Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-04-02
2004-01-13
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S700000
Reexamination Certificate
active
06677765
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the detection of ammonia or to the detection of the ill effects of ammonia in flue gas.
BACKGROUND
Ammonia is in common use today as a reactant for the removal of nitrogen oxides from gas streams. When it is injected it reacts with nitric oxide (NO) to form N
2
and H
2
O and thereby reduces the emissions of the undesirable nitrogen oxides. It is usually used in concentrations about as large as the NO concentration.
Two common methods are used to speed the reactions between ammonia NH
3
and NO. In one case a high temperature is used. Temperatures of about 1600° F. to 1900° F. are used to speed the reaction. After this reaction the gases, if they are from combustion in a boiler, pass through several heat exchange devices and they eventually exit the stack at about 270° F. to 370° F. The gases from some older boilers may exit the stack at higher temperatures, but for efficiency it is necessary to have low stack temperatures. This type of process is known as Thermal deNOx or Selective Non-Catalytic Reduction (SNCR). In another case a catalyst is used to speed the reaction. Even so the catalyst bed may be at around 700° F. This process is known as Selective Catalytic Reduction (SCR). Subsequent to the reduction, the gas is cooled to the same temperatures as in the Thermal deNOx systems.
In either of these processes some of the NH
3
passes through the reaction zone and out of the stack along with the flue gas. It is undesirable to have NH
3
in the flue gas as it is seen as an undesirable emission and in many places there are regulations limiting the NH
3
emissions. The odor of ammonia is objectionable. When the ammonia is too high some will be absorbed by the fly ash. The ash then has the odor of ammonia and must be disposed of rather than used in concrete. This adds an expense to the operation of the boiler.
NH
3
also reacts with chlorine or hydrochloric acid, either of which may be in flue gas from the combustion of coal and are usually in the combustion of refuse and some wood waste. The reaction forms ammonium chloride (NH
4
Cl). NH
4
Cl forms as a very fine particle or fume, which makes an objectionable visible emission. Also, the NH
4
Cl can plug various heat exchange devices and stop the flow of the flue gas.
Most importantly the NH
3
reacts with sulfur trioxide (SO
3
) to form ammonium sulfate ((NH
4
)
2
SO
4
) or ammonium bisulfate (NH
4
)HSO
4
. They both can plug heat transfer devices especially the regenerative air heater, which has many small passages. This plugging can restrict the flow of the air and the flue gas so severely that the boiler must be taken off-line and the air heater cleaned. The ammonium bisulfate is much the worst offender of the two as it is very sticky through much of the exhaust gas temperature range. The problem may be exacerbated by the SCR process that converts some of the SO
2
to SO
3
. Also, the ammonia may react with SO
2
and oxygen to form these ammonium sulfates. Since the SO
2
is much more abundant (about 50 to 1) than the SO
3
all of the ammonia present in flue gas is likely to react and form an ammonium sulfate.
By whatever process the ammonia reacts or how ammonium sulfates are formed, all of the ammonium salts that are present in the flue gas can cause fouling of heat exchanger surfaces and plugging of the heat exchangers. This shuts down the operation. Ammonium bisulfate is the worst offender since it melts at 296° F. Ammonium sulfate is solid to 955° F. where it decomposes and ammonium chloride is solid to 662° F. where it decomposes. Thus, while any of these salts can cause fouling, only the ammonium bisulfate will exist in the liquid state in the boiler and the liquid is the source of the greatest fouling problem.
The fouling of heat transfer surfaces by liquid ammonium bisulfate and the solid particles that are imbedded in the liquid can become very severe at temperatures above 296° F. The ammonium bisulfate decomposes at high temperatures, maybe as high as 914° F. and the fouling problem could extend to temperatures this high.
Ammonium bisulfate, which is often called ammonium acid sulfate is acidic and can cause corrosion especially in the presence of water. Thus, the molten ammonium bisulfate may cause a water dew-point at temperatures significantly above its melting point of 296° F. In that case, the water present in the flue gas is induced, by the ammonium bisulfate, to condense at higher and higher temperatures where it should not normally condense. This water dew point further aggravates the fouling tendencies of the ammonium bisulfate in that it allows for the condensation of a sticky water-soluble material which in turn causes fly ash to also accumulate (or foul) at temperatures above 296° F. How far above this temperature that fouling occurs is a measure of the fouling tendency caused by an excess of ammonia.
To eliminate the plugging it is necessary to eliminate the formation of ammonium sulfates in the flue gas. The sulfur as well as chlorine are in the fuel whether it is coal, oil, waste or other combustible material. Consequently, to prevent heat exchanger plugging as well as to reduce emissions of ammonia and prevent this source of corrosion it is necessary to reduce the amount of ammonia (i.e., ammonia slip) that passes through the SNCR or SCR process. To do this, it is very important to be able to measure either the ammonia slip or the resulting fouling tendency. Typically, ammonia is introduced into flue gas at multiple locations around the circumference of the stack to reduce NO
x
. The problem with reducing the ammonia is that it is essential for the NO
x
reduction processes. Because ammonia is introduced at multiple locations and because of turbulence and cross currents in the flue gas, the concentration of ammonia may be too high in one location and not high enough in another part of the process. A measurement device is needed to find the ammonia concentrations on a spatial basis and in real time. Instruments that are available for the measurement of ammonia have not been reliable and wet chemical analysis of the gas for ammonia is too slow for control purposes. If there were a reliable instrument and method for measuring ammonia slip on a spatial basis in real time, the ammonia slip measurement could then be used to optimize the spatial injection of ammonia into any combination of SNCR and SCR processes.
SUMMARY OF THE INVENTION
We provide a method of measuring ammonia in flue gas by using a cooled probe to measure conductivity (and corrosion) caused by condensed ammonium bisulfate. The process will work for any fuel with a significant concentration of sulfur, i.e., where there is potential for the fouling and corrosion problem to occur. The method and probe may also reveal those circumstances where there is no problem in fouling or corrosion occurring. It will be most useful in furnaces or boilers where the operator is injecting ammonia to reduce NO
x
emissions. The use of this method and device will allow the boiler operator to use as much ammonia as is required to substantially eliminate NO emissions without fear of fouling the boiler back passes and without excessive ammonia emissions.
We prefer to provide a tubular probe having spaced apart bands or patches of the same material as the probe body. The bands or patches are attached to the probe body by an electrically insulating, high temperature material. At least one thermocouple is attached to the probe. A series of cooling tubes are provided within the probe body to direct cold air to the regions near each band. One or more probes is placed in the furnace or boiler above the ammonia injection zone. When ammonium bisulfate forms on the probe it completes an electrical circuit between the probe body and the bands. Hence, the presence of ammonium bisulfate can be detected by a change in resistance between the bands and the probe body. The ammonium bisulfate will also cause corrosion of the probe. Electrochemical noise is generated during the corrosion process. A monitor connected to the probe
Breen Bernard P.
Eden David
Gabrielson James E.
Benson Walter
Buchanan Ingersoll P.C.
ESA Corrosion Solutions, LLC
Le N.
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