Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Patent
1992-09-14
1994-11-29
Wieder, Kenneth A.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
324639, 324647, G01N 2200
Patent
active
053693698
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to the measurement of the unburnt carbon content of fly ash produced by a coal fired boiler.
BACKGROUND ART
In the combustion of pulverised coal for steam generation in coal-fired power stations there are certain fixed losses determined for example, by plant design, and certain controllable losses caused by operating under non-ideal conditions. The controllable losses comprise: gases;
In practice the controllable losses show a minimum as a function of oxygen in the flue gas and it is preferable to operate near this minimum. One way this can be achieved is by basing control of the boiler on the measurement of oxygen and carbon monoxide in flue gas. Most large boilers today are equipped with oxygen analysers which measure O.sub.2 at one point in a duct. A problem with these analysers is that the reading is drastically distorted by air infiltration into the furnace and in the convection passages downstream of the burners. Also, as measurements are made at one point, sampling errors are large.
Carbon monoxide in flue gas stays at very low levels at high excess air and rises as excess air is reduced. Infrared CO analysers are available which direct the IR beam across the stack, thus minimising sampling errors. However, optimising excess air using CO monitors generally produces a large amount of unburnt carbon in the ash, because CO levels are very low at optimum excess air.
An alternative technique is to base control of the boiler on the determination of unburnt carbon in the fly ash. A 500 MW power station burning black coal of 20% ash will produce about 2500 tonnes/hr flue gas, and 37 tonnes/hr fly ash. The carbon content of this fly ash will be normally in the range 2-5 wt % although it may contain up to 15 wt % carbon. Typically the fly ash concentration in flue gas is about 20 g/m.sup.3. Present instruments for the determination of the carbon content of the fly ash rely on extracting a sample, typically less than 1 gram, from the duct and analysing this on a batch basis typically at 10-20 minute intervals.
One prior art carbon concentration monitor [Rupprecht and Patashnick Co., Inc, NYSERDA Report 86-2, January 1986] is based on a microbalance and small furnace. The instrument collects a 10-50 mg sample of fly ash from the outlet duct of a boiler and determines the unburnt carbon in this sample from the mass loss after heating at 750.degree. C., this measurement cycle being repeated at approximately 15 minute intervals. One disadvantage of this analysis technique is that it is very difficult to collect a representative sample of such small size, and therefore sampling uncertainty significantly limits the accuracy of the unburnt carbon determination. The analysis accuracy for replicate samples in laboratory tests was approximately .+-.0.5 wt % at 2.3 wt % carbon.
Another commercially available device [Energy and Environmental Research Corporation, 18 Mason, Irvine, Calif., USA; December 1987] for the determination of unburnt carbon in fly ash collects an approximately 1 gram sample from the duct using an isokinetic sampler and analyses this for unburnt carbon content from the measured surface reflectance of the sample. The sample collection and measurement cycle is repeated at approximately 5 minute intervals. In a plant test of the instrument at the Nefo power plant, Denmark, the analysis accuracy was approximately .+-.1 wt % at less than 3 wt % carbon and .+-.0.5 wt % at greater than 3 wt % carbon. The analysis accuracy is limited by sampling uncertainty, due to the sample size and measuring principle (i.e. surface reflectance) used, and the sensitivity of the reflectance measurement to coal type.
A device based on a measurement of the capacitance of a fly ash filled capacitor has been proposed for the determination of carbon in fly ash in Australian Patent 562440. In this arrangement ash is taken from an ash hopper using a screw conveyor, fed into a measuring chamber into the electric field established by the electrodes of a capacitor and the change in capacita
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Commonwealth Scientific and Industrial Research Organisation
Tobin Christopher
Wieder Kenneth A.
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