Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2002-05-29
2003-11-18
Richter, Sheldon J. (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S678000
Reexamination Certificate
active
06647726
ABSTRACT:
BACKGROUND AND SUMMARY OF INVENTION
The present invention relates to a method of producing additional electrical energy in a boiler plant of a pulp mill. Black liquor preferably having a dry solids content of over 80% and combustion air is fed into the furnace of a chemical recovery boiler for burning black liquor and recovering chemicals therefrom. The flue gases generated in the combustion are led to an economizer of the recovery boiler, in which economizer feed water for the boiler is heated, and after the economizer to a device that cleans the flue.
Feed water flows from the economizer to the steam-generating bank of the boiler, and into the superheater for producing steam preferably having a pressure of more than 80 bar. The steam flows from the recovery boiler to a steam turbine for producing electricity. Steam discharging from the turbine is utilized for preheating the feed water flowing to the economizer. In the present application, an economizer is understood as a heat exchanger or plurality of heat exchangers in which thermal energy is transferred from the flue gases to the feed water. The scope of the invention is not limited by the use of this term to any particular design of a gas-to-liquid heat exchanger.
Waste liquor, so-called black liquor is generated when producing chemical pulp in the paper and pulping industry, waste liquor is usually combusted in a boiler recovering chemicals and heat. In a conventional recovery boiler, process chemicals are recovered by spraying the black liquor into the furnace. When being introduced into the furnace, the black liquor dries fast and burns in the beginning under reducing conditions and later under oxidizing conditions in the furnace, whereby flue gases and melt are generated. The melt is discharged from the furnace. The combustion air is fed into the recovery boiler usually at multiple levels so that in the beginning the conditions prevailing are reducing and later oxidizing. In the hot furnace the water, the volatile parts of the dry solid matter and the gasifying parts are evaporated from the liquor drops. The heat contained in the flue gases is recovered by means of heat transfer surfaces inside the boiler, e.g., in the superheater, in the steam-generating bank and in the economizers into the water flowing therein, which water exits the superheater in form of high-pressure steam. The flue gases of the recovery boiler are discharged from the economizer into the gas cleaning device. The ash of the waste liquor droplets, i.e., the inorganic matter in the waste liquor, gathers at the bottom of the boiler, forming a so-called char bed. The char bed is reduced and melted. The melt flows onto the bottom of the furnace, wherefrom it is discharged and introduced back to the digestion process via various process stages.
The production of sulfate pulp is the most important method of producing pulp in the world. During the years, the need for heat and electricity in a sulfate pulp mill has continuously decreased as a result of development work and at present the production of sulfate pulp is more than self-sufficient when energy, or at least process heat, is concerned. Energy is produced in a pulp mill mainly by combustion of black liquor in a recovery boiler and bark and wood wastes in an auxiliary boiler and oil or gas in a lime sludge reburning kiln. The bark of the raw wood material and the organic matter in the black liquor usually cover the whole energy requirement. There are also plants in which wood or bark is used as fuel for the lime reburning kiln, either as such after drying or after drying and gasifying.
At the present time, the production of energy is effected as follows: the recovery boiler and the auxiliary boiler, in which the bark generated in the mill is combusted, produce superheated high-pressure steam. The produced steam is led via one or more back pressure steam turbines and the steam from the discharge end is used to cover the heat requirement of the mill. The turbine and the generator connected thereto produce the electricity needed by the mill. Electricity is usually produced by a back pressure turbine having one or more bleedings. The back pressure used is 3-6 bar (abs.) and the bled pressure 10-16 bar (abs.). The production of electricity may also be effected by means of a condensing turbine. There are also pulp mills devoid of a gas turbine. In such a case the electricity required is supplied from outside the mill.
The recovery boiler has developed to be a reliable process for regeneration and energy production. The ratio of heat and electricity obtained by means of the recovery boiler is disadvantageous in present-day sulfate pulp mills. The heat production need may be covered by the recovery boiler better than before, due to decreased heat consumption by the processes, but the electricity yield as well as the efficiency of condensation solutions is low. Because the flue gas particles of the recovery boiler containing alkalis, sulfur and chlorine are easily fusible and tend to fasten onto the heat transfer surfaces, the recovery boiler has a risk of high-temperature corrosion and clogging. The principal way to avoid corrosion has been to choose the temperature and pressure of the steam produced to be low enough to decrease the detrimental effects of molten salt.
In steam boiler plants, the higher the steam pressure and temperature in the boiler may be raised and the lower the pressure of the steam taken from the turbine to be used for the needs of the mill, the higher the overall electrical efficiency of the plant is, i.e., the ratio between the net production of electric power and the consumption of process heat. There is a need to raise the overall electrical efficiency of the recovery boilers nearer to that of the conventional coal-fired power plants, i.e., the pressure and the temperature of the steam produced by the recovery boilers should be raised to as high a level as possible. In other industrial boilers, the conventional steam pressure/temperature is, e.g., 130 bar/540° C. In primary power plants producing only electricity, the pressure and the temperatures of the steam are even distinctly higher than that, and also intermediate superheating is practiced in them. In recovery boilers, the typical pressure of fresh steam has been 60-90 bar, but nowadays, due to many improvements of apparatuses and processes as well as better materials, significantly higher pressures and temperatures, even e.g., 120 bar and 520° C., may be achieved.
At present, the flue gases being discharged from the economizer of the recovery boiler are cooled by feed water to a temperature of less than 200° C., preferably 150-170° C. Higher final temperatures of flue gases would lead to a lower efficiency due to greater flue gas losses, as, due to impurities contained in the flue gas, the recovery boilers have not been provided with flue gas-heated preheaters of air to decrease the final temperature of the flue gas. The electricity production efficiency of the recovery boiler and the steam turbine assembly might be improved by more effective use of bled steams of the steam turbine for preheating the feed water of the boiler, for preheating the combustion air and for drying the fuel to be burned in the lime reburning kiln or the bark boiler. In conventional power plants, the preheating of the feed water is divided into a low pressure (condensate/additional water) and high-pressure parts, the feed water tank being located at the boundary thereof. In recovery plants, only the feed water tank is used for preheating the feed water. Back pressure production makes the low pressure preheating typically inappropriate and high-pressure preheating raises the temperature of the feed water so that it would lead to increased temperatures of discharging flue gases, which in turn would decrease said efficiency, unless it would be possible to cool the flue gases after the economizer in an economical way.
EP patent 724683 presents an installation of a steam boiler provided with a furnace and a steam turbine, in which the preheating of the fe
Saviharju Kari
Simonen Jorma
Andritz Oy
Nixon & Vanderhye P.C.
Richter Sheldon J.
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