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Innovative supercritical boilers for near term global markets

Exploitable results

In recent years, comprehensive investigations were performed on optimisation of HP separators in the Siemens BENSON test rig. The HP separator must have a high separation efficiency in re-circulation mode on the one hand, and a low pressure drop in BENSON boiler operation on the other. Investigations focused on the impact of fluid mass velocity, the inlet diameter, the length of the flow path over which separation occurs and the vortex breaker on the pressure drop and efficiency. In the course of these investigations it was possible to find a common optimum for the mechanical design parameters. Using these results, the next step was to define the configuration and dimensions of the separator for the operating conditions considered for the project ISB-2000.
Two innovative 600MWe class supercritical coal-fired boiler variants of the Mitsui Babcock two-pass boiler design were generated as reference designs, both incorporating novel cost reduction features and the most advanced commercially available materials for boiler pressure parts: Traditional spiral wound smooth bore tube furnace arrangement. Vertical internally ribbed tube furnace arrangement with low fluid mass flux. The 'state-of-the-art' reference designs are applicable for the major coal-fired advanced power generation markets of Asia, China in particular, and also emerging major players such as the US. The designs maximise energy efficiency, reduce fuel consumption and inherently deliver greater CO2 reduction capability, whilst at the same time being realisable in the near-term with boiler steam conditions of approximately of 300bar/600°C/620°C. For improved plant cost, operability and flexibility, the supercritical boiler reference designs developed incorporate several innovative features which include the latest commercially available materials, vertical internally ribbed tubing, improved steam/water separator design, improved mechanical details and integrated fuel re-burn for control of NOx emissions. These improvements considered to be relatively incremental and hence of limited technical and economic risk, will enable the supercritical plant to compete far more effectively with sub-critical plant in the Asian market. For demonstration of this advanced innovative technology, it will be necessary to focus on securing a final customer and candidate site. To overcome this barrier, collaboration with local Chinese design institutes and boilermakers is required as well as taking advantage of the collaboration agreements which already exist between Chinese authorities and the European Commission and UK industry.
The HF boiler is a revolutionary steam generator concept, the fundamental feasibility of which has already been confirmed in several studies. One of the key drivers to develop the HF boiler is the aim to reduce the height of the overall boiler and length of the interconnecting pipe work between boiler and turbine. The HF boiler almost halves the overall boiler height from about 63m for a typical two-pass once through supercritical boiler down to about 35m. The design of the furnace layout has been the subject of extensive CFD analysis to optimise burning performance and establish heat flux data as a basis for thermo-hydraulic calculations of the evaporator. During the course of the ISB 2000 project it was verified that the HF boiler can be designed for materials that are available today and the steam parameters in question. The new thermo-hydraulic concept for a vertical-tubed furnace allows the design of a boiler with a dramatic reduction in overall height, as indicated above. The advantages of this horizontal, low profile design include reduced cost of structural steelwork, straightforward installation, short installation time due to parallel installation of the furnace, lateral pass and second pass.
The results consists on the methodology used to analyse the design of NOx formation in pulverised coal fired boilers. The methodology starts from the definition of a basic furnace configuration based on previous experience. This definition may have several options, regarding the position, shape and orientation of flow ports. For each possible configuration, CFD (Computational Fluid Dynamics) based on models are used to analyse the flow, gas and particle mixing, heat transfer and the formation of NOx. Several post-processing techniques were developed to evaluate the contribution from individual burners for carbon-in-ash, gas mixing parameters and tools were produced to perform mass and energy balances in arbitrary volumes. The set of tools with an appropriate judgement of the results, allow the selection of the most suitable configuration in terms of carbon-in-ash, NOx, formation and radiation heat transfer distribution. The methodology can be used as a service to projects for new boilers or for the retrofit of existing boilers.
Using rifled furnace tubing can considerably enhance heat transfer in fossil-fuelled steam generators. As the heat transfer characteristics of rifled tubes depend both on the tube geometry selected and on the planned service conditions, it is most practical to validate steam generator design by the performance of suitable experiments. A rifled tube was prepared and instrumented in the BENSON test rig at SIEMENS Power Generation for this purpose. Accounting for one-side heating in boiler design, a test tube has been installed which was heated with a sophisticated circumferential heat flux profile. This profile is equal to that of a real boiler furnace wall at its heated side. During the evaluation of the steady state tests tube friction factors and friction pressure losses were determined for the purpose of investigating pressure drop. Wall temperature profiles, heat transfer coefficients and critical boiling conditions were determined in order to investigate heat transfer. Summarizing the heat transfer and pressure drop experiments it can be concluded that the tested rifled tube represents an evaporator tube with an excellent heat transfer behaviour.
The two variants of the 600MWe class two-pass coal-fired supercritical boiler generated, i.e. traditional spiral wound furnace and vertical internally ribbed tube furnace, incorporate improved mechanical details to reduce capital costs. As a novel cost reduction feature both boilers have been designed without a vestibule between the furnace and the second pass. This novel feature introduces the need for innovative support of the common rear furnace wall and second pass front wall. The spiral wound furnace requires a vertical support strap arrangement to carry the vertical boiler loads of the lower furnace to the upper furnace and the slings. A welded strap design is chosen for the front and sidewalls, but further consideration is required with regard to supporting the rear furnace wall. A conceptual hot strap support design has therefore been developed to carry the load of the lower furnace when subjected to the hot flue gases of the second pass. The vertical tube furnace does not require a support strap arrangement to carry the vertical loads of the lower furnace and therefore the hot structure support arrangement has followed conventional buckstay design, as in natural circulation sub-critical boiler support arrangements. The novel cost reduction feature is applicable to both sub-critical and supercritical two-pass coal-fired utility boiler designs and is considered to be of limited technical and economic risk.