FI124679B - Method and apparatus for controlling the combustion in a boiler - Google Patents

Description

The present invention relates to a method for real-time monitoring of a combustion event, for example in a fluidized bed boiler, according to the preamble of claim 1. The invention is particularly suitable for controlling the combustion event of fuels of variable composition and difficult to burn. These include waste fuels and biomass.

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The invention also relates to an arrangement for carrying out the method.

Fluidized bed combustion in its various forms is the most common combustion method in small and medium-sized boilers. In such a boiler, air, air / flue gas mixture or other gas is blown from the bottom of the boiler to a bed of sand or other non-combustible material, which then floats above the bottom of the boiler. Fuel is fed to this 20 beds. Fluidized bed combustion is suitable for a variety of fuel types, both at different times and at the same time. Due to the heat stored in the fluidized bed, the combustion event is stable even with varying fuel quality, which has a particular advantage in the combustion of biofuels. Hulidioksi-

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As a result of the 25 trade, demand for biofuels on the market is increasing and their prices are rising. The competitiveness of peat is also burdened by the trade in carbon dioxide. At the same time, a large number of different waste fuels and cultivated

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LO biomasses, which are significantly more problematic to burn

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g 30 than peat] a wood. Thus, fuel price differentials are widening and economic reasons compel production plants to switch to increased use of problematic fuels.

Combustion of biofuels can be problematic due to the high alkali metal and chlorine content of the fuel. Volatile alkalis, such as potassium and sodium, react readily in the vapor phase with chlorine to form corrosive and dirty compounds that also increase the sensitivity of the fluidized bed to Sint. From the vapor phase, the alkali metals also condense from the hot flue gases to the cooler surfaces of the superheaters. As a result, a dirty layer that is difficult to clean is formed on the surfaces. To control fouling, fuels containing harmful compounds must be kept low enough. The most difficult situation arises if the alkali chlorides condense from the gas phase to the surfaces of the thermal surfaces, whereby the chlorine component, which is the worst in terms of corrosion, can be enriched in the dirt layer and damage the metal protecting oxide layer.

The alkali concentration can be controlled by analyzing the fuel or by taking samples of the flue gases and analyzing the samples in the laboratory. This is often sufficient if the fuel composition is stable and well known. When fuel quality and contents vary, there is a need for a real-q or at least a reasonably fast concentration.

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ok for 25 measurements. US 5432090 discloses a method for real-time measurement of the cp 1 alkali content of a boiler

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x by measuring the intensity and spectrum of the light emitted by the flame. US 5432090 and US 6298913 have

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g illustrates similar systems in which the flue gases in m § 30 are conveyed to a flame which is monitored optically. In the solution described in Japanese Patent Publication No. JP 2004286380, the combustion event is indirectly monitored by a corrosion sensor 3 and the fuel supply is changed based on sensor detection. In the solution of EP 0 603 571 the analysis is made from ash.

5 It is quite difficult to measure directly the gas components of the flue gases as elements. Thus, another method of measuring their number would be necessary.

It is an object of the present invention to provide a method 10 for indirectly measuring the gas phase properties of a boiler, in particular its soiling properties.

In particular, it is an object of one embodiment of the invention to provide a method by which the amount of evaporated alkali metal 15 can be analyzed for boiler flue gases.

It is an object of one embodiment of the invention to provide a method for controlling the alkali metal content of combustion gases.

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It is another object of the present invention to provide a method for controlling sulfur emissions from a combustion event by dispensing volatile alkali.

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The invention is based on taking a sample of the flue gases, cp ^, which is introduced into the scrubber and measuring at least one chemical or physical property of the detergent medium, whereby cc provides an indication of changes in the gas phase concentrations.

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Specifically, the method 00 of the invention is characterized by what is set forth in the characterizing part of claim 1.

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The arrangement according to the invention, in turn, is characterized by what is stated in the characterizing part of claim 10.

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The invention provides considerable advantages.

Immediate benefits can be achieved through improved boiler utilization and reduced maintenance costs. However, the greatest benefit is achieved in mixed combustion where the optimization of the fuel mixture ratio can be done safely by measurement based chemical indices. This will maximize the share of the cheapest fuel grades, which can yield up to tens of percent of fuel costs. The co-incineration of waste fuels is of particular benefit.

Another use of the invention is to control sulfur emissions by means of volatile alkalis (K, Na). The uncontrolled use of alkaline dosing currently prevents application because even a slight overdose leads to contamination of thermal surfaces.

δ c \ j ok 25 As emissions and availability requirements become more stringent, and as fuel options and fuel qualities increase, the quality of the fuel is no longer described by its name, but the chemical composition of the fuel must be known to

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g the combustion properties of the substance are determined. fuel

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§ 30 and chemical analysis of its ash is done in the laboratory, o 00 When chemical analysis data is available, the calorific value of the fuel, the composition of the flue gases 5 and other information related to the combustion process can be calculated. Based on the chemical analysis of the parameters may also be determined with the boiler flue gas side problems can be predicted. The problems monitored by the indices 5 are e.g. fouling of thermal surfaces, corrosion of thermal surfaces and sintering of fluidized bed. Other things that can be determined by chemical analysis include: calorific value of the fuel, parameters describing the combustion conditions (such as adiabatic combustion temperature) and flue gas emissions prior to purification.

The invention enables the development of a continuously evolving fuel chemistry management system. At its core is a measurement technology that can real-time determine the gas chemistry of combustion products. The sites to be measured are selected from different parts of the boiler hearth and flue. The measurement data is analyzed on the basis of known theory and practical comparative data in gas chemistry. Benchmarking is both generic and boiler-specific so that new experiences can be directly utilized.

By means of the invention it is possible to provide continuous control of the fuel mixture feed so that contamination, utilization of a cheap amount of fuel and, for example, control of sulfur emissions

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Qk 25 is optimized. Significant savings in cp ^ are achieved with cheaper fuels. Contamination control allows longer operating cycles and service intervals. invention

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method is particularly needed for biofuels; and

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g in waste fuel installations. The above-

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For these reasons, their use will continue to increase and in the Nordic countries small and medium-sized boiler plants using these fuels will be based on fluidized bed technology.

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Thus, the invention is particularly applicable to such plants, but it can of course be utilized in other types of boilers as well.

The invention will now be explained in more detail by means of the accompanying drawing, which schematically shows an arrangement according to the invention.

Problems with the heating surfaces of the steam boiler are due to reactions of alkali-10 (Na, K) and chlorine. The idea of the invention is to evaluate, through an indirect measurement procedure, how much of these components are gaseous in the furnace. In addition, efforts are made to determine the concentration of sulfur oxides, which are also gases, in the furnace. The reactions between gases can be estimated by their concentrations (partial pressures of gases in the furnace) and reaction equilibrium. The calcareous alkaline earth metal (Ca) is in the solid phase of the furnace and its reactions cannot and need not be evaluated by this method. Instead, the volatile metals 20 (zinc, aluminum, chromium, copper) can be estimated. These metals can cause problems because they partially vaporize in the furnace and react with gaseous chlorine to cause fouling and harmful emissions.

c \ j 1 2 5 oo ^ cp ^ The flue gas discharge line 2 leaves the furnace 1 on a ceramic or x 3 filter. The filter 3 separates solids from the flue gases cc, so that only m g of the harmful gaseous substances, particularly alk. Next, the flue gases are led to the water circuit 00 00 where the water is circulated by pump 4. The water is cold so that the substances in the gas phase are condensed in the water. The circulation speed of the water 7 and the amount of gas taken out of the furnace are kept constant, whereby the electrical conductivity of the circulating water is changed as a function of the alkali vapor content of the flue gases. The change in electrical conductivity can further be used as a control criterion for controlling the composition of the fuel mixture or mixing an additive with the fuel. The change in electrical conductivity is measured by sensor 5 and the measurement result is processed by computer 6. Water can be added to the circuit via valve 7.

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Instead of just a conductivity sensor, other sensors can be incorporated into the system to provide a more complete fuel chemistry management system. Usually the most important measured value is pH and, in addition, the conductivity of electricity-15. Other measurable values include al-calicity. In principle, any quantity measurable from the medium by the analytical methods can be measured from the scrubber detergent medium. For example, if the detergent medium is water, standard assays and sensors for water chemistry may be used for analysis. Particularly preferably, the process water from the plant is used as the medium for the scrubber, whereby the values measured for the process water can be considered as reference values for the water treated with the scrubber. The scrubbing medium 't q may be other than water and the washing may be performed in several steps, if necessary with a variety of media. To improve the selectivity of the measurement and increase the sensitivity,

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sex detergent can be treated by various methods (such as ion exchange, reverse osmosis, buffering, chemical dosing, stripping, etc.).

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§ 30 o ^ The flue gases are taken from the part of the boiler furnace or flue duct where the measurement data is desired, for example, when measuring the concentrations of precious metals, they are sampled in the area where they are in gas, but combustion reactions have already taken place. The sampling point is particularly influenced by the temperature, which is different in different parts of the furnace and flue gas duct. Most reactions in the furnace and flue gas are temperature dependent, so the sampling point must be selected so that the sample is obtained at a temperature suitable for the reactions. In the gas phase, alkali metals react with chlorine to form corrosive compounds. On the other hand, if there is enough sulfur in this area, they will react with sulfur and form harmless compounds. In addition to alkali metals, it is possible to measure the proportion of chlorine, sulfur and sulfur oxides in the sample for a more complete analysis. In this case, of course, the apparatus 15 must be provided with the necessary sensing.

The invention can be applied to both single-fuel and multi-fuel boilers. In single-fuel boilers, the invention 20 can control the feed of additives and, in multi-fuel boilers, also the mixture ratios of the fuel mixture. In both cases, the objective is that the process conditions at the incineration process are somehow maintained in an area where no harmful compounds of chlorine and alkali metals are formed.

In either case, the combustion temperature may additionally or alternatively be lowered. The method operates such that when the sensor or sensor system detects a change in pH or at least an increase in electrical conductivity from the boiler-specific maximum to be determined, the control system starts to take steps to change the process conditions 00. These measures may include reducing the amount of chlorine-containing fuel, reducing the amount of alkali metal-containing 9 fuel, modifying the additive material, or modifying the combustion temperature. The method does not require very accurate measurement data, but it is sufficient to provide an indication that the process is moving or has entered a harmful area, after which a correction can be made to the control values. Because the measurement is continuous, the effect of the control change is immediately visible. Adjustment of the combustion event to the desired process values can be made based on the measurement result using conventional control techniques.

The advantages of the invention in multi-fuel combustion are illustrated by the following example. It uses waste (REF) fuel in conjunction with peat.

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Peat (example price 7 € / MWh) can burn a certain amount of cheap (example price 0 € / MWh) REF fuel with a maximum chlorine content of 0.15%. Due to the risk of corrosion of the boiler surfaces, the maximum proportion of REF-20 fuel must be limited to 10%. The system can set a maximum safety percentage for REF fuel, so its amount can be limited or increased according to the momentary changes in peat quality and REF quality. If

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qq 25 plants have a 100 MW fuel output and an annual operating time of cp ^ 6000 h, would result in an increase in REF fuel share from 10% to 20%, saving € 420,000 / year.

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Further savings can be made eg. lower maintenance costs

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g and better usability.

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In addition to the above, the present invention has other embodiments.

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The flue gas can be mixed with clean water or other suitable washing liquid or even gas instead of process water. Most importantly, changes in the composition of the sa-5 gas can be measured in the wash fluid. By flue gas scrubber is used herein a device in which a quantity of sample gas is mixed with the assay liquid used as a detergent medium. Instead of direct mixing, the flue gases can be led into contact or interaction through, for example, a semipermeable membrane, whereby complete mixing does not occur.

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Claims (14)

A method for controlling the combustion of a boiler, comprising removing from the boiler a combustion gas for analyzing the amount of alkali metal vaporized, characterized in that the gas is sampled to measure the amount of alkali metals in the boiler furnace or flue gas duct where they have gas an apparatus for mixing a sample of 10 combustion gases with an analytical fluid used as a detergent in a scrubber and measuring at least one chemical or physical property of the scrubber detergent to indirectly evaluate the properties of the gas phase. Method according to claim 1, characterized in that the combustion gas is drawn from the boiler (1) in a continuous flow. Method according to one of Claims 1 to 2, characterized in that the concentration of the detergent in the detergent medium is at least one contaminated or discharged. 't A method according to any one of claims 1 to 3, characterized in that the mixture of detergent and flame retardants is determined to detect at least pH of at least one contamination or change in the concentration of the pollutant C 1 x emissions. 30 Method according to one of Claims 1 to 3, characterized in that at least one conductivity of the mixture of detergent and flue gases is determined to detect a change in the content of at least one harmful substance. Method according to one of Claims 1 to 5, characterized in that at least the pH or electrical conductivity of the mixture of detergent and combustion gases is determined to detect a change in the concentration of alkali metals. Method according to one of Claims 1 to 7, characterized in that the flue gases are filtered before being mixed with the detergent medium. Process according to one of Claims 1 to 7, characterized in that process water is used as the washing medium. Method according to one of Claims 1 to 8, characterized in that the flue gases are contacted with the detergent medium, for example through a semipermeable membrane. δ CvJ οό 25 An arrangement for detecting changes in the composition of the flue gas for analyzing the amount of CvJ x alkali metals vaporized from the boiler flue gases comprising at least one sensor (5), a device for measuring sensor data and a flue gas scrubber, known from LO § 30 (2) for taking a sample to measure the amount of alkali metals in the combustion gases of the boiler (1) from the part of the furnace furnace or flue gas duct where they are in gas, but combustion reactions have already taken place and placed in a device mixing the combustion gas with detergent and the sensor (5) is adapted to measure chemical or physical changes in the scrubbing medium caused by the flue gas. Arrangement according to Claim 10, characterized by 10 water circuits (4, 7 8) arranged to flow a constant amount of water, at which at least one sensor (6) is arranged, and 15. means (2) for deriving a constant flow amount. flue gas into the water cycle. Arrangement according to one of Claims 10 to 11, characterized in that the at least one sensor (6) is a pH sensor. Arrangement according to one of Claims 10 to 12, characterized in that the at least one sensor (6) is an electrical conductivity sensor. c \ j co 25 cp Mediator x ma according to one of Claims 10 to 13, characterized by a filter (3) for removing solids from the flue gas stream before it is introduced into the water cycle. m m CD m g 30 o c \ j