Outline of once-through boiler technologies

The Shinchi Unit No.2 Boiler for Soma Joint Electric Power Company, representing the world's first application of the vertical waterwall design to a 1,000 MW Supercritical Sliding-Pressure Operation Boiler, began its commercial operation in July, 1995, and has been operating successfully since that time.Table 1 shows the major specifications for this boiler, andFigure 1 presents a side view of its general arrangement.
This paper describes the design features and operating results of the Shinchi Unit No 2 Boiler, focusing on the unique vertical waterwall design.
Other features include advanced technologies utilized in order to burn a wide variety of coals while maintaining a superior performance on environmental protection. As noted inTable 2, which outlines the construction and trial operation schedule, a total of 13 coals (seeTable 3) were used during the trial operation.

In a supercritical boiler with sliding pressure operation, measures must be taken to deal with the deterioration of the heat transfer coefficient of water and steam in the partial load region where the fluid in the tubes becomes a two-phase flow at subcritical pressure. The spiral tube wall design shown inFigure 2 (a), using inclined boiler tubes, has been conventionally employed in order to increase mass velocity in tubes to the level required for maintaining proper heat transfer-nucleate boiling. The accompanying reduction in the number of tubes increases flow velocity by 2 to 3 times, for example, but this spiral tube wall design has relatively complicated structure, higher system pressure loss, and the higher auxiliary power consumption.
In considering these issues, MHI's attention was drawn to the superior heat transfer characteristics of rifled tubes (that promote heat transfer by means of a spirally grooved internal surface). Steam film is dispersed by means of the grooves on the inside surface of the rifled tubes, this inhibits film boiling and allows nucleate boiling to be maintained to a high steam quality level, thereby holding down metal temperatures. Accordingly, as illustrated inFigure 2 (b), the metal temperature of rifled tubes is kept sufficiently low and design flow velocity can be reduced to between 1,500 and 2,000 kg/(m2os) without encountering difficulty.
Following confirmation through laboratory testing, a vertical waterwall with rifled tubes was combined with a circular firing for practical application.

In addition to improved safety, this furnace design offers the following advantages:

The furnace design described here is already used for Kawagoe Unit No 1 and Unit No.2 (gas-fired 700 MW boilers) of Chubu Electric Power Co., Inc., for Matsuura Unit No 1 (coal-fired 700 MW boiler) of Kyushu Electric Power Co., Inc., and for Hekinan Unit No 1 (coal-fired 700 MW boiler) of Chubu Electric Power Co., Inc., accumulating a great mount of experiences. The experience gained with these installations resulted in verification in advance and design improvements that were subsequently incorporated into the design for the world first 1,000 MW of this type.
Various tests conducted during the trial operation period for this 1,000 MW boiler demonstrated that reliability was equal or better than spiral type. As shown inFigure 3, which gives fluid temperature distributions at the furnace outlet for 100% ECR in the supercritical region and for 50% ECR in the subcritical region, stable and consistent furnace outlet fluid temperature distributions were achieved.
MHI is now aiming for achieving even better performance at Tohoku Electric Power Co., Inc. Haramachi Unit No.1 and Chugoku Electric Power Co., Inc. Misumi Unit No.1 1,000 MW boilers, and MHI recommends the use of this design to the customers because of its number of advantages.

Figures 4(a) and4(b) illustrate the Low NOx Burner and the Total Low NOx Combustion System combining the in-furnace NOx reduction system, advanced MACT, and high-fineness MRS pulverizer, used in order to reduce NOx emissions and unburnt carbon in fly ash.

The results of firing tests conducted to adjust the Total Low NOx Combustion System are displayed inFigure 6. Against targets of 180 ppm for NOx and unburnt carbon in fly ash of under 5%, all test coals were well under. This was achieved even for N coal, which tends to have high NOx and unburnt carbon in fly ash figures, with test results of 150 ppm for NOx and 3% for unburnt carbon.

Figure 7 provides boiler performance test results at the acceptance test. A low excess air combustion (10 to 15% at rated capacity) was achieved due to the effectiveness of the MRS pulverizer in reducing the number of coarse particles. Meanwhile, boiler efficiency reached 91.6% with SL coal from US, against planned efficiency of 89.2%