogen Sulfur Ash Moisture Heating Valve (8372 Btu/lb) 52.65 3 Si02 4.59 A1203 0.75 1.33 Fe203 Ti02 9.7 20.0 p205 CaO- 19,459 J/g MgO Na20 K20 so3 15.8 12.1 9.9 0.8 1 .o 17.5 6.2 8.8 0.0 27.2 Other important considerations are the operation of the combustor and how opera- tional parameters affect the performance of the bed material, sulfur retention on coal ash and bed material, and heat transfer. The most important operational para- meters of the AFBC for the tests to be discussed here are listed in Table 2. Run Number Coal Type Bed Material Average Bed Temperature (OF) Superficial Gas Velocity (M/sec) Excess Air (%) Additive Ash Reinjection (%) Coal Feed Rate (kg/hr) TABLE 2. AFBC OPERATIONAL PARAMETERS 21 81 Beulah Quartz 1467 1.8 25.49 None None 53 2281 Beulah Limestone 1460 2.0 22.65 * 100 57 2481 Beulah Limestone 1450 1.8 24.38 * 100 48 *Addition of supplemental bed material to maintain bed depth. The tendency for the bed to agglomerate has been shown through extensive testing to depend on the following parameters: 1. Bed temperature (higher temperature increases tendency) 2. Coal sodium content (increased coal sodium content shows increased severity of agglomeration) 175
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GFETC constructed a 0.2 square meter experimental AFBC. A detailed description<br />
<strong>of</strong> <strong>the</strong> unit is given by Goblirsch and o<strong>the</strong>rs (1).<br />
<strong>at</strong>ed over a wide range <strong>of</strong> conditions as listed below:<br />
The combustor can be oper-<br />
Average bed temper<strong>at</strong>ure -- 700 to 982OC<br />
Superficial gas velocity -- 0.9-2.7 m/sec<br />
Excess air -- 10 to 50%<br />
Ash reinjection<br />
(% Of primary cyclone c<strong>at</strong>ch) -- 0 to 100%.<br />
The nominal <strong>coal</strong> feed r<strong>at</strong>e is 80 kg/hr <strong>at</strong> 1.8 m/sec superficial gas velocity and 20%<br />
excess air.<br />
This paper discusses <strong>the</strong> performance <strong>of</strong> quartz or limestone as a bed m<strong>at</strong>erial<br />
during <strong>the</strong> combusting <strong>of</strong> high sodium North Dakota lignite. The lignite is from <strong>the</strong><br />
Beulah mine <strong>of</strong> Mercer County, North Dakota. The composite <strong>coal</strong> and <strong>coal</strong> ash anal-<br />
ysis is summarized in Table 1. The lignite was partially dried before this series <strong>of</strong><br />
tests; its as-mined moisture content was 3630, and its he<strong>at</strong>ing valve 15,000 J/g.<br />
TABLE 1. TYPICAL COAL AND COAL ASH ANALYSIS<br />
OF HIGH Na BEULAH LIGNITE<br />
Ultim<strong>at</strong>e Analysis , As Fired Coal Ash Analysis, % <strong>of</strong> Ash<br />
Carbon<br />
Hydrogen<br />
Nitrogen<br />
Sulfur<br />
Ash<br />
Moisture<br />
He<strong>at</strong>ing Valve<br />
(8372 Btu/lb)<br />
52.65 3 Si02<br />
4.59 A1203<br />
0.75<br />
1.33<br />
Fe203<br />
Ti02<br />
9.7<br />
20.0<br />
p205<br />
CaO-<br />
19,459 J/g MgO<br />
Na20<br />
K20<br />
so3<br />
15.8<br />
12.1<br />
9.9<br />
0.8<br />
1 .o<br />
17.5<br />
6.2<br />
8.8<br />
0.0<br />
27.2<br />
O<strong>the</strong>r important consider<strong>at</strong>ions are <strong>the</strong> oper<strong>at</strong>ion <strong>of</strong> <strong>the</strong> combustor and how opera-<br />
tional parameters affect <strong>the</strong> performance <strong>of</strong> <strong>the</strong> bed m<strong>at</strong>erial, sulfur retention on<br />
<strong>coal</strong> ash and bed m<strong>at</strong>erial, and he<strong>at</strong> transfer. The most important oper<strong>at</strong>ional para-<br />
meters <strong>of</strong> <strong>the</strong> AFBC for <strong>the</strong> tests to be discussed here are listed in Table 2.<br />
Run Number<br />
Coal Type<br />
Bed M<strong>at</strong>erial<br />
Average Bed Temper<strong>at</strong>ure (OF)<br />
Superficial Gas Velocity (M/sec)<br />
Excess Air (%)<br />
Additive<br />
Ash Reinjection (%)<br />
Coal Feed R<strong>at</strong>e (kg/hr)<br />
TABLE 2. AFBC OPERATIONAL PARAMETERS<br />
21 81<br />
Beulah<br />
Quartz<br />
1467<br />
1.8<br />
25.49<br />
None<br />
None<br />
53<br />
2281<br />
Beulah<br />
Limestone<br />
1460<br />
2.0<br />
22.65<br />
*<br />
100<br />
57<br />
2481<br />
Beulah<br />
Limestone<br />
1450<br />
1.8<br />
24.38<br />
*<br />
100<br />
48<br />
*Addition <strong>of</strong> supplemental bed m<strong>at</strong>erial to maintain bed depth.<br />
The tendency for <strong>the</strong> bed to agglomer<strong>at</strong>e has been shown through extensive<br />
testing to depend on <strong>the</strong> following parameters:<br />
1. Bed temper<strong>at</strong>ure (higher temper<strong>at</strong>ure increases tendency)<br />
2. Coal sodium content (increased <strong>coal</strong> sodium content shows increased<br />
severity <strong>of</strong> agglomer<strong>at</strong>ion)<br />
175
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