ARTICLE pubs.acs.org/EF
Staged-Air Ratio Optimization for a New Down-Fired Technology within a Cold Small-Scale Model of a 350 MWe Utility Boiler Min Kuang, Zhengqi Li,* Pengfei Yang, Jinzhao Jia, and Qunyi Zhu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, People’s Republic of China ABSTRACT: In this paper, a new combustion technology based on the concept of multiple injection and multiple staging was developed especially for a down-fired pulverized-coal 350 MWe utility boiler with particularly high NOx emissions and severe asymmetric combustion. Compared to the prior technique, the new technology creates a completely unique combustion system necessitating a burner redesign and reconfiguration. To establish efficient furnace operating conditions for the reconfigured furnace, an appropriate range for the staged-air ratio must be ascertained. For this purpose, cold airflow experiments were conducted by recording aerodynamic field measurements within a small-scale model at various staged-air ratio settings (viz., 0%, 10%, 20%, 25%, 30%, and 35%). Aerodynamic fields and distributions of velocities throughout the furnace were measured, in addition to decay and penetration depths of downward airflows and overfire air. At lower staged-air ratios of 0%, 10%, 20%, and 25%, a well-formed symmetric flow field appeared in the lower furnace and the furnace throat region. Velocity distribution, as well as the decays in the downward airflow and OFA jets, also displayed well-defined symmetries along the furnace center in the zones near the front and rear walls. At higher ratios of 30% and 35%, a deflected flow field developed in the lower furnace, as well as in the furnace throat region, although this airflow was redirected higher up the front wall than for the rear wall. To establish a strongly symmetric flow field and appropriate penetration depths, a staged-air ratio of 25% was found optimal for the newly reconfigured furnace.
1. INTRODUCTION Reserves of anthracite and lean coal are abundant and globally distributed. With their low volatiles content, anthracite and lean coal present difficulties in ignition and burnout.1,2 Down-fired boilers are designed to burn anthracite and lean coal. Four types of down-fired utility boilers are manufactured: the Foster Wheeler (FW), the Babcock & Wilcox (B&W), the Stein, and the Mitsui Babcock Energy Limited (MBEL) down-fired boilers. Differences between these four types—in particular, the burner arrangements and air distribution—have been described in detail in previously published literature.3 Most of these boilers suffer from similar problems of poor stability, low burnout, and high NOx emissions. Actual operating data usually show 8%-15% carbon in the fly ash, a boiler efficiency of ∼85%-92%, and NOx emissions that are >1100 mg/m3 (at 6% O2 dry).4-16 For FW boilers, reports have appeared on aerodynamic characteristics,4,5 combustion,4,6 slagging,7 and NOx reductions by overfire air (OFA) application.8,9 Fan et al. investigated the combustion characteristics and NOx formation within a 300 MWe B&W down-fired boiler using a numerical simulation approach.10,11 Burdett conducted industrial tests to investigate the effects of air staging on NOx emissions from a 500 MWe Stein down-fired boiler unit.13 However, little research has been reported on MBEL downfired boilers, which suffer from similar problems of (a) large differences in combustion characteristics between front- and rear-wall zones (viz, asymmetric combustion);14,15 (b) high NOx emissions typically in the range of 1100-1500 mg/m3 at 6% O2, but sometimes as high as 1700 mg/m3;14,16 and (c) serious slagging in the lower furnace.16,17 Severe asymmetric combustion could produce large differences in the heat load r 2011 American Chemical Society
between the zones near the front and rear walls. This could create adverse effects in boiler operations and should be eliminated. According to previous results obtained from a 300 MWe MBEL boiler, Li et al. discovered that a deflected flow field appearing in the lower furnace resulted in large combustion differences between zones near the front and rear walls, and NOx emission could only be reduced by 13% when the staged-air dampers were opened from 30% to a fully open setting.14 Moreover, adjustment of the feed direction of staged air to a declination of 45°, instead of the original horizontally fed setup, inhibits generation of this deflected flow field.3 With regard to low-NOx combustion technology especially designed for a down-fired boiler, to date, there have only been reports on low-NOx combustion retrofit in FW boilers, one called “vent-to-OFA” and the other “combined high efficiency and low-NOx technology”.8,9 For MBEL down-fired boilers, not a single technology has been reported that gives significant reductions in NOx emissions, or anything comprehensive enough that would also eliminate asymmetric combustion and minimize slagging in the lower furnace. In this paper, we propose a new deep-air-staging technology specifically for a MBEL downfired pulverized-coal 350-MWe utility boiler. An exclusive combustion system, composed of a staged-air declination combined with a large increase in staged-air mass flow rate and overfire air application, as well as a burner redesign and reorganization, was installed in the boiler. Much work must be accomplished in the new area, such as a series of optimization experiments in staged Received: December 24, 2010 Revised: February 21, 2011 Published: March 10, 2011 1485
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Table 1. Operating Airflow Parameters of the Boiler during Cold Small-Scale Experimentsa Secondary Air a
Staged Air b
a
b
velocity
momentum flux
velocity
momentum flux
velocity
momentum flux
velocity
momentum flux
case
(m/s)
ratio (kg m/s2)
(m/s)
ratio (kg m/s2)
(m/s)
ratio (kg m/s2)
(m/s)
ratio (kg m/s2)
0%
82.05
21272.09
65.43
116.33
10%
69.59
15383.96
55.49
84.13
20.60
798.91
16.13
4.56
20%
57.13
10447.84
45.56
57.14
40.72
3195.64
32.26
18.24
25%
50.90
8336.78
40.59
45.59
50.90
4993.19
40.32
28.50
30%
44.67
6463.72
35.62
35.35
61.08
7190.19
48.38
41.04
35%
38.44
4828.66
30.65
26.41
71.26
9786.65
56.45
55.86
0
0
0
0
Note that (1) “a” and “b” denote air velocities for the design operations at full load and for the cold small-scale airflow experiments with different stagedair ratios, respectively, and (2) secondary air velocities correspond to both inner and outer secondary air velocities. a
air (viz, its declination angle and ratio) and overfire air (including its ratio, ejected angle, and positioned location), as well as on the new burner arrangement. Here, our attention is focused on the impact of staged-air ratio on the aerodynamic characteristics, and thus a staged-air ratio optimization is reported for this new technology. Optimal settings are clearly necessary for good boiler operations for several reasons: (1) a predetermined significant reduction in the particularly high NOx emissions is very likely to be affected at low staged-air ratios, because of relatively shallow staging conditions in the lower furnace; and (2) high carbon content in the fly ash and other less certain factors, such as recurrence in the flow-field deflection, may appear at especially high staged-air ratios, because of large values in the transverse momentum component of staged air. A previous article18 reported on a similar staged-air optimization for a 300-MWe MBEL down-fired boiler with 45° staged-air declination. Significant differences do exist between that investigation and the present one, mainly in two respects: (1) The previous optimization was conducted with prior MBEL art that has little relevance to the new technology under study here; and (2) The two boilers are quite distinct, with numerous differences, including electrical capacity (300 MWe vs 350 MWe), furnace configuration (especially in regard to the boiler nose), the arrangement of the fuel-lean flow nozzles, and secondary air velocity (∼30-35 m/s vs 45-50 m/s). For these details and others, see Figure 1 and Table 1 (each presented later in this work). Aerodynamic characteristics are of great importance in combustion in general and pulverized-coal combustion in particular; different fields equate to variations in the process. However, it is very difficult to collect aerodynamic parameters, such as velocities and turbulence intensities, in full-scale furnaces. Therefore, to understand aerodynamic behavior, most experiments are performed in small-scale models.3-5,11,13,19-22 The results of small-scale cold experiments show certain differences from those performed in full-scale hot furnaces. However, using certain modeling similarity criteria, aerodynamic characteristics from both can be correlated. Thus, pulverized coal combustion can be analyzed in this manner. Furthermore, in cold modeling, certain optimized parameters can be obtained with the assistance of numerical simulations. To determine the impact of stagedair ratios on the aerodynamic characteristics and then to establish an appropriate staged-air ratio range for the MBEL down-fired
boiler with the new technology, cold airflow experiments within a small-scale model furnace were conducted to investigate the aerodynamic field at different staged-air ratio settings. The results of these experiments can lead to (i) beneficial modifications of MBEL down-fired boilers already in service and (ii) new designs.
2. EXPERIMENTAL SETUP 2.1. Utility Boiler with the Prior MBEL Art. Figure 1 presents a schematic of the furnace and combustion system of the boiler. The arches separate the furnace into two regions: a rectangular upper furnace and an octagonal lower furnace with four wing walls. A total of 16 cyclones symmetrically arranged on the arches divide the primary air/ fuel mixture into fuel-rich and fuel-lean flows needed in regulating fuelbias combustion. The fuel-rich flow is vertically channeled through nozzles near the front and rear walls, while the fuel-lean flow is injected at a 15° angle inclined toward the furnace center through nozzles centered over the furnace. The fuel-rich flow nozzles and secondary-air ports are positioned closely together in an alternating manner. There are eight burners lining the front and rear arches—four fuel-rich flow nozzles and four fuel-lean flow nozzles—and eight secondary-air ports feed each burner. The air box is partitioned into two parts: one on an arch and the other below an arch. Each part has eight small boxes, with each box on the arch corresponding to one of the burners, and each box below the arch feeds a group of staged-air ports. Near-wall air is partitioned air fed to air boxes on the arches and designed to mitigate slagging on the front and rear walls. The distribution of the vertically ejected secondary air and the horizontally fed staged air is adjustable by damper openings associated with each box. 2.2. Combustion System with the New “MIMSC” Technology. As illustrated in Figure 2, the method that realizes this new technology is as follows: (1) Fuel-rich flow is channeled vertically down through nozzles centered over the furnace, while fuel-lean flow is similarly injected near the front and rear walls. The fuel-rich flow nozzles are short, rectangular in shape, and arranged in pairs side by side, to maintain strength in the fuel-rich flow; (2) Two rows of short secondary-air ports are located on arches across the breadth of the furnace, viz, one row of inner secondary-air ports positioned between the fuel-rich and fuellean flow nozzles, and another row of outer secondary-air ports located near the front and rear walls. Accordingly, secondary air is fed in a two-step process near the arches, and a first combustion stage forms in the burner region. The near-wall 1486
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Figure 1. Schematics of the furnace and combustion system of the down-fired boiler (dimensions given in millimeters). air is then canceled by outer secondary air adjacent to the front and rear walls; (3) Staged-air ports are arranged uniformly on front and rear walls across the breadth of the furnace, and a high staged-air ratio is established to lower the stoichiometry in the zone above the staged air. Next, a second combustion stage is formed. Again, staged air is fed at a set declination of 45°; and (4) Direct-flow slit OFA ports are arranged symmetrically on the front and near arches across the breadth of the furnace with a certain declination in the secondary-air box near the furnace throat to lower the stoichiometry to close to 1.0 in the zone below the OFA. The OFA ratio (viz, the ratio of OFA mass flow rate to the total air mass flow rate into the furnace) is set at 20%. With this OFA introduction a third combustion stage forms in the throat region. It is worthwhile recalling that the proposed technology is based on the air-driving mechanism derived from Bernoulli’s principle, which,
simply stated, becomes: the higher the airflow velocity, the lower the airflow static pressure. Thus, for the two parallel airflows injected into the furnace, a static pressure difference arises, depending on the difference in airflow velocities. Because of this static-pressure difference, the lower-velocity airflow is deflected toward and gradually mixes with the higher-velocity airflow. Carried by the higher-velocity airflow, the lower-velocity airflow can now penetrate further into the furnace. Here, the technology resolving the above-mentioned problems is based on three levels of air driving and a comprehensive deep staged combustion. Because of the presence of multiple airflow injections under deep air-staging, we shall call the new technology “multi-injection multistage combustion” (MIMSC). To understand the underlying principle behind MIMSC technology pictorially, a schematic diagram of the multiinjection airflows is presented in Figure 2.
2.3. Cold Small-Scale Aerodynamic Experiments and Measuring Methods. Figure 3 shows the experimental system with the 1487
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Figure 2. Schematics of the combustion system of the down-fired boiler applying the MIMSC technology. MIMSC technology. It consists of an induced-draft fan, a small-scale furnace model, and an IFA300 constant-temperature anemometer. The small-scale model furnace is a 1:15-scaled version of the original. X0 is the horizontal distance between the front and rear walls in the lower furnace. H0 is the vertical distance between the outlet of the fuel-rich flow nozzle and the upper edge of the dry bottom hopper, while H is the vertical distance from the measurement point to the outlet of the fuelrich flow nozzle. All airflow rates into the small-scale furnace were measured by Venturi tube flowmeters. Before each cold airflow experiment, all Venturi tube flowmeters were first calibrated with a L-shaped standard Pitot tube and then set to the required readings to provide airflows with estimated velocities, according to their flow coefficients. The Pitot tube was used a second time to check the actual velocities of the airflows emanating from the Venturi tube flowmeters. With the comparison of the estimated and actual velocities, measurement errors of the Venturi tube flowmeters are determined to be
