Seven years have passed since the world’s first low mass flux vertical tube once-through furnace was put into operation by Doosan Babcock at the Yaomeng Thermal Power Plant Unit 1 in China. That boiler replaced a boiler of another design that had become unreliable. The operating experience with the Posiflow design has been so positive that the owner has since ordered and commissioned a replacement for Unit 2’s boiler. Here’s what makes this furnace design unique.
Europe has seen an increase in demand for new coal-fired power stations over the past few years — no doubt spurred by an aging coal fleet in Western Europe and increased power demand in Eastern Europe. Many of the new coal-fired plants use supercritical boiler technology with a spiral wound furnace design. Future coal-fired power stations are considering even higher steam conditions to increase plant efficiency in order to reduce CO2 emissions per megawatt-hour produced.
Spiral Tube Furnace Design
Spiral wound furnaces have tubes that are wound at an angle of 10º to 25º around the furnace perimeter from the lower furnace inlet headers to above the burner zone. This arrangement means that each tube in the furnace passes through the various heat zones so that the heat absorption for adjacent tubes is reasonably uniform (Figure 1).
1. Improved furnace design. The typical spiral wound boiler uses tubes set on an angle of 10º to 25º from horizontal in the lower furnace section and vertical tubes in the upper furnace. Source: Doosan Babcock
A high mass flux (mass flow across a unit area expressed in units of kg/m2s) is required to maintain effective heat transfer across boiler load range as well as to avoid stratification of water and steam at minimum furnace flow. The potential of stratification or inadequate heat transfer at low loads limits the minimum once-through load (Benson load) of spiral furnaces. Also, high mass flux causes a high pressure drop in the tubes, which means feedwater pump power draw is correspondingly high.
Figure 2 illustrates a flow response calculated for various heat absorptions for a boiler wall circuit with a high mass flux, spiral wound furnace. The left graph shows that the dynamic loss (or friction loss) component is much greater in magnitude than the static loss (or hydrostatic loss). The graph to the right shows that the result of an increase in heat absorption is a reduction in tube flow. Taken together, the images in Figure 2 illustrate that furnace wall tube metal temperatures are adversely affected as tube flow is reduced when there is an above-average heat supply.
2. Negative flow response characteristic for a high mass flux system. The data were taken from a high mass flux design boiler operating at 100% MCR with a water mass flux of 1,800 kg/m2s. In this design, water flow must decrease with higher heat input to maintain system pressure loss. Source: Doosan Babcock
For spiral wound furnaces, response time is restricted by the way the furnace is supported. Because spiral wound tubes can not support their own weight, vertical support straps are required on the outside of the furnace. Thermal stresses between the tubes and the support straps limit the start-up speed of the boiler as the strap temperature lags behind the tube temperature.
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