India has a number of coal-fired power plants in the 60-210 MWe range, with relatively small mills supplying a single burner row. This results in a typical burner capacity of 30MWt. Ignition of the coal flame is achieved with a centrally mounted oil burner which is also used for flame support when coal combustion is unstable. The quality of some Indian coals is relatively poor, with ash contents in excess of 40 per cent, so the quantity of support fuel consumed is considerable, with associated high costs.

There is, therefore, a significant opportunity for the supply of improved coal burners with enhanced stability and turndown capabilities, enabling operators to reduce the need for oil support fuel. Also in view of the nature of Indian coal, there is a particular requirement to reduce the number of fuel system components in the burner exposed to wear, eg the coal spreader in conventional burners.

The HISAT concept

Initially Mitsui Babcock Energy Ltd (MBEL) considered two development strategies to address the Indian market:

  • Development of the MBEL low NOx axial swirl burner to include the required stability and turndown objectives.
  • Development of a hybrid burner designed to achieve high rates of mixing of fuel and air in the near burner zone by aerodynamic means.

    The ‘hybrid burner’ strategy was selected as it offered the most direct means of achieving the objectives of the development. In addition, there is no current requirement for specific NOx reduction within the Indian market. It was also an opportunity to build on MBEL’s experience in developing a similar burner for ESKOM’s Lethabo station in South Africa, which was developed to achieve stable combustion down to 40 per cent burner load without the use of support fuel. The Lethabo burner was also designed to minimise wear components, generating the burner flame recirculation zone aerodynamically rather than mechanically and thus eliminating the wear-susceptible coal spreader. Generation of swirl in the combustion air stream was achieved by an axial generator rather than a conventional radial swirl generator. This enabled the generation of higher levels of swirl (thus enhancing flame stabilisation) for a similar system pressure loss compared to the conventional system.

    The Lethabo burner also incorporates a modified type of PF (pulverized fuel) inlet elbow, to reduce component wear in that part of the burner.

    The Lethabo plant has operated successfully for 13 years firing fuels with ash contents over 40 per cent (GCV around 16 MJ/kg) with wide turndown capability and minimal oil support.

    Prototype design

    The prototype HISAT burner was designed with the objective of guaranteeing turndown to 50 per cent load in a wall-fired configuration with Indian coals having fuel fineness in the range 65-70 per cent passing 75 microns. There was also to be minimum modification required to existing plant. Following the Lethabo concept, the large and strong internal recirculation zone at the flame centre and the rapid mixing regime was to be achieved through the following burner design features:

  • Incorporation of a large central blockage area within the secondary air stream to
  • promote the formation of central low pressure zones and encourage recirculation of hot flue gases to the flame root to provide the necessary ignition source.
  • Promotion of a relatively thin layer PF stream on a large diameter thus increasing the surface-area/volume ratio to enhance mixing rates with the external combustion air stream.
  • Selection of full load secondary and primary air stream velocities that result in sufficient velocity difference at the required turndown load to ensure high inter-stream shearing and mixing rates thus promoting rapid fuel and air mixing and resulting in intense, stable combustion.

    These features were incorporated as far as possible in the HISAT burner. In addition, to enhance burner stability, two additional design modifications were considered:

  • Incorporation of a flameholder device similar to those fitted on the MBEL low NOx burners. This device has the effect of ensuring rapid ignition of the fuel within the low stoichiometry central zone of the low NOx burner. There was some doubt as to the effectiveness of this device in the HISAT design as the fuel stream trajectory and shape and the recirculation zone formation are significantly different in the fast mixing rate HISAT burner compared to the controlled mixing rate low NOx burner. In addition there was some concern on the effects of high wear rates on a flameholder fitted to the HISAT burner.

  • Introduction of swirl to the primary-air/fuel stream. MBEL’s previous test experience with the low NOx axial swirl burner suggested that the introduction of swirl to the primary-air/fuel stream results in a wide rapidly mixing flame firmly stabilised within the burner quarl. It was felt that by introducing swirl to the primary-air/fuel stream of the HISAT burner, the effects of the lower air stream velocities and core air bluff body ratio (compared with the final version of the Lethabo burner (the Mark IV)) on flame stability and turndown could be partially mitigated and the burner performance could be moved closer to that of the Lethabo Mark IV burner. However, there was a requirement to maintain the pressure loss through the burner inlet elbow/primary air tube at a maximum value of 1.2 kPa (12 mbar) to avoid extensive draught plant modification or replacement. To achieve the degree of swirl required at low pressure loss, it was necessary to adopt a modified burner inlet elbow consisting of a banjo type elbow with an entry tangential to the outside wall of the elbow. Internally the banjo elbow was fitted with a scroll to produce the swirling PF flow and maintain low pressure loss. It was recognised that the incorporation of a scroll introduced a further wear component into the primary- air/fuel stream and the adoption of tangential entry to the elbow would require site PF pipework modification.

    CFD modelling

    The FLUENT fluid dynamics program was used primarily to investigate two design options of the basic HISAT burner, the burner elbow and the flameholder.

    The results of the comparison of conventional splashplate and banjo/scroll elbows. As expected, the splash plate elbow produced a maldistributed flow within the burner primary-air/fuel annulus with a significant proportion of the flow passing along that side of the annulus opposite the inlet pipe. This would result in a concentrated fuel stream with a high axial velocity on one side of the burner at the exit, potentially leading to reduced ignition/mixing rates of fuel and air and possibly an adverse effect on flame stability. There is also some evidence from previous work on the Lethabo burner that such a maldistribution, combined with the high swirl combustion air stream can result in undesirable furnace wall deposition.

    The banjo elbow produces a much more evenly distributed primary-air/fuel stream at the burner outlet with a significant tangential velocity component. These two factors aid the rapid and even mixing of fuel and air streams in the quarl zone where the streams are able to expand through centrifugal flow.

    Using the data produced by the FLUENT model it was decided that the banjo/scroll type of elbow was necessary to ensure good mixing of the fuel and air in the quarl zone.

    The position and size of the FLUENT calculated recirculation zones for the banjo elbow burner with and without a flameholder. With the flameholder the recirculation zone is distorted with its ‘centre’ significantly displaced off the burner centreline. It is also apparent that the strongest recirculating flows do not occur within the burner quarl and the recirculating gases, necessary to ensure stable ignition of the fuel, are significantly further away from the end of the primary-air/fuel pipe. By comparison the aerodynamic pattern resulting from the case with no flameholder fitted is substantially symmetric about the burner centreline and the recirculating flow projects back into the throat with high recirculation velocities within the burner quarl zone. On the basis of this the flameholder option was rejected.

    Combustion tests

    The prototype HISAT burner was tested at the Renfrew Large Scale Burner Test Facility in June 1998. Most of the testing was performed on Thoresby run of mine coal, with an as fired ash content of 30.5 per cent and a gross calorific value of 22.5 MJ/kg Although this is to the lower end of the range of ash content of typical Indian fuels (resulting in less onerous stability/ turndown requirements on the burner), the fuel also has a lower volatile content than the Indian fuels, imposing a more onerous ignition condition.

    The tests consisted of a series of light-up, load increase, swirl variation and load turndown tests during which the following parameters were measured:

  • Fuel and air flows to the burner.
  • O2, CO and NOx levels at the furnace exit.
  • O2 and CO levels on the centreline of the flame.
  • Flame temperatures, from the end of the primary air tube axially along the burner centreline (using a simple chromal K thermocouple mounted within an uncooled carrier tube). These data enabled us to examine the position of the flame ignition point for different burner settings and to compare this with similar measurements made on the Lethabo burner.
  • Flame images. Video recordings were made with the help of equipment and personnel from Imperial College, London. These gave better results than the normal means of recording flame images, which tends to give poor quality images when firing high ash fuels (due to the high flux of highly reflective ash material within the furnace).
  • Flame detector signals. These were continually monitored during the tests to ensure that adequate signals were obtainable. Two positions were used: the conventional position on the burner back-plate, viewing via an opening in the secondary air swirl generator blades; and a temporary position viewing through the oil burner ignitor carrier tube, along the burner centreline.

    The tests yielded the following conclusions:

  • Flame ignition. Coal ignited successfully from oil support at both maximum and minimum secondary air swirler settings.
  • Optimum secondary swirler setting. The optimum setting of the secondary air swirl generator was found to be minimum swirl.
  • Flame stability and turndown. At the optimum setting of minimum secondary air swirl the coal firing load could be reduced to 40 per cent of full load with the flame remaining stable and without the need for oil support.
  • Burner pressure drop. At optimum burner geometry (without secondary air barrel) and at minimum secondary air swirl the burner pressure drop was 13 mbar. This is about equivalent to a projected plant pressure drop across the burner of 10.4 mbar when leakage through out of service burners is taken into account.
  • Flue gas emissions and combustion efficiency. At 29 MWt and a furnace exit oxygen of 3 per cent, the emissions when firing Thoresby coal at the optimum swirler setting (minimum) were NOx 765 vppm and CO 30 vppm.
  • Flame monitoring. Flame monitoring signals of 8-10 V were obtained from the Mitsui Babcock dual signal flame detector over the load range of the burner when the flame monitor was positioned to view through the secondary air annulus.

    Towards commercial application

    The prototype development burner has now been engineered to a full commercial product capable of offering significant improvements in plant operational flexibility, furnace stability and the reduction of support fuel consumption, with resultant economic benefits.

    A number of retrofit proposals are being pursued and it is anticipated that the HISAT burner will achieve its first commercial application in the next few months.

    Scaling up for future burner tests

    the-hisat-test-programme-described-in-the-main-article-was-among-the-last-to-be-performed-on-mitsui-babcock-s-50-mw-renfrew-burner-test-facility-work-is-now-underway-on-constructing-a-3-75m-replacement-able-to-test-burners-of-up-to-80mw-thermal-capacity-the-new-facility-to-be-commissioned-in-the-second-half-of-1999-will-help-develop-prove-and-demonstrate-the-new-combustion-equipment-required-to-meet-the-progressively-reduced-emission-limits-that-will-be-set-for-fossil-fuel-fired-combustion-plant-in-t

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