Coalex탄과 국내 무연탄 혼소시의 연소 특성

김성철, 이현동, 김태형, 김영주, 박권순

전력연구원 발전연구실

Combustion Characteristics of Blended Coal for Coalex Coal & Anthracite

Sung-Chul Kim, Hyun-Dong Lee, Tai-Hyeung Kim,

Young-Zoo Kim, Kwon-Soon Park

Power Generation Laboratory

Korea Electric Power Research Institute

INTRODUCTION

Coal burning power plants are comprised of 27.7% for the total installed capacity of 47,053MW in KEPCO. As of 2000, coal consumption in coal fired power plants amounts to 30.8 million tons. It is anticipated that the share of the plants will be gradually increased up to 36% and coal consumption will increase 47% by the year of 2005. Coal is relatively inexpensive and thus, effective for utility power generation. However, the coal-fired power plants generate a considerable amount of emissions and has been a target of environmental regulations[1]. Therefore, coal selection should be based not only on the cost of the coal per unit combustion energy but also the cost caused by any adverse impacts on environmental problems. To meet the environmental regulations, coal blending is practiced at power plants..  However, coal blending can affect the boiler performance, therefore, blending is conducted in such a way that properties of blended coals are close to those of the design coal for the power plant in consideration. It is thus required in advance to evaluate coals to see if the coals satisfy the coal specifications, to find optimum combustion conditions for power plants to be used, and to meet stringent environmental regulations.  Combustion characteristics of the coals have been determined by utilizing several indirect methods including proximate and ultimate analyses, thermogravimetric analysis, and drop tube furnace tests. However, it is believed that direct combustion test with pilot-scale furnace would be the best way to simulate combustion phenomena of real utility boiler[2,3]. Combustion tests have been conducted by blending of bituminous coal imported from Australia and korean domestic anthracite under the opposed-wall firing mode.

The main purpose to construct this facility is to evaluate the coal combustion characteristics on utility boiler. Both heat and mass balances were performed to provide information to design the furnace[4]. The facility can be divided into three major systems; coal handling, burner and heat removal, and flue gas treatment systems.  In addition, instrumentation and distributed control system is implemented.

COMBUSTION TEST RESULTS

Prior to test run of the facility, the test coals are analyzed, and the results are shown in Table 1.

Table 1. Analysis of the test coal

Property		Bituminous (Coalex)	Anthracite (Korea)
Proximate analysis (wt %)	Moist F.C V.M Ash	1.72 54.83 27.15 16.30	2.81 48.53 5.97 42.69
Ultimate Analysis (wt %)	C H O N S Ash	68.12 4.17 9.32 1.36 0.45 16.59	51.05 0.52 3.88 0.31 0.65 43.93
Ash analysis (wt %)	SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3	64.68 23.63 3.69 0.43 0.36 0.07 1.59 0.18	54.57 27.75 5.29 2.04 1.01 0.36 4.77 1.93
Calorific value (kcal/kg) HGI IDT(□)		7,023 52 1,550	4,596 80 Ｂ1,550

According to the results, Coalex coal, imported from Australia, has the characteristics of bituminous coal that has fixed carbon content of 54.83%, volatile matter of 27.15%.  On the other hand, anthracite has 48.53% fixed carbon, 5.97% volatile matter and 42.69% ash.

Combustion tests were conducted using the blended coal between Coalex coal and Anthracite. Coal feeding rate is 120~130kg/hr (Input calorie base 842,760kcal/hr) at the inside pressure of the furnace of +5~10□Aq, and combustion test runs are conducted by changing the excess air ratio, 2.0% and 4.0%.

Heat loss from the furnace is measured and the results are summarized in Table 2.  The amount of unburned carbon loss is decreased as the excess air ratio is increased, meanwhile unburned carbon loss is increased as the anthracite coal blending ratio is increased. However, flue gas loss is increased as the excess air ratio is increased. Sum of flue gas heat loss and unburned carbon loss is increased in proportion to the increase of anthracite blending ratio.

Measured temperature histories of flue gas after heat exchanger are given in Fig. 1. The results show that flue gas temperature increased from 100℃ to 150℃ under the excess O2 2.0% condition. Temperature of flue gas at the excess air ratio on 4.0% is higher than that of 2.0%, and this provide the increase of heat loss by increase of the amount of flue gas.

Table 2. Comparison of Heat loss with blending ratio

Blending ratio	Excess air ratio (%)	Heat loss (kcal/kg)
		Flue gas	Unburned carbon	Sum
100:0	2.0	230.3	60.3	290.6
	4.0	240.0	47.5	287.5
90:10	2.0	244.4	79.5	323.9
	4.0	343.8	73.2	417.0
80:20	2.0	232.1	158.1	390.2
	4.0	290.1	101.1	391.2

(a) Excess O2 2.0%                         (b) Excess O2 4.0%

Fig. 1. Measured temperature histories of flue gas

To measure the emission impacts, SOx/NOx analyzer and opacity meters are installed at the outlet of the furnace, and the inlet/outlet of electrostatic precipitator(EP). The analytical results of opacity and SOx/NOx emission levels summarized in Table 3.  From the results of EP inlet/outlet opacity, efficiency of electrostatic precipitator is estimated to be approximately up to 99.76%. Opacities at the outlet of electrostatic precipitator are increased with the increase of anthracite coal blending ratio. Sulfur content of Coalex is 0.45% and anthracite is 0.65%, but SOx emission is almost constant. From this result, conversion ratio of SOx of anthracite is lower than that of bituminous coal. NOx emission of blended coal(Coalex 80% + anthracite coal 20%, O2 4%) decreases 26ppm compared to Coalex coal itself firing on 4.0% excess O2 condition and NOx emission is increased approximately 10~30ppm as the excess O2 ratio increases from 2.0% to 4.0%, regardless of blending ratio.

Table 3. Comparisons of emission characteristics with blending ratio

Blending ratio	Excess O2 ratio(%)	EP inlet opacity(%)	SOx(ppm)	NOx(ppm)
100:0	2.0	27.0	252	142
	4.0	26.0	247	176
90:10	2.0	40.0	253	134
	4.0	41.0	247	141
80:20	2.0	40.0	245	126
	4.0	40.0	238	150

CONCLUSIONS

Combustion test in the pilot-scale test facility have been conducted by blending Coalex coal and anthracite. Coal feeding rate is 120 ~ 130 kg/hr at the inside pressure of the furnace of +5 mmAq, and combustion test runs are conducted at the excess air ratio of 2.0 % and 4.0 %.  Total heat loss caused by flue gas and unburned carbon is increased in proportion to the increase of anthracite blending ratio. Flue gas temperature increased from 100℃ to 150℃ and temperature of flue gas at the excess air ratio of 4.0% is higher than that of 2.0 %. Opacities at the outlet of electrostatic precipitator increase with the increase of anthracite blending ratio. Sulfur content of Coalex is 0.45% and anthracite is 0.65%, but SOx emission concentration is almost constant. From this result conversion ratio sulfur to sulfur dioxide for anthracite is lower than that of Coalex coal . NOx concentration of blending coal (Coalex 80% + anthracite 20%, O2 4%) is decreased 26 ppm compared to Coalex itself firing and increases approximately 10 ~ 30 ppm as the excess O2 ratio increases from 2.0% to 4.0%.

REFERENCES

1. U.S. Department of Energy, "Clean Coal Technology Demonstration Program:         Project Fact sheets," Report No. DOE/FE-0351, 1996.

2. Johnson, T. R. and Scotter, J. G., "Application of Pilot-Scale Coal Testing to         Utility Boilers," EPRI Report CS-5946, 1988.

3. Baker, J., et al., "Construction and Calibration of the Australian Coal Combustion     Testing and Research Facility,"Commonwealth of Australia Report No.               NERDDP/EG/87/613, 1986.

4. Li, K. W. and Priddy, A. P., Power Plant System Design, Wiley & Sons, New       York, NY, pp. 138-187, 1985.