Ultra Supercritical Boiler
Ultra Supercritical Boiler
Ultra Supercritical Boiler
Abstract
Once-through utility (OTU) boilers have the advantage that they do not rely on the density
difference between water and steam to provide the driving force for water circulation to cool and
protect the furnace enclosure evaporator tubes. With the boiler feedpump providing the driving
force for circulation, the OTU boiler can be operated at pressures well above the critical point
[3202 psia (220 bar)], and, in combination with elevated steam temperatures [>1100°F (593°C)],
can significantly increase the efficiency of a Rankine cycle power plant. This increase in plant
efficiency reduces fuel consumption for a given output, thus proportionally reducing all pollutant
and waste streams including CO2 emissions. However, the benefits of elevated steam
conditions must be balanced with plant cost, reliability, and operational flexibility. The
advantages of ultra-supercritical boiler technology have been demonstrated in the high gas
temperature, high heat flux environment of suspension-fired boilers with units in operation
having capacities in excess of 1000 MW e with steam at approximately 1130°F (610°C), 4350
psia (300 bar). The process environment and features of the circulating fluidized bed (CFB)
boiler has the potential to extend the limits of the OTU technology and also further reduce
emission levels. Described in this paper is the design of an 800 MW e ultra-supercritical CFB
boiler, which is part of an on-going CFB boiler conceptual design study program jointly funded
by the US Department Energy and Foster Wheeler. The feasibility of achieving the future target
of 700°C (~1300°F) steam temperatures, in the relatively low [~1600°F (871°C)] gas-side
temperature environment of the CFB furnace, is also discussed.
Introduction
A U.S. Department of Energy and Foster Wheeler technology R&D partnership is
completing a conceptual design study (Ultra-Supercritical CFB Boiler Conceptual
Design Study, DE-FC26-03NT41737) which is jointly funded by the U.S. Department of
Energy and Foster Wheeler. The two primary objectives of the study are:
(1) to determine the economic viability of Ultra-Supercritical OTU CFB Technology and
(2) identify pathways for the diffusion of Ultra-Supercritical OTU Technology into CFB
Technology.
Design Basis
The site conditions, fuel, sorbent, and steam cycle conditions, as well as the
emission levels to which the CFB boiler was designed are summarized in Table 1.
Also included are significant heat and material balance, and plant performance
parameters. The emission targets are typical of current regulation expectations. An
SNCR is included for NOx control. Sulfur capture is achieved with limestone addition
to the furnace. The limestone consumption and plant efficiency are values without
an ash hydration system, a backend polishing scrubber, or flue gas heat recovery
system (Ref. 2) included. These enhancements are beyond the scope of this study,
and are options which can be included for improved performance.
SH I
To turbine
From turbine
RH I
Water/Steam
separators RH II
INTREX To turbine
Furnace
To flash tank
INTREX-
chambers
Economizer II
Economizer I
HP-heaters
From feed tank
Feed water pump
(ft.)
0 0 5
HEIGHT(ft.)
120
0
because the overall increase in INTREXTM
FURNACE HEIGHT
100 % % % 0
duty lowered the furnace exit gas temperature
80 %
which resulted in a lower temperature
FURNACE
difference for heat transfer in the upper cells. 60
2,200
PC FEGT
TEMPERATURE (F)
2,000
~850 F
1,800
1,000
30 40 50 60 70 80 90 100
LOAD (% MCR)
The Case 2 800 MWe design shown in Figures 3 and 4 was modified with the
following changes:
With these changes, the full load furnace exit gas temperature is 904°C (1659°F),
evaporator inlet water is sufficiently subcooled over the load range, maximum
evaporator outlet superheat is about 200 kJ/kg (86 Btu/lb), and full reheat steam
temperature is maintained down to approximately 45% load [717°C (1323°F) at 40%
load].
Conclusions
The 800 MW e Ultra-Supercritical boiler described in this paper integrates the current
state-of-the-art for both CFB (Second Generation Compact) and OTU (BENSON
Vertical) boiler technologies. The integration of these technologies provides fuel
firing flexibility, low grade fuel firing capability, low pollutant emissions, and high
efficiency for cost effective power production. Conceptually the CFB process can be
used to achieve the future high temperature goals for ultra-supercritical boiler
technology if features such as described in this paper (internal and external solids
circulation, stacked-bed INTREXTM heat exchanger, parallel pass HRA) are used.
Component selection options and several means to adjust where and how much
heat is absorbed give the CFB boiler the advantage to push OTU technology to its
limits for cost effective and environmentally friendly power production.
Acknowledgement
The financial support of this study by U.S. Department of Energy National Energy
Technology Laboratory under Contract DE-FC-26-03NT41737, and Foster Wheeler
North America Corp., is appreciatively acknowledged.
Nomenclature
CFB Circulating fluidized bed
ECO Economizer
FEGT Furnace exit gas temperature
HDR Header
HP High pressure
HRA Heat recovery area
INTREXTM Integrated recycle heat exchanger
OTU Once-through utility
P.A. Primary air
PC Pulverized coal
PSH Primary superheater
RH Reheater
S/C Stripper cooler
SNCR Selective non-catalytic reduction
SH-I Primary superheater in outboard HRA pass
SH-III Intermediate superheater in lower INTREXTM cell
SH-IV Finishing superheater in upper INTREXTM cells
References
1. S. J. Goidich, S. Wu, Z. Fan, A. C. Bose, "Design Aspects of the Ultra-
Supercritical CFB Boiler," International Pittsburgh Coal Conference,
Pittsburgh, PA, September 12-15, 2005.