NB/T 10127-2018Performance design standard for furnaces and burners of large-capacity pulverized-coal-fired boilers (English PDF)

Foreword

This document was issued on 25 December 2018 by the National Energy Administration of the PRC and takes effect on 1 May 2019.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.060.30, Chinese classification J98.

It replaces JB/T 10440-2004, which is superseded.

This standard was drafted in accordance with the rules given in GB/T 1.1-2009 'Directives for standardization — Part 1: Structure and drafting of standards'.

This standard supersedes JB/T 10440-2004 'Performance design standard for furnaces and burners of large-capacity pulverized-coal-fired boilers'.

Compared with JB/T 10440-2004, the main technical changes of this standard are as follows:

— two clauses, 'General requirements' and 'Design conditions', have been added;

— the former Clause 5 'Determination of the effective furnace volume and calculation formulas of furnace thermal characteristic parameters' has been split, each part now forming a clause of its own;

— the 600 MW unit content of former Table 2 'Recommended ranges of furnace thermal characteristic parameters for tangential firing (BMCR condition)' has been modified and 1 000 MW unit content has been added;

— the 600 MW unit content of former Table 3 'Recommended ranges of burner operating parameters for tangential firing with direct-firing pulverizing system (BMCR condition)' has been modified and 1 000 MW unit content has been added;

— the 600 MW unit content of former Table 6 'Recommended ranges of furnace thermal characteristic parameters for opposed firing (BMCR condition)' has been modified and 1 000 MW unit content has been added;

— the 600 MW unit content of former Table 7 'Recommended ranges of burner operating parameters for opposed firing with direct-firing pulverizing system (BMCR condition)' has been modified and 1 000 MW unit content has been added;

— the 300 MW unit content of former Table 9 'Recommended ranges of furnace thermal characteristic parameters for W-flame firing (BMCR condition)' has been modified and 600 MW unit content has been added;

— former Table 10 'Recommended ranges of operating parameters of double-cyclone separator burners for W-flame firing (BMCR condition)' and Table 11 'Recommended ranges of operating parameters of dual-register swirl burners for W-flame firing (BMCR condition)' have been merged into Table 10 'Recommended ranges of burner air distribution parameters of W-flame boilers with direct-firing pulverizing system (BMCR condition)'; the 300 MW unit content of Tables 10 and 11 has been modified and 600 MW unit content has been added.

This standard was proposed by and is under the jurisdiction of the National Technical Committee on Boilers and Pressure Vessels of Standardization Administration of China (SAC/TC 262).

Drafting organizations of this standard: Shanghai Puhua Coal Combustion Technology Research Center, Shanghai Power Equipment Research Institute Co., Ltd., Harbin Boiler Company Limited, Shanghai Boiler Works Co., Ltd., Dongfang Electric Corporation Dongfang Boiler Group Co., Ltd., Babcock and Wilcox Beijing Co., Ltd., Wuhan Boiler Co., Ltd., Wuxi Huaguang Boiler Co., Ltd., Xi'an Thermal Power Research Institute Co., Ltd., Ha'erbin Dianzhan Shebei Chengtao Sheji Yanjiusuo Youxian Gongsi, Tsinghua University, Harbin Institute of Technology, Xi'an Jiaotong University and Huazhong University of Science and Technology.

Main drafters of this standard: Shi Hongfei, Hu Rende, Chen Duanyu, Zhang Yanjun, Zhang Jianwen, Zhang Shanying, Han Zhenjie, Wu Huadong, Mao Junhua, Lu Junfu, Zhang Hai, Sun Shaozeng, Chen Xiubin, Wang Yueming, Yu Deting, Che Defu and Zheng Chuguang.

The previous edition of the standard superseded by this standard is: JB/T 10440-2004.

Publication data

NB/T 10127-2018 is an energy industry standard of the People's Republic of China, issued by the National Energy Administration on 2018-12-25 and implemented from 2019-05-01. It is classified under ICS 27.060.30 and CCS J98.

The standard was approved by National Energy Administration Announcement No. 16 of 2018, which, in accordance with the relevant provisions of the Notice of the National Energy Administration on Issuing the Administrative Measures for Industry Standardization in the Energy Sector (Trial) and its Implementing Rules (Guonengju Keji [2009] No. 52), approved and published 204 industry standards, comprising 32 energy standards (NB) and 172 electric power standards (DL).

In the list of industry standards annexed to the announcement, NB/T 10127-2018 appears as item 28, superseding JB/T 10440-2004, with Xinhua Publishing House as publisher, approval date 2018-12-25 and implementation date 2019-05-01.

1 Scope

NB/T 10127-2018 sets the performance design rules for the furnaces and burners of large pulverised coal boilers - units of 300 MW and above - replacing JB/T 10440-2004. The furnace is where a large boiler's reliability is decided: size it too small and the ash slags on the walls and the superheater overheats; fire it badly and nitrogen oxide emissions rise, combustion is incomplete and the flame impinges on the tubes. The standard gives the scope, the normative references and the defined terms and symbols, then the coal properties that govern design - heating value, volatile content, ash content and fusion temperature, slagging and fouling indices - and how they are classified. It sets out the choice of firing system: tangential corner firing, wall firing with front or opposed burners, and down-shot W-flame firing for low-volatile anthracite. Furnace sizing follows, with the heat release rates per unit volume, per cross-sectional area and per burner zone, the furnace height and the residence time, and the limits on each by coal type. Burner design covers the arrangement, the primary, secondary and over-fire air proportions and velocities, the low-NOx staging, and the flame stabilisation needed for low-load operation. The furnace exit gas temperature, the thermal calculation and the performance guarantees and their testing complete the standard.

1.1 This standard specifies the firing modes of large-capacity pulverized-coal-fired boilers, the method for determining the effective furnace volume, the selection of furnace thermal characteristic parameters and burner design parameters, the evaluation indices of furnace and burner design performance, and other requirements related to type selection and design.

1.2 This standard is applicable to the performance design of furnaces and burners of large-capacity pulverized-coal-fired boilers matched with generating units of the 300 MW class and above (maximum continuous evaporation of the 1 000 t/h class and above); pulverized-coal-fired boilers for generating units with a capacity below 300 MW may also refer to it. This standard is not applicable to pressurized combustion boilers, cyclone-fired boilers or wet-bottom (slag-tap) pulverized-coal-fired boilers.

2 Normative references

The following documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including all amendments) applies.

GB/T 211 Determination of total moisture in coal

GB/T 212 Proximate analysis of coal

GB/T 213 Determination of calorific value of coal

GB/T 214 Determination of total sulfur in coal

GB/T 219 Determination of fusibility of coal ash

GB/T 476 Determination of carbon and hydrogen in coal

GB/T 1574 Test method for analysis of coal ash

GB/T 2565 Determination of grindability index of coal — Hardgrove method

GB/T 2900.48-2008 Electrotechnical terminology — Boiler

GB/T 3715 Terms relating to properties and analysis of coal

GB/T 10184 Performance test code for utility boiler

GB/T 16507 Water-tube boilers

GB/T 34348 Technical specification for utility boilers

TSG G0001 Boiler safety technical supervision regulation

TSG G0002 Boiler energy conservation technical supervision administration regulation

3 Terms and definitions

For the purposes of this document, the terms and definitions given in GB/T 2900.48 and GB/T 3715 and the following apply. For ease of use, some of these terms and definitions are repeated below.

3.1 effective furnace volume V: the geometric volume of the space within the furnace boundaries in which fuel combustion and effective radiant heat transfer take place. [GB/T 2900.48-2008, definition 4.3.25]

3.2 coal flammability index RW: an index characterizing how easily a coal ignites and burns stably.

3.3 coal burnout index RJ: an index characterizing how easily a coal burns out.

3.4 coal slagging index Rz: an index characterizing the slagging tendency of coal ash.

3.5 boiler minimum stable load ratio without auxiliary fuel support (BMLR): under the design coal and the conditions specified in the contract, the ratio of the minimum stable combustion load of the boiler without auxiliary fuel support to the boiler maximum continuous rating (BMCR).

4 General requirements

4.1 The safety technical requirements for the furnace and burners shall comply with TSG G0001.

4.2 The basic energy conservation requirements for the furnace and burners shall comply with TSG G0002.

4.3 The performance design of the furnace and burners shall comply with the requirements of GB/T 16507 and GB/T 34348 and shall satisfy boiler performance and operating needs.

4.4 The furnace arrangement and the firing mode shall be suited to the characteristics of the coal fired in the boiler, taking comprehensive account of stable ignition, complete combustion, prevention of fouling, slagging, corrosion and wear, and low pollutant emissions.

4.5 The furnace design shall ensure good aerodynamic conditions in the furnace and a uniform flue gas temperature field at the furnace outlet; the flue gas temperature deviation between symmetrical points on the two sides of the flue at the same elevation at the furnace outlet should not exceed 50 degrees C.

4.6 The design of the furnace and burners shall ensure a high burnout rate and a low minimum stable load ratio without auxiliary fuel support for the coal fired in the boiler.

5 Design conditions

The design conditions generally include, but are not limited to, the following:

a) boiler parameters;

b) coal quality data of the coal fired in the boiler (including coal quality data of the design coal and of the check coal), including at least the following items:

1) the name and origin of the coal; if it is a blended coal, in addition to the coal quality data of the blend, the coal quality data of each component coal and the blending ratio shall be obtained;

2) results of the proximate analysis of the coal;

3) results of the ultimate analysis of the coal;

4) results of the calorific value measurement of the coal;

5) results of the coal ash analysis;

6) results of the grindability index test of the coal.

c) the coal flammability index RW and the coal burnout index RJ shall be determined according to the method in Annex A;

d) the coal ash slagging index Rz shall be determined according to the method in Annex B;

e) relevant meteorological conditions, mainly the annual average atmospheric pressure at the boiler site (or the altitude of the site) and the corresponding atmospheric temperature;

f) the expected mode of operation (including base-load or peak-regulating operation, etc.);

g) the technical standards for design and manufacture.

6 Selection of firing mode

6.1 The selection of the firing mode is based mainly on coal quality characteristics. Coal quality characteristics shall be determined according to GB/T 211, GB/T 212, GB/T 213, GB/T 214, GB/T 219, GB/T 476, GB/T 1574 and GB/T 2565, together with the results of the special tests given in Annex A and Annex B.

6.2 The ignition stability of coal should be characterized by the coal flammability index RW. The boundaries for classifying ignition difficulty by RW determined according to Annex A are as follows:

— RW < 4.02: extremely difficult-to-ignite coal;

— 4.02 <= RW < 4.67: difficult-to-ignite coal;

— 4.67 <= RW < 5.00: medium-ignitability coal;

— 5.00 <= RW < 5.59: easy-to-ignite coal;

— RW >= 5.59: extremely easy-to-ignite coal.

The value of RW is related to the volatile matter on a dry ash-free basis Vdaf of the coal. When a test value of RW cannot be obtained and calculation formulas or charts using RW as a parameter are to be used, RW may be estimated from Vdaf; however, for coals with ash content (Aar) greater than 35% or moisture (Mar) greater than 40%, the ignition stability class determined from the estimated RW shall be lowered by one grade accordingly, for example an easy-to-ignite coal is lowered to a medium-ignitability coal.

Based on pre-furnace coal data from more than 30 large-capacity boilers, the fitted relationship between RW and Vdaf is given by formula (1): RW = 3.59 + 0.054 Vdaf, where RW is the coal flammability index and Vdaf is the volatile matter on a dry ash-free basis, in %.

6.3 The burnout difficulty of coal is characterized by the coal burnout index RJ. The boundaries for classifying burnout by RJ determined according to Annex A are as follows:

— RJ < 2.5: extremely difficult-to-burn-out coal;

— 2.5 <= RJ < 3.0: difficult-to-burn-out coal;

— 3.0 <= RJ < 4.4: medium-burnout coal;

— 4.4 <= RJ < 5.29: easy-to-burn-out coal;

— RJ >= 5.29: extremely easy-to-burn-out coal.

6.4 The slagging tendency of coal ash is characterized by the coal ash slagging index Rz. The boundaries for classifying slagging tendency by Rz determined according to Annex B are as follows:

— Rz < 1.5: low-slagging coal; 1.5 <= Rz < 2.5: medium-slagging coal; Rz >= 2.5: severely slagging coal.

6.5 The firing mode and pulverizing system of pulverized-coal-fired boilers are selected as follows:

a) For extremely easy-to-ignite coal (lignite with RW >= 5.59 or Vdaf >= 37%), tangential firing or opposed firing with a direct-firing pulverizing system should be adopted. When the as-received moisture Mar of the lignite fed to the boiler exceeds 30%, a fan mill direct-firing pulverizing system with furnace flue gas drying should be adopted in view of drying and explosion prevention needs; however, for lignite with as-received moisture Mar < 35% and net calorific value Qnet,ar > 10 MJ/kg, medium-speed coal mills may also be adopted, with a higher hot air temperature (not lower than 380 degrees C) or other measures ensuring the drying capacity of the pulverizing system.

b) For easy-to-ignite and medium-ignitability coal (bituminous coal with 4.67 <= RW < 5.59 or 20% <= Vdaf < 37%), tangential firing or opposed firing with a direct-firing pulverizing system should be adopted; when the coal is highly abrasive, a direct-firing system with ball-tube mills shall be adopted.

c) For difficult-to-ignite coal (lean coal with 4.24 <= RW < 4.67 or 12% <= Vdaf < 20%), tangential firing or opposed firing should be adopted, with an intermediate bin pulverizing system with ball-tube mills and hot-air pulverized coal conveying, a double-inlet double-outlet ball-tube mill direct-firing pulverizing system, or a medium-speed mill direct-firing pulverizing system (when medium-speed mills are adopted, rotary classifiers should be fitted).

d) For difficult-to-ignite coal (coal with 4.02 <= RW < 4.24 or 8% <= Vdaf < 12%), when strong peak-regulating low-load capability, high combustion efficiency or medium or higher coal ash slagging tendency is required, W-flame firing should be adopted; after technical and economic comparison, tangential or opposed firing may also be adopted.

e) For extremely difficult-to-ignite coal (anthracite with RW < 4.02 or Vdaf < 8%, or RJ < 2.5), W-flame firing should be adopted, preferably with an intermediate bin pulverizing system with ball-tube mills and hot-air pulverized coal conveying, a double-inlet double-outlet ball-tube mill direct-firing system, or a semi-direct-firing pulverizing system with high-temperature hot air replacement.

f) For coals liable to explosion, the necessary explosion prevention measures shall be considered when selecting the pulverizing system.

7 Method for determining the effective furnace volume

7.1 For tangentially fired boilers, the furnace outlet flue gas window section is generally defined as the imaginary plane formed vertically upward from the tip of the rear wall furnace nose to the roof tubes, as shown in Figure 1. If the transverse pitch of platen heating surfaces arranged within this imaginary plane (i.e. on the furnace side) is <= 457 mm, that platen zone shall be deducted from the effective furnace volume. For opposed-fired boilers and W-flame boilers, platen heating surfaces with a transverse pitch > 457 mm generally extend beyond the vertical plane above the nose tip, as shown in Figures 2 and 3; in this case the furnace outlet flue gas window may be moved back along the horizontal flue to the section where heating surfaces with a transverse pitch <= 457 mm appear, but it shall not go beyond the plane extended from the rear water wall (for W-type boilers, the rear water wall of the upper furnace). For tower-type boilers, the furnace outlet flue gas window is the horizontal imaginary plane formed by the centrelines of the first row of tubes, encountered along the flue gas path, whose horizontal transverse pitch is <= 457 mm, as shown in Figure 4.

7.2 In the cold ash hopper zone of the furnace, only the upper half of its height is included in the effective volume; the lower half is a dead zone and is not included in the effective furnace volume.

7.3 The furnace cross-sectional area is calculated as the rectangular plane enclosed by the centrelines of the water wall tubes. When the design includes large corner cuts (short side of the furnace corner cut l11 > square root of (l1 x l2)/10, or for W-type boilers l11 > square root of (l8 x l2)/10), as shown in Figure 3, the effective furnace volume shall be calculated as the actual volume enclosed by the corner-cut walls.

Figure 1 — Schematic diagram of the furnace structural dimensions of a tangentially fired boiler (showing the platen superheater, the furnace outlet flue gas window, the burners, the cold ash hopper calculation section and dimensions l1 x l2, l3, l4, l5, l6, l12, l13, l14, l15 and angle beta). Note: the rear pass of the tangentially fired boiler shown is a single-flue arrangement; tangentially fired boilers with dual flues and parallel damper temperature regulation also exist.

Figure 2 — Schematic diagram of the furnace structural dimensions of an opposed-fired boiler (showing the platen superheater, the furnace outlet flue gas window, the cold ash hopper calculation section and dimensions l1 x l2, l3, l4, l5, l6, l12, l13, l15 and angle beta).

Figure 3 — Schematic diagram of the furnace structural dimensions of a W-flame boiler (showing the platen superheater, the furnace outlet flue gas window, the upper furnace l7 x l2, the lower furnace l8 x l2, the cold ash hopper calculation section and dimensions l3, l6, l9, l10, l11, l12, l13, l14, l15 and angle beta).

Figure 4 — Schematic diagram of the furnace structural dimensions of a tower-type boiler (showing the furnace outlet flue gas window, the platen superheater, the cold ash hopper calculation section and dimensions l1 x l2, l3, l4, l5, l6, l13, l15 and angle beta). Note: the tower boiler shown uses tangential firing; tower boilers with opposed firing and W-type firing also exist.

The symbols used in Figures 1 to 4 are explained as follows:

l1 — furnace depth, the distance between the centrelines of the front and rear water wall tubes, m;

l2 — furnace width, the distance between the centrelines of the left and right side water wall tubes, m;

l3 — for tangentially fired and opposed-fired boilers, the distance from the centreline of the uppermost primary air nozzle (for anthracite-fired tangentially fired boilers with an intermediate bin hot-air pulverized coal conveying system, if the tertiary air nozzles are arranged above the elevation of the primary air nozzle centre, the uppermost tertiary air nozzle) to the centreline of the lowest tubes of the platens, m; for tower-type boilers, the distance from the centreline of the uppermost primary or tertiary air nozzle to the centreline of the lowest tubes of the horizontal tube bank in the furnace, m; for W-flame boilers, the vertical distance from the upper break point of the arch top to the nose tip or the platen bottom, m;

l4 — distance between the centrelines of the uppermost primary or tertiary air nozzle (see explanation of l3) and the lowest primary or tertiary air nozzle, m;

l5 — distance from the centreline of the lowest primary or tertiary air nozzle to the knuckle point of the cold ash hopper, m;

l6 — furnace height, the distance from the bottom slag outlet of the furnace to the centreline of the furnace roof tubes; for tower-type boilers, the distance from the bottom slag outlet to the centreline of the lowest tubes of the horizontal flue gas window at the furnace outlet, m;

l7 — upper furnace depth of a W-flame boiler, m;

l8 — lower furnace depth of a W-flame boiler, m;

l9 — lower furnace height of a W-flame boiler, from the bottom slag outlet to the upper break point of the arch top, m;

l10 — upper furnace height of a W-flame boiler, from the upper break point of the arch top to the centreline of the furnace roof tubes, m;

l11 — short side of the furnace corner cut, m;

l12 — height of the furnace outlet flue gas window, m;

l13 — distance from the knuckle point of the furnace cold ash hopper to the bottom slag outlet, m;

l14 — furnace nose depth, the distance from the nose tip to the centreline of the rear water wall tubes, m;

l15 — clear width of the slag outlet, the horizontal clear distance of the bottom outlet of the cold ash hopper, m;

beta — angle between the slope of the cold ash hopper and the horizontal plane, in degrees.

8 Selection of furnace thermal characteristic parameters and burner design parameters

8.1 Calculation of the main furnace thermal characteristic parameters

8.1.1 Furnace volumetric heat load qv. The furnace volumetric heat load qv is calculated by formula (2): qv = Nr / V, where qv is the furnace volumetric heat load, in kW per cubic metre, and V is the effective furnace volume, in cubic metres.

Nr is the boiler heat input, in kW. In this standard it is the product of the boiler design calculated coal consumption Bj at the boiler maximum continuous rating (BMCR) condition and the as-received net calorific value of the design coal Qnet,ar, see formula (3): Nr = Bj x Qnet,ar, where Qnet,ar is the as-received net calorific value of the coal, in kJ/kg.

Bj is the boiler design calculated coal consumption, in kg/s, calculated by formula (4): Bj = B x (1 - q4/100), where B is the boiler design coal consumption, in kg/s, and q4 is the heat loss due to unburned carbon in solid residues, in %.

8.1.2 Average residence time of pulverized coal tau. Selecting the furnace volumetric heat load from the standpoint of combustion characteristics is mainly a matter of controlling the residence time of the pulverized coal in the furnace (replaced by the average flue gas residence time tau). This standard takes the average residence time of pulverized coal as the residence time tau over the section from the centre of the uppermost primary air nozzle or tertiary air nozzle to the lower edge of the platens. The calculation method for tau is given in Annex C.

8.1.3 Furnace cross-sectional heat load qF. The furnace cross-sectional heat load qF is calculated by formula (5): qF = Nr / (1 000 F), where qF is the furnace cross-sectional heat load, in MW per square metre.

F is the furnace cross-sectional area, in square metres. Except where there are large corner cuts (see 7.3), it is calculated by formula (6): F = l1 x l2; for W-flame boilers the lower furnace cross-sectional area shall be taken, calculated by formula (7): F = l8 x l2, where l1 is the furnace depth (distance between the centrelines of the front and rear water wall tubes), l2 is the furnace width (distance between the centrelines of the left and right side water wall tubes) and l8 is the lower furnace depth of a W-flame boiler, all in m.

8.1.4 Burner zone wall heat load qHr. The burner zone wall heat load qHr is calculated by formula (8): qHr = Nr / (1 000 FHr), where qHr is the burner zone wall heat load, in MW per square metre.

FHr is the area of the furnace belt enclosed by the distance between the centrelines of the uppermost and lowest primary air nozzles or tertiary air nozzles plus 3 m, in square metres, calculated by formula (9): FHr = 2(l1 + l2) x (l4 + 3) x zeta, where l4 is the distance between the centrelines of the uppermost and lowest primary or tertiary air nozzles (see explanation of l3), in m.

zeta is the refractory belt correction factor, calculated by formula (10): zeta = 1 - 0.535 Fw / [2(l1 + l2)(l4 + 3)], where Fw is the area of the refractory belt, in square metres; for the derivation see Annex D.

The burner zone wall heat load is not calculated for W-flame boilers.

8.1.5 Requirements for tower-type boilers. Tangential, opposed and W-flame firing may all adopt the tower-type arrangement. Except for the furnace outlet flue gas window, which requires a special definition (see Clause 7), the selection of the other thermal characteristic parameters and burner design parameters depends on the firing mode adopted.

8.2 Selection of furnace thermal characteristic parameters and burner design parameters for tangential firing

8.2.1 The principles for selecting the thermal characteristic parameters are as follows:

a) the thermal parameters may be determined by analogy using the design data of a number of boilers already in operation;

b) the influence trends of unit capacity, coal ignition and burnout characteristics and coal ash slagging tendency on the values of the furnace thermal characteristic parameters are given in Table 1;

c) the furnace volumetric heat load should be selected moderately. An excessively high heat load impairs burnout and raises the furnace outlet flue gas temperature, which may cause local slagging of heating surfaces; an excessively low heat load causes an imbalance in the distribution between radiant and convective heating surfaces, with the economizer heating surface even approaching zero, which not only increases boiler manufacturing cost but is also detrimental to boiler operating performance;

d) qF and qHr together constitute the combustion intensity of the main combustion zone; therefore, when one of them (for example qF) is fixed, qHr may be increased in order to improve combustion stability;

e) the average distance between the centres of the primary air nozzles is related to the heat input of a single primary air nozzle; as the heat input increases, the average distance should be increased;

f) for lignite-fired boilers with low-temperature combustion using a pulverizing system with hot furnace flue gas drying, the average residence time of pulverized coal over the distance from the centreline of the uppermost primary or tertiary air nozzle to the lower edge of the platens shall be selected so as to ensure sufficient burnout, while also considering the increase in radiant heating surface area caused by the low combustion temperature;

g) for tangential firing with an intermediate bin pulverizing system, the location at which tertiary air is introduced into the furnace shall be considered, and measures shall be taken to enhance pulverized coal burnout and reduce NOx emissions.

Table 1 — Influence trends of unit capacity, coal ignition and burnout characteristics and coal ash slagging tendency on furnace thermal characteristic parameters (columns: qv, qF, qHr, l3, tau).

Table 1, increasing unit capacity: qv decreases, qF increases, qHr, l3 and tau show no trend.

Table 1, decreasing coal ignition performance: qF increases, qHr increases, qv, l3 and tau show no trend.

Table 1, decreasing coal burnout performance: qv decreases, qF increases, qHr increases, l3 increases, tau increases.

Table 1, increasing coal ash slagging tendency: qv decreases, qF decreases, qHr decreases, l3 increases, tau increases.

8.2.2 The recommended ranges of furnace thermal characteristic parameters (BMCR condition) for tangential firing with a direct-firing pulverizing system are given in Table 2, which lists values for the 300 MW, 600 MW and 1 000 MW classes by coal type.

Table 2, furnace volumetric heat load qv (kW per cubic metre): anthracite/lean coal 85 to 116 (300 MW), 80 to 95 (600 MW), 70 to 80 (1 000 MW); bituminous coal 90 to 118, 80 to 105, 70 to 90; lignite 75 to 90, 70 to 80, (60 to 70).

Table 2, furnace cross-sectional heat load qF (MW per square metre): anthracite/lean coal 4.5 to 5.2 (300 MW), 4.6 to 5.3 (600 MW), 4.6 to 5.5 (1 000 MW); bituminous coal 3.8 to 5.1, 4.2 to 5.4, 4.5 to 5.5; lignite 3.5 to 4.3, 3.7 to 4.5, (4.0 to 4.8).

Table 2, burner zone wall heat load qHr (MW per square metre): anthracite/lean coal 1.4 to 2.2 (300 MW), 1.8 to 2.0 (600 MW), 1.5 to 2.2 (1 000 MW); bituminous coal 1.1 to 2.1, 1.3 to 2.1, 1.2 to 2.1; lignite 1.0 to 1.5, 1.1 to 1.6, (1.1 to 1.6).

Table 2, distance l3 from the centre of the uppermost primary or tertiary air nozzle to the lower edge of the platens (m): anthracite/lean coal 17 to 21.5 (300 MW), 20 to 24 (600 MW), 22 to 28 (1 000 MW); bituminous coal 16 to 20, 17 to 22, 20 to 28; lignite 18 to 24, 22 to 25, (22 to 26).

Table 2, residence time tau from the centre of the uppermost primary or tertiary air nozzle to the lower edge of the platens (s): anthracite/lean coal 1.8 to 2.3 (300 MW), 1.9 to 2.5 (600 MW), 2.1 to 2.6 (1 000 MW); bituminous coal 1.6 to 2.2, 1.6 to 2.3, 2.3 to 2.8; lignite 2.0 to 2.6, 2.2 to 2.8, (2.2 to 3.0).

Note 1 to Table 2: the distance l3, the residence time tau and the volumetric heat load qv do not take account of the influence of low atmospheric pressure; for the correction method see 9.14.

Note 2 to Table 2: anthracite/lean coal refers to coal with RW < 4.67 (Vdaf < 20%).

Note 3 to Table 2: bituminous coal refers to coal with 4.67 <= RW < 5.59 (20% <= Vdaf < 37%).

Note 4 to Table 2: lignite refers to coal with RW >= 5.59 (Vdaf >= 37%).

Note 5 to Table 2: qHr for anthracite/lean coal does not take account of the refractory belt correction factor.

Note 6 to Table 2: the data in brackets in the table are reference values.