Developing a high stability burner for the Indian retrofit market21 May 1999
The HISAT (High Stability and Turndown) burner is designed to be able to turn down to 50 per cent of maximum load without oil support and remain stable firing poor quality Indian coals. It has been successfully tested at Renfrew using UK coal considered representative of that found in India.
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:
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.
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:
These features were incorporated as far as possible in the HISAT burner. In addition, to enhance burner stability, two additional design modifications were considered:
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.
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:
The tests yielded the following conclusions:
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.
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