OPERATIONAL EXPERIENCE FROM THE FIRST 300 MWEL CFB PLANT IN THE P. R. OF CHINA

G.-N. Stamatelopoulos, J.-C. Semedard, S. Darling

ALSTOM Power

INTRODUCTION Fluidized bed combustion plants have been in successful operation for many years in the capacity range from 50 to 350 MWel. Steam generators with circulating fluidized bed (CFB) combustion have found acceptance throughout the world over the last few years, in particular for the power generation, but also as industrial power plant and combined heat and power station. The reason for this success was on the one hand that they could meet the considerably stricter anti-pollution legislation without add-on equipment, and on the other hand that fluidized bed combustion plants allow the use of a broad fuel range, including various sludges and production residues and different types of coal and biomass. CFB plants are also successful in China for the same reasons. The increase in the energy consumption in China necessitates that new power stations are provided. As the Chinese fuel market is characterized by a high share of low-volatile and high-sulfur coals, the fluidized bed technology is an excellent technology for this purpose. The construction of the 300 MWel demonstration power plant Baima in the province of Sichuan is an important milestone in this respect. Following a detailed evaluation process in the years from 1998 to 2002 the delivery contract was awarded for the boiler scope at the beginning of 2003, making the Baima plant the first 300 MWel CFB plant in China and one of the biggest worldwide. The paper focuses on the design principles applied for this boiler, describes the project schedule and realization and gives first operational feedback from the commissioning phase and the commercial operation of the plant. Furthermore, the paper will give an overview of the other 300 MWel CFB boilers under construction, or in operation in China and discusses the further increase in capacity up to 600 MWel, which will further improve the economic efficiency of these plants in combination with the introduction of supercritical steam parameters and the once-through technology.

CFB DESIGN PRINCIPLES CFB references for the utilization of a big variety of fuels exist today. Coal and lignites with water content up to 60 %, ash content up to 40 % and sulfur up to 14 % (daf) have been utilized successfully. References exist also for the utilization of slurry, sewage sludge, petroleum coke and bark. Besides the ability to burn a wide range of fuels including difficult to burn fuels and opportunity fuels, CFBs have achieved impressive environmental performance, especially low NOX emissions and economic in-furnace desulfurization by limestone addition. While post-combustion control equipment can be used with CFB technology to meet special emissions requirements, the very low NOX emissions that CFB combustion achieves without any secondary measures can be attributed to the following reasons:

• relatively low and uniform furnace temperature of approx. 850 °C to 900 °C; • air staging in the lower furnace by primary air and secondary air introduction at appropriate levels,

resulting in substoichiometric combustion conditions in the lower furnace hopper; • equal distribution of fuel and air by the intense mixing inside the furnace; • positive impact of high efficiency cyclones that result in isothermal conditions in the primary loop.

Sulfur capture is achieved through inherent capture by the fuel bound calcium and injection of prepared and dried limestone in the furnace. The crushed limestone is pneumatically fed into the CFB through adequate injection points. The particles size distribution (PSD) is an important factor in the sulfur capture process efficiency, together with carefully controlled temperature and high cyclone efficiency. When equipped with high efficient cyclones, CFB boilers are typically able to capture up to 95 % of the SO2 generated by the combustion of the fuel at moderate limestone addition rates. The cyclone sizing and geometry, which includes the design of the inlet duct, is at the heart of Alstom CFB combustion technology: the capture efficiency of the separation system is the decisive factor in maintaining the bed density and retaining the fine calcium rich particles in the primary loop. A high bed density in turn

ensures a high heat transfer and a uniform temperature in the furnace, a high contact between CaO particles and SO2 rich flue gas for optimum sulfur capture efficiency and of course the best possible combustion by keeping the fuel particles in the furnace for the longest possible time. It also has a beneficial impact on NOX emissions, as explained above. The products from the CFB furnace consist of the fuel ashes, the sulfated lime as well as some excess lime (unutilized CaO). Considering the ashes, the main differences between PC and CFB ashes are:

• total ash flow for CFB is larger due to limestone addition and desulfurization, • CFB produces less fly ash and more bottom ash than PC fired boiler, • fineness of fly ashes for CFB and PC fired furnaces is similar, • bottom ash particle sizes differ for CFB and PC (slag) • CFB ashes contain higher SO3, and CaO than PC fired ashes.

Despite the differences in composition, particle size and properties, various applications for CFB ashes have been established, as reported in many VGB research projects [1]. Commonly CFB ashes are used for re-filling of mines, as roadbed material, or as additive for cement.

THE BAIMA PROJECT IN CHINA Baima, one of world’s largest CFB boilers, is an anthracite-fired 300 MW power plant being built in the Sichuan province of the People’s Republic of China. The Baima Project includes a Transfer of Technology agreement for CFB boilers between 200 and 350 MWel from Alstom to China’s three major boiler manufacturers (Dongfang Boiler Group, Harbin Boiler Company, Shanghai Boiler Works) and to the seven members of the Chinese Electric Power Design Institute. The choice of Alstom CFB technology was based on a series of evaluations conducted several years by Chinese experts assessing both technical and economical criteria. The contract for the design and equipment supply for the Baima 300 MWel CFB boiler was signed on March 31st, 2003. According to the contract, Alstom supplied the CFB, with most of the manufacturing being done in China and subcontracted to Dongfang Boiler Group. Alstom worked closely with Dongfang Boiler, during the detailed engineering phase. The Baima plant was constructed for Sichuan Baima CFB Demonstration Power Plant Co. Ltd., whose main shareholder is State Power Grid and Sichuan Bashu Development Company. The site is located in Sichuan Province and benefits from the proximity of the Dongfang Boiler workshop. In selecting CFB technology for Baima, the owners benefit both in terms of specific operational aspects and cost. One main advantage is the ability to burn the low volatile anthracite without oil support even at low loads; the minimum load without oil support guaranteed for Baima was 35 %. This feature entails considerable fuel oil savings during part load operation. Post-combustion equipment (such as SO2 scrubbers and SNCRs) are avoided, thus positively impacting CFB investment cost and ease of operation.

BOILER DESIGN This 1025 tonnes/hr natural circulation CFB boiler was designed to fire a Chinese anthracite coal at steam conditions as shown in Table 1. The anthracite coal is characterized by a relatively low volatile matter content (8.5 as received) and a high ash content (35 % as received). The lower calorific value is 4420 kcal/kg. The detailed analysis is given in Table 1. Table 1: Baima CFB Boiler Main Data

Steam conditions at MCR Units

Main steam flow t/h 1025 Main steam pressure Bar 174 Main steam temperature °C 540

RH steam flow t/h 844 RH steam pressure Bar 37 RH steam temperature °C 540

Feed Water temperature °C 281

Design coal analysis Carbon % wt AR 49.2 Volatile matter % 8.55 Sulfur % 3.54 Ash % 35.27 Moisture % 7.69 Lower Heating Value kcal/kg 4420

Sulfur % 3.54 to 4.30Emissions level SO2 mg/mN³ ≅ 6 % O2 600 NOX mg/mN³ ≅ 6 % O2 250 Particulates mg/mN³ 100

The emission requirements for the gases discharged at the stack are shown in Table 1. No post-combustion cleaning equipment is required. The design utilizes the concepts developed and well proven by Alstom over several years of successful operation at the Provence and Red Hills plants, i.e. a pant-leg furnace, four high efficiency cyclones and four external fluidized bed heat exchangers (FBHEs), two for bed temperature control and two for reheat steam temperature control [2]. The arrangement of the CFB boiler is shown in Fig. 1.

Fig. 1: Baima CFB Arrangement

OPERATIONAL FEEDBACK The trial run was successfully passed on April 17th, 2006 and the boiler has been in commercial operation since then, following grid demand mainly between 160 and 300 MW. The coal fired was of lower quality than expected, with 42 to over 50 % ash containing stones and with a varying quality, despite coal mixing in the coal yard. Fig. 2 gives an overview of the ash content varying with time. Coal flow can vary from 130 t/h up to 210 t/h for the same power output of 300 MW in a few minutes. In spite of the obvious difficulty to optimize operation with such variable coal properties good combustion and desulfurization results were achieved.

Fig. 2: Ash content variation with time

The variability of the coal quality is also due to the high stone content of the coal. This required some adjustments of the coal preparation system, but even with those adjustment the bottom ash was very coarse – with a D50 of 1 to 3 mm – resulting in limitations in the ash extraction system. As the customer was not able to deliver better coal quality, the decision was made to replace the four fluidized bed ash coolers (FBACs) with four, more adapted to the coarse bottom ash, rotary ash coolers of Chinese design. Each rotary ash cooler was installed in the

place of the existing FBACs, which were dismantled. The inlet of the ash cooler was kept through the cone valve and a vent to the flue gas duct was installed. Finally the FBHE drains were adapted. The water quality and pressure were taken into consideration and this led to a thick shell for the cooler. Fig. 3 shows the design principle of the rotary ash coolers. Such coolers are widely used in China, but also in Europe for industrial applications and solids cooling.

Fig. 3: Rotary Ash Cooler

The operation of the rotary ash cooler was satisfactory and no operational problem arose, whatever the coal quality. The ash temperature could be kept within acceptable values and the interface with the boiler in terms of expansion and cone valve operation performed well.

Another area in the CFBs which is often linked with quality issues is the refractory in the furnace. Therefore, special care was taken in order to ensure high quality for the delivery and erection of the refractory, high portions of which were supplied directly from China. Those efforts included clear specifications during the design phase and close supervision and day-to-day quality control with on site replacement, if necessary during erection. Dry out definition and supervision was also in Alstom’s responsibility. Those efforts resulted in a very successful outcome and very few incidents with refractory failures that allowed the operator to follow a low maintenance regime. Fig. 4 gives an idea of the refractory shape after 4500 hours of operation.

Fig. 4: Status of refractory after 4500 hours

Performance tests were carried out in April 2007 and the major results compared to the contractually agreed values are summarised in Table 2. Table 2: Performance Tests Results Baima

Parameter Units Contractual Value Performance Test Efficiency based on LHV % 91.8 93.6 Min. Load without oil support % 35 35 Self consumption kW 9500 8600 SO2 Emissions at 6 % O2 mg/mN³ 600 600 Ca/S Ratio - 1.8 1.6 NOX Emissions at 6 % O2 mg/mN³ 250 100

The combustion efficiency is also very good and results in less than 3 % unburned carbon LHV basis.

DEVELOPMENT OF LARGE CFBS IN CHINA Besides the Baima demonstration project a number of 300 MWel CFBs are under construction, or already in operation in China. Table 3 shows those references. Table 3: 300 MWel CFBs in China

Name Capacity [MW]

Province

Projects under construction, or in operation Baima 1 x 300 Sichuan Kaiyuan 2 x 300 Yunnan Qinhuangdao 2 x 300 Hebei Xialongtan 2 x 300 Yunnan

Xunjiansi 2 x 300 Yunnan Pingshuo 2 x 300 Shanxi Meng Xi 2 x 300 Guangdong

Projects under Development Xuancheng 2 x 300 Anhui Huangjiaozhuang 2 x 300 Sichuan Huaibei 4 x 300 Anhui Pingshi (Shaoguan) 2 x 300 Guangfong

The deployment of the large utility-class scale CFBs in China is based on a single technology, that is transferred from Alstom to the Chinese boiler manufacturers. Alstom technology is the basis for all CFBs sold by the licensees in the 200 to 350 MWel size range. It is expected that experience in diverse coals, operation modes and site conditions will be gained in China, and will benefit markets in Asia, Europe and elsewhere.

In the coming years, feedback and experiences from all three boiler manufacturers can be shared by utilities since the CFB technology used is identical. Development based on this technology will also be shared. In order to accelerate the Transfer of Technology implementation, it has been decided that each boiler manufacturer will design and manufacture its first order in co-production with Alstom.

KAIYUAN: in October 2003, Alstom received from Harbin an order for the first of the co-production projects. This is the Kaiyuan plant (Fig. 5), in Yunnan Province. It consists of two lignite-fuelled 300 MWel CFBs. Because the units are located 1200 m above sea level, the CFBs are the largest ever constructed in terms of physical dimensions.

Fig. 5: Kaiyuan boiler in operation

The CFB boilers were designed to fire Yunnan lignite with 35 % moisture and 11 % ash. Unit 1 passed successfully the trial run in June 2006 and Unit 2 in August 2006; both are now in commercial operation. Table 4 gives an overview of the performance tests results of Yunnan lignite fired 300 MW CFBs. Table 4: Performance results of Yunnan CFB boilers (3 units: Kaiyuan, Shiaolongtan, Xiunjiangsi)

Parameter Units Contra-ctual Value

Performance

Efficiency (LHV) % 92.8 > 93

SO2 at 6 % O2 mg/mN³ 400 400 Ca/S Ratio - 2.0 2.0 NOX at 6 % O2 mg/mN³ 350 132 Carbon Loss % 0.4 0.13

FURTHER SCALE-UP AND SUPERCRITICAL DESIGN The greatest challenge for the increase of capacity to 400 up to 600 MWel is to apply the excellent operating experience gained from the CFB plants of capacity class 250 to 300 MWel to the larger plants and to accompany this capacity increase with the introduction of supercritical technology. The increase in capacity and the introduction of supercritical steam parameters in CFB plants impact some key components such as the furnace, cyclones and FBHEs.

Furnace For this size a concept with a dual grate (pant leg) is required in order to ensure adequate fluidisation and complete combustion. Three cyclones and up to three FBHEs are arranged on each side of the furnace. Fig. 6 shows the overall arrangement of a six-cyclone plant. In view of the enlargement of the furnace the following furnace limitations are to be kept:

• The furnace width is limited by the max. size of the buckstays, which is required to withstand the furnace internal pressure.

• The furnace depth is designed in such a manner that the secondary air can penetrate deeply into the furnace in any operating case. With the conventional single grate concept this requirement leads to widths of max. 10 m. In order to avoid this limitation for large furnaces, the double hopper concept was introduced.

Fig. 6: Arrangement of a supercritical fluidized bed boiler for 600 MWel

A staging of the secondary air is especially effective for high reactive fuels. The more homogeneous the oxygen distribution, the less NOX und SO2 is produced and the more effective becomes the limestone consumption and fuel burnout. Each grate section of the dual grate is provided with separate fuel and air feeding points. The secondary air is injected from the enclosing wall and also from the interior hopper wall. The separate control of the primary and secondary air in each hopper section helps to achieve a uniform fluidization, homogeneous stoichiometries and a uniform bed material inventory.

Cyclone For supercritical steam parameters the cyclones plus inlet and outlet ducts are provided with a tubed design and cooled by the superheated steam. This leads to a minimization of the refractory lining in the cyclone area. Experience with the manufacture and erection of tubed cyclones has already been gained with subcritical plants – as e.g. at Zeran B in Poland.

FBHE For the optimum utilization of the heat input to the furnace, FBHEs are used as a supplement to the heating surfaces in the furnace. In the FBHEs the circulating ashes with temperatures from 845 to 900 °C are cooled to approx. 600 °C, and the heating surfaces can be arranged as superheater, reheater or even as evaporator. The ash flow into the FBHE is controlled by an ash valve. Besides the possibility of controlling the furnace and reheater temperature without spray attemperation, another advantage of the FBHE is the high heat transfer coefficient from the ash particles to the tube banks, which leads to compact heating surfaces. Modular design enables the increase of the thermal output without any modification of the design concept. Thus, the scale-up risks are avoided. With the increase of the plant capacity the number of the FBHEs is increased. The number of FBHEs can be at most equal to the number of the cyclones.

SUMMARY CFB technology can contribute to an efficient, environmentally compatible and economic utilization of coal and other fuels. Experience gained at Baima and Kaiyuan firing anthracite and lignite paves the way towards a large diversity of the future fuels burned in large CFBs in China. It appears from the variability of Chinese coals and the potential presence of stones that careful attention must be paid to the coal range definition for proper design of the coal preparation and ash evacuation equipment for optimized operation. In recent years, CFB plants have become established in the capacity range from 250 to 350 MWel. A further capacity increase up to 600 MWel is possible and remains to be realized. The introduction of supercritical steam parameters further improve efficiency and additionally allows the reduction of CO2 emissions. In this context, CFB technology takes advantage of the design and operating experience of pulverized fuel firing plants. Particularly in the area of the materials and once-through technology the experience from pulverized fuel firing plants can be directly applied to the fluidized bed.

REFERENCES [1] Theis, K.: Kurzbericht über die Tätigkeit des VGB 2002/2003. VGB PowerTech 9/2003, pp. 46-77 [2] Semedard, J.-C.; Skowyra, R. S.; Stamatelopoulos, G.-N.: Circulating fluidised bed technology applied

to diverse fuels in Asia: Demonstrating successful Projects, Power-Gen Asia 2006, August 2006, Hong-Kong,