Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2000-04-13
2002-05-07
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S670000
Reexamination Certificate
active
06381962
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and apparatus for generating electric power by combusting waste products. More particularly, it relates to a method and apparatus for generating electric power at high efficiency from a high-temperature and high-pressure steam which is produced by using the heat of exhaust gas generated when various waste products are combusted, while avoiding heat exchanger problems caused by corrosion due to high-temperature molten salt.
BACKGROUND ART
It is expected that in the 21st century, the treatment of various waste products will change greatly from simple incineration to dioxin-free recycling system which can recover energy at high efficiency. Because more efforts will be directed to sorting waste products, the recycling technology according to the type of waste product will be desired. For example, a gasification and melt combustion system which is capable of simultaneously coping with the control of dioxin and melting of ash by using combustion heat generated when waste products are combusted is becoming the main option for treating general waste products. A chemical recycling technology which is capable of converting plastic-based waste products into raw material for chemicals is becoming the main option for treating plastic-based waste products. Highly efficient power generation by combustion of waste products will require a power generation efficiency of 30% or higher.
Present thermal recycling systems for generating electric power by using thermal energy produced by the combustion of wastes is generally carried out by recovering the combustion heat of waste products in the form of steam using a waste heat boiler and supplying the steam to a steam turbine to generate electric power by a generator driven by the steam turbine.
One example of a conventional power generation system based on waste incineration is shown in
FIG. 4
of the accompanying drawings. As shown in
FIG. 4
, waste products are combusted in an incinerator or a gasification and melt combustion furnace
11
, and the heat of generated exhaust gas is recovered by a waste heat boiler
13
, which produces superheated steam. The superheated steam is supplied to a steam turbine
15
to which a generator is directly coupled, thus generating electric power. The generated electric power is consumed in the waste incineration facility, and is sold to the power company. The exhaust gas that has passed through the waste heat boiler
13
flows through a preheater
16
(such as an economizer) and a bag filter
17
, and then is discharged through a stack into the atmosphere as low-temperature clean gas.
The efficiency of power generation in the conventional steam-turbine power generation system greatly depends on the temperature of the superheated steam supplied to the steam turbine. The efficiency of power generation is remarkably increased as the temperature of the superheated steam is increased. In the conventional practical system for power generating by waste incineration, the temperature of the superheated steam has been about 400° C. at maximum for the following reason, and the efficiency of power generation has been about 20% at most.
Heretofore, the temperature of the superheated steam has not been increased beyond about 400° C. because of corrosion caused by corrosive gas such as hydrogen chloride produced during waste combustion and corrosion caused by a high-temperature molten salt. As for the waste heat boiler, since saturated steam having a relatively low temperature, e.g. about 310° C. flows through water pipes even under the pressure of about 100 kg/cm
2
, corrosion of the water pipes can be prevented even if metal pipes are used. However, in the case of superheated steam, since the temperature of superheated steam is a high temperature of 400° C. or higher, the surface of the heat transfer metal pipes will be damaged by corrosion caused by corrosive component such as high-temperature molten salt.
The mechanism of corrosion is complicated and the corrosive reaction is affected by a combination of various factors. The most important factor for the corrosion of the heat transfer tube is whether the heat transfer tube is exposed to an environment containing molten salt of NaC1 and/or KC1, rather than the concentration of HC1. Under this environment, salts are melted, adhere to the heat transfer tubes and accelerate the corrosion thereof.
FIG. 5
shows different forms of corrosion depending on the temperature of exhaust gas (represented on the horizontal axis) produced by combustion of wastes and the surface temperature of a heat transfer tube (represented on the vertical axis), deduced from the long experience of the inventors and a corrosion test using a municipal waste incinerator. As shown in
FIG. 5
, there are four forms, i.e., “intense corrosion region”, “corrosion progress region”, “corrosion retardation region”, and “corrosion-free region”. When the temperature of superheated steam is 400° C., the surface temperature of the heat transfer tube is about 430° C. which is higher than the temperature of superheated steam by about 30° C. At this temperature, the temperature of exhaust gas of 600° C. or thereabout is considered to be a boundary temperature separating the “corrosion progress region” and the “corrosion-free region” from each other. This coincides with the fact that when the temperature of exhaust gas entering a boiler bank where steam pipes are gathered closely is 600° C. or higher in the waste heat boiler of a municipal waste incinerator, salts adhere to heat transfer tubes causing exhaust gas passages to be clogged. Therefore, the boundary temperature between when salts are melted or solidified is considered to be about 600° C. This temperature corresponds to the melting point of complex salts. The melting point of NaC1 is 800° C., and the melting point of KC1 is 776° C. However, salts turn into complex salts after being melted, and their melting point is lowered, e.g., to the range of 550 to 650° C. This melting point varies with the properties of the waste products which differ from place to place. For example, this boundary temperature may be lower than 600° C. in local cities close to seashores, because salts are present at higher concentration in the waste products. Even if the temperature of exhaust gas is in the range of 500 to 600° C., when the surface temperature of the heat transfer tube is equal to or higher than about 430° C., its environment belongs to the “corrosion retardation region”, and the heat transfer tube suffers slight corrosion, less intense than molten-salt corrosion. Accordingly, the selection of the superheater tube material for use at the boundary temperature is of importance. Inasmuch as the surface temperature of the heat transfer tube is about 30° C. higher than the temperature of the superheated steam, the temperature of the superheated steam, which is about 400° C., is considered to be an allowable temperature limit for preventing corrosion. If the temperature of the superheated steam is 400° C., the pressure of the steam is suppressed to about 3.9 MPa on account of the problem of a turbine drain attack. Hence, the efficiency of power generation is only about 20% in the waste incinerating power generating system.
In order to obtain superheated steam at a temperature of 400° C. or higher while avoiding the “corrosion progress region”, it is necessary to install superheated steam pipes in the condition that the temperature of exhaust gas is in the range, of 500 to 600° C. Such a method is disadvantageous, however, in that since the temperature difference between the exhaust gas and the superheated steam is small, in order to obtain a desired amount of heat transfer, the required heat transfer surface is too large, resulting in a large-size heat recovery facility.
On the other hand, attempts have heretofore been made to develop corrosion-resistant metallic materials for the purpose of higher power generation efficiency by increasing the steam temperature without corrosion of the heat transfer tube. However, suc
Hirose Tetsuhisa
Nakata Nobuo
Ohshita Takahiro
Takahashi Koichi
Ebara Corporation
Nguyen Hoang
Wenderoth , Lind & Ponack, L.L.P.
LandOfFree
Method and apparatus for generating electric power by... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for generating electric power by..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for generating electric power by... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2876449