Power plants – Motive fluid energized by externally applied heat – Power system involving change of state
Reexamination Certificate
1999-08-24
2001-10-09
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Power system involving change of state
C060S039182, C060S039464, C110S343000, C110S216000, C110S229000, C110S245000
Reexamination Certificate
active
06298666
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a system for recovering heat from combustion gases or combustible gases produced by partial burning of combustibles. In particular, the invention relates to a heat recovery system that can be applied to the treatment of municipal solid wastes (so-called municipal wastes” or MW) or waste plastics.
The reduction of dioxins and the rendering of soot and dust innocuous are two essential requirements that must be met by recent waste incineration systems. In addition, it has been proposed that new thermal recycling systems be established that can treat wastes not only as materials to be disposed of but also as alternative energy sources.
Advanced power generation systems using municipal wastes have been developed with a view to generating electricity at a higher rate of efficiency than conventional systems in the process of burning solid wastes. According to a modified version of this system that utilizes reburning and superheating, the steam produced in a waste heat boiler is superheated to a higher temperature with a clean hot combustion gas produced by reburning combustion gas from a combustion furnace using high-grade fuel of different origin, for example, kerosine or natural gas. Such an independent superheater is used for the purpose of enhancing the efficiency of power generation with steam turbines. The advanced system of power generation from municipal waste utilizing such superheating method is under active development as being suitable for incineration facilities of a comparatively small scale.
Gases produced in the combustion of municipal wastes generally contain HCl which is generated by the combustion of polyvinyl chloride, and if the surface temperature of heat transfer pipes for heat recovery exceeds about 400° C., corrosion of these pipes due to HCl becomes pronounced. To avoid this problem, the temperature of superheated steam must be held lower than 400° C., but as a result increased efficiency of power generation with steam turbines cannot be achieved.
However, a recent study has revealed that the main cause of corrosion of heat transfer pipes is in fact the deposit of molten salts on the pipes. Municipal wastes have high concentrations of salts such as NaCl (m.p. 800° C.) and KCl (m.p. 776° C.) and, as the combustion proceeds, these salts form a fume and are deposited on the heat transfer pipes, the temperature of which is low. Since this deposit accelerates the corrosion of the heat transfer pipes, the maximum temperature of the superheated steam that can be used in the existing power generation systems using municipal wastes has been about 300° C., which will ensure that the surface temperature of heat transfer pipes can be held below about 320° C.
Table 1 compares the features of various thermal recycling systems. Obviously, for successful high-efficiency power generation and RDF (refuse-derived fuel) power generation, the use of higher-grade materials as heat transfer pipes is not sufficient and conditions preventing the above discussed corrosion problem must first be realized.
TABLE 1
Power Generating Method
Details
Features
Comments
Conventional power
The heat of combustion is recovered
Steam pressure is low because
Once a superheated steam
generation
by a waste heat boiler to generate
the superheated steam
temperature of 400° C. is
electricity using back pressure steam
temperature has conventionally
assured, high steam
turbines.
been set to a low level. As a
pressures also will be
result, the power generating
attained.
efficiency is also low. In recent
years, heated steam at a
temperature of 400° C. has been
attempted.
Highly efficient generation
New material developments have
No additional load on the
The development of
by new material
led to materials for incineration
environment, assist fuel is not
materials resistant to molten
development
furnaces and superheaters that are
required.
salt corrosion encounters
resistant to corrosive components
both technical and economic
such as hydrochloric acid which are
difficulties. It is therefore
generated in the combustion of
more important to create
refuse/wastes. This has led to
conditions that will avoid
improvements in steam conditions
corrosion.
and enhancement of power
generating efficiency.
RDF Power Generation
The addition of lime and the like to
As it is difficult to generate
Though hydrochloric acid
the waste material to produce a
electricity at a high efficiency
formation is decreased, the
solid fuel not only has the advantage
in a small-scale plant, only
measures against molten salt
of helping to prevent putrefaction
solid refuse material is
corrosion are practically at
but also helps to create more
produced. The RDF is
the same level as before. It
favorable steam conditions with a
therefore collected for
is therefore necessary to
view to achieve a higher level of
generating electricity at high
create conditions that will
power generating efficiency by
efficiency in a large-scale plant.
obviate corrosion as
dechlorination and desulfurization.
described above.
Advanced Refuse Power
Combined cycle power generation
The most effective practical use
The use of large amounts of
Generation
with gas turbine. Power is
is to introduce such a system in
high quality fuels and the
generated with a gas turbine, and
large-scale incineration systems.
economic feasibility of the
waste heat from the gas turbine is
This process requires gas
process are problems. The
utilized to superheat the steam from
turbine fuel such as natural gas.
key is whether the unit price
the refuse waste heat boiler. By this
of produced electricity is
means, the efficiency of power
increased.
generation is enhanced.
Reburning by use of an
This is included in an Advanced
This method offers a high fuel
The use of large amounts of
Additional Fuel
Refuse Power Generating system.
utilization efficiency and is
high quality fuels is
The steam from the waste heat
effective in small-scale
expensive. The key is to
boiler is superheated by using
incineration plants.
ensure that the price at
additional separate fuel in order to
which the power sold is
enhance the power generating
greater than the fuel costs.
efficiency of the steam turbine.
The advanced systems of power generation from MW involve huge construction and fuel costs and hence require thorough preliminary evaluation of process economy. Deregulation of electric utilities is a pressing need in Japan but, on the other hand, the selling price of surplus electricity is regulated to below (particularly at night). Under these circumstances, a dilemma exists in that high-efficiency power generation could increase fuel consumption and the deficit in a resultant corporate balance sheet. Some improvement is necessary from a practical viewpoint. Therefore, what is needed is the creation of an economical and rational power generation system that involves the least increase in construction cost and which also consumes less fuel, namely, a new power generation system that can avoid the corrosion problem.
The mechanism of corrosion is complicated and various factors are involved in the reaction. However, it can at least be said that the key factor in corrosion is not the HCl concentration in the gas, but whether or not NaCl (m.p. 800° C.) and KCl (m.p. 776° C.) are in such an environment that they take the form of a fume (molten mist). These salts are fused to deposit on heat transfer pipes and thereby accelerate corrosion. The molten salts will eventually become complex salts which solidify at temperatures as low as 550-650° C. and their solidification temperatures vary with the composition (or location) of municipal wastes which, in turn, would be influenced by the quantity and composition of the salts.
These are major causes of the difficulties involved in the commercial implementation of advanced or high-efficiency power generation systems using MW.
Table 2 lists representative causes of corrosion and measures for avoiding corrosion.
TABLE 2
Causes of Corrosion
Corrosion-Preventing Method
1.
Acceleration of corrosion due
Use of medium-temperature exh
Fujinami Shosaku
Hirose Tetsuhisa
Irie Masaaki
Miyoshi Norihisa
Oshita Takahiro
Ebara Corporation
Nguyen Hoang
Wenderoth , Lind & Ponack, L.L.P.
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