Method of producing cement clinker and electricity Field of the invention The invention relates to a method of producing cement clinker and electricity, comprising feeding cement raw meal constituents and carbon rich material in a circulating fluidized bed boiler furnace, calcining the cement raw meal constituents in the circulating fluidized bed boiler furnace and producing steam with the flue gases of the fluidized bed, discharging calcined material in a rotary kiln, clinkering the calcined material in the rotary kiln and subsequently cooling the clinker, further comprising the gas and the solids out of the fluidized bed entering a cyclone system, the solids being separated therein being returned to the bottom boiler furnace, whereby part of those solids are first cooled down in a solids heat exchanger in producing steam, whereas the gas and the fly ash escaping the cyclone system being passed to a conventional boiler back pass with at least one heat exchanger and through a filter, further comprising the produced steam being fed to a steam turbine island comprising a steam turbine being drivingly connected to a generator.

Discussion of Background U. S. Patent 4,425, 163 describes a method for producing cement clinker, in which raw cement powder is calcined to a level of at least 95% in a circulating fluidized bed and then clinkered in a rotary kiln. The circulation system comprises a fluidized bed reactor, a cyclone separator and a recycling duct.

The carbonaceous fuel supplied to the calcining fluidized bed reactor is burnt near stoichiometrically in two stages with fluidizing gas and secondary gas. This method requires a preheating of the raw material in a suspension type heat exchanger.

A method of producing cement clinker and electricity as described above in the "field of the invention"is known from the article, Challenges of circulating fluid bed reactors in energy and raw material industries"by Lothar Reh, published in DECHEMA Deutsche Gesellschaft fur chemisches Apparatewesen, Chemische Technik und Biotechnologie e. V. , Frankfurt am Main. This article is based on a lecture held on the 6th International Conference on Circulating Fluidized Beds, in Wurzburg, Germany on August 22-27,1999. This article describes trigeneration of cement, electricity and heat from high-ash coal using Circulating Fluidized Bed-Technology. Limestone and high-ash coal are separately prepared and ground before feeding them into the boiler. These two basic raw materials are proportioned online. Sulfur is completely bound into the bed material, which has the chemical composition of clinker. The hot bed material is discharged directly into a small rotary kiln. This direct use of ash substituting the clay component in cement avoids disposal of ash completely. This known method requires high performance cyclones in order to catch and recycle the pulverized limestone and/or lime and the grinding of the high-ash coal prior to its entry in the boiler.

Summary of the invention Accordingly, the object of the invention is to provide a novel low energy intensive method and plant for integrating a cement clinker-burning unit into a hydrocarbon, especially coal-fired power plant, using circulating fluidized bed technology.

This is achieved, according to the invention, by following means: - feeding into the circulating fluidized bed boiler furnace the limestone component of the cement raw meal constituents in crushed form for forming bed and circulating material, - milling the hot bed material and re-injecting at least the coarser part of the milled solids to the fluidized bed boiler furnace, - separating the circulating bed material with the cyclone system so as the lime-rich fly ash obtained from calcination of limestone from the ground part of the cement raw mix escapes the cyclone system, - further separating the lime-rich fly ash from the flue gas before flue gas cooling and forwarding it to the clinkering kiln.

The capacity of a new greenfield plant can thus be increased by de-coupling the raw meal preheat and the calcination process steps from the clinkering process step, resulting in a lower overall cost by energy, mass and equipment integration. The power plant supplies the raw meal as a by-product for the cement production, re-using solids residue and thus minimizing the amount of residues from the power plant. By minimizing the balance of plant of both the power and the cement plant, a decrease of investment and operation cost is obtained. Fuel utilization of the power plant is increased whereas energy losses, external electricity supply and auxiliaries consumption of the cement plant are reduced.

Milling limestone requires dry material. The new method allows the use of high moisture limestone, since only crushing of the limestone is required. No milling of limestone to high fineness is required.

Compared to a progressive slow heating rate at low temperatures, a very high degree of calcination (above 90 %) is obtained by a high heating rate at high temperature, followed by a high residence time at this high temperature. The preheating and calcining step is thus increased in compactness.

If the cement clinker and electricity producing is performed simultaneously, there is a continuous raw meal feed to the cement plant which avoids intermediate material storage.

The discharged hot bed material from the circulating fluidized bed furnace is classified and cooled before milling. This allows reliable operation of the downstream milling equipment and gives an opportunity for further steam production. If the hot bed material is milled together with additives, mixing issues in the cement plant are avoided due to the use of the power plant as mixing device. Moreover there is only need for milling the bottom lime since thermal milling occurs in the circulating fluidized boiler. The mixing of all clinker constituents in the circulating fluidized bed avoids separately dedicated equipment in the cement plant.

If fuel fines processed for the circulating fluidized bed boiler are also used for the cement clinkering in the kiln, no dedicated fuel preparation for the cement plant is needed. Moreover the clinkering kiln may have a minimum length if rotary type, due to the calcined preheated and properly sized mixed meal.

As the flue gas discharged from clinkering process is passed through a scrubber, wherein gypsum is produced, there is a complete capture of SO2 from the combined power/cement plant. This is also true for the boiler flue gas.

Another advantage of the invention is to be seen in a significantly smaller number of primary cyclones leading to lower capital cost, because the primary cyclones may have a larger cut size than the above mentioned prior art devices. The primary cyclone system has a lower pressure drop, which leads to lower power consumption and lower operation cost. It also has a higher reliability due to the absence of fines in the material collected in the cyclone, which becomes easier to discharge. Another advantage is seen in the fact that there is no need to grind the fuel feed. Lastly the nitric oxide emissions from the CFB furnace are lower due to the positive catalytic effect of bed chemistry, CaO rich, on fuel nitrogen species, leading to N20 emission (another greenhouse gas).

Brief description of the invention A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing, which illustrates diagrammatically an exemplary embodiment of the invention with coal as hydrocarbon. Only the elements essential for understanding the invention are shown. Arrows illustrate the flow direction of the working media.

Description of the preferred embodiment Referring to the drawing, the equipment necessary for performing the co- production of cement and electricity consists mainly of a circulating fluidized bed boiler furnace 1, called hereafter CFB, a cement plant and a steam turbine island 42. The equipment necessary for performing the cement production comprises mainly three blocks, namely the same CFB acting as a raw mix preheater and calciner, a clinkering kiln 16 and a clinker cooler 20. The basic idea is to have used the circulating fluidized bed furnace as steam boiler and as calciner/preheater/mixer for the cement raw mix. The rotary clinkering kiln may be replaced by another high temperature reactor, i. e. a fluid bed type with partial bed agglomeration, for additional embodiment.

This clinkering kiln has an inlet end 43 and a discharge end 44 with a combustion zone near the discharge end. In the rotary kiln 16, the preheated and precalcined raw mix is burned into cement clinker. For the combustion in the kiln 16, a certain amount of fuel, i. e. coal is injected at the discharge end 44 via a burner 18 together with primary air 15. This coal can be taken via line 55 from the carbon rich material feed line 2 after the coal mill of the power plant.

Via air intake lines 23, ambient air is introduced in the system by fans 22 to the clinker cooler and heated therein by cooling down the cement clinker. The heated air exits the clinker cooler in a first stream 25 and is supplied to the kiln 16 as kiln combustion (secondary) air. The cement clinker is then forwarded from the discharge end 44 of the kiln via discharge line 19 into the clinker cooler 20, which might be a moving grate. The cooled clinker is finally supplied via line 21 to a clinker grinder, which is not shown.

So far, cement production and the apparatus involved are known. In prior art plants, the gaseous medium used for preheating the raw mix in a preheater tower consisting of a plurality of cyclones is the kiln gas exhausting the inlet end 43 of the kiln. It is now intended to give the kiln gases a separate treatment.

This is made possible by feeding the kiln of the cement plant with preheated calcined raw meal, consisting essentially of lime and combustion residues like ash, carbon and desulfurization residues. Thus there is a direct transfer of separated material to the clinkering kiln without intermediate cooling and preheating and without recarbonation.

The flue gas stream exiting the kiln 16, relatively concentrated in SO2, can be cost-effectively treated by cleaning it in a wet limestone scrubber to valuable product gypsum, which might be added to the final cement product, if not detrimental. A 1 00%-bypass off gas line 45 is connected to an off gas cooler 46 and an off gas filter 47. This filter could be an electrostatic precipitator wherein the entrained particles are separated, the coarse particles being returned to the process and the fine particles being separately disposed. An off gas fan 48 and a scrubber 49 follow the electrostatic precipitator downstream. The gas exit of the scrubber is connected to stack 41. CaC03 with SOZ and oxygen form gypsum CaS04 in the limestone scrubber. This gypsum might be used as an additive to the ground clinker in the final cement product. By this way of treating the exhaust gases, it is possible to suppress the alkali and chloride circulation and the other volatile matters in the cement clinker combustion device, as well as to reduce their concentrations in the cement clinker to produce low alkali cement quality for example. These components are contained in the now bypassed kiln gases and thus are not internally recirculated. This will improve substantially the kiln operation, i. e. no ring formation.

The remaining air of the clinker cooler 20 is not discharged-like in prior art plants-but is now used for steam production. Via clinker cooler line 67 it is forwarded to a de-duster 68, which in this example is a cyclone. After having been dedusted, the air is forwarded, for example, with a clinker cooler air fan 69 into the boiler back pass. The solids separated in the cyclone 68 might be added to the cement clinker in line 21. The dedusted air may then be injected into the boiler 1 (not shown).

The drawing illustrates in its left part in a simplified block diagram how the calcined raw meal, consisting essentially of lime and combustion residues like ash, carbon and desulfurization residues, is produced in the CFB furnace of a power plant.

Via air intake line 29 a combustion air fan 27 sucks the major portion of ambient air in the system. This air is preheated in an air preheater 28 arranged in the boiler back pass. Via line 30 the air is fed to the CFB furnace 1, in which it penetrates via a primary air supply 5 and a secondary air supply 4.

Depending on the apparatus type, the gas/air mixture can be introduced into the CFB furnace on different levels. In the example shown on the drawing, in which the CFB furnace is an upright circulating fluidized-bed steam generator with a flow stream from bottom to top, the fluidizing air is introduced at the bottom through an air distributor. The secondary air is fed through one or more elevations of ports in the lower furnace. The CFB furnace is provided with four. other inlets. One cooled solids return line 7, one main solids return line 12, one inlet 2 for the carbon rich material which is coal in this example and one inlet 3 for the raw cement mix, containing the limestone.

Coal is introduced mechanically or pneumatically to the lower portion of the CFB reactor via supply 2. This coal can be either crushed or pulverized. Like the air, coal may be injected on different levels of the reactor. If the coal is in form of crushed material with a size of approximately 6-mm, it can be fed by gravity.

Combustion takes place throughout the furnace, which is filled by bed material.

Flue gas and entrained solids leave the furnace and enter one or more primary cyclones 8, where the solids are separated.

The primary cyclone 8 is designed to separate the circulating coarse calcined mix from the flue gases. Since the mean size of the ash and the lime is typically smaller than 50pm it will escape the cyclone, while the char and the crushed lime/limestone, which is far greater in size will be retained in the cyclone. Thus the fly ash escaping the cyclone consists predominantly of lime and is forwarded with the flue gas in the flue gas and fly ash duct 32.

The solids separated in the cyclone 8 are recycled to the furnace via cyclone downcomer 6. The major portion is directly returned to the bottom furnace via main solids return line 12. Some solid may be diverted via line 10 to an external fluidized-bed heat exchanger 9, performing an additional heat exchange duty for furnace temperature control, and then returned to the CFB furnace bottom via cooled solids return line 7. The bed temperature in the CFB furnace 1 is essentially uniform and is maintained at an optimum level for calcination, sulfur capture and combustion efficiency by heat absorption in the furnace, and in the fluid bed heat exchanger 9 if necessary. In the present example the heat exchange is supposed to occur in furnace evaporator walls 36. Superheating of the steam and-for large steam turbine units with a reheat cycle-reheating is performed preferably by further heat removal from the hot solids absorption in the fluidized-bed heat exchanger. This heat exchanger 9 is containing immersed tube bundles 33. The flow rate of the solids through apparatus 9 via line 10 can be used to control the steam temperature. The produced steam is fed to the steam system and the turbine island 42 comprising at least one steam turbine driving a generator producing electrical power.

Sulfur compounds in the fuel or in the cement raw materials are mainly released in the CFB furnace 1 as SO2. There is no possibility to have part of fuel sulfur not released in the furnace, like in prior art suspension cyclones where the raw meal is injected at flue gas low temperature, which prohibits sulfur capture. In traditional CFB steam production units, the amount of limestone needs to be minimized-Ca/S molar ratio typically around 2 for 95% sulfur capture-to minimize operating costs. In the present method, by contrast, Ca/S molar ratios are far greater than 3, well between 10 and 50 as example, which increases-up to 100 % capture efficiency-the sulfur capture from flue gas exiting the system via stack 41. No attendant increase in operating costs results since a very high amount of calcium relative to sulfur is inherent in the cement clinker making process.

The CFB is now used for a simultaneous coproduction of steam and calcined raw mix for the cement production, in which coproduction the ashes of the power production are used to replace part of the cement raw mix in the cement production. Indeed coal ashes are similar in composition to calcined clays.

Moreover all of the coal residues are converted into clinker ; the sulfur is absorbed partly by clinker, notably by Na2O and K2O sulfates.

As described above, one inlet 3 for the raw cement mix is provided in the furnace 1. Via this line 3 crushed limestone is introduced to form the bed of the circulating material. At temperatures above about 800°C, limestone CaCO3 is fully calcined into CaO. CaO combines with SO2 released from coal combustion and oxygen to form gypsum CaSO4.

According to the invention, the bottom part of the bed material is milled and returned to the bed. This bed material consists of ash particles as well as of calcined lime particles of larger particle size. These solids are acting as heat carrier and are contacting and igniting fresh fuel particles. Ash and lime particles may agglomerate due to their continuous motion and contact with each other to a size such they are less susceptible to fluidization. As the coarser lime particles they tend to descend toward the bottom of the fluidized bed furnace and come to rest upon the air distribution plate.

These coarse solids are removed from a solids exit port 70 in the furnace that is positioned at a level just above the air distribution plate. They move into an inclined solids duct 71 and pour into a vessel which is a fluidized bed classifier and cooler 72. The coarse solids particles flowing through a solids inlet port meet a rising current of air, the classifier air supply 73, introduced through nozzles in the lower portion of the vessel. The rising air strips the fine ash and lime particles from the feed. Entrained by the air, the fine particles exit the vessel through an air/solids outlet port 74, traverse a return duct 75 and pass into the furnace 1. This particle size classification action takes place in the upper part of the vessel 72, while the coarser particles fall counter-current to the air stream into the bottom region, which is the cooler region. In this cooler region is a fluidized bed of coarse particles sustained by the airflow through the nozzles. An ash disposal below the level of the nozzles receives a buildup of particles that dropped out of the fluidized bed. During the residence time of the solids in the fluidized bed, they undergo a substantial cooling due to the injected air. By cooling the solids the air injected is heated up and forwarded to the boiler together with the fines. Thus the sensible heat that would have been lost in disposal of hot ashes is regained. Control of the cooled particulate removal from the vessel is effected by a valve 76 which is operated to drop the accumulated solids into a duct 77 connected to a mill 78, which might be a ball mill.

The solids undergo a milling process in which 90% of the material is milled to a size <90 microns. They are mixed with additives introduced in the mill 78 by a line 14. These corrective additives are used, if any essential chemical compound needed in the mixture of coal ash and limestone like iron oxide or silica content are not present in the required amount. The solids are returned to the fluid bed via line via line 79. Via line 80 some of these solids might be added to the calcined raw mix in line 54 as corrective medium for the calcination degree and for temperature moderation.

The flue gas and the fly ash exit the gas outlet of the separation device 8 via the flue gas and fly ash duct 32 into the boiler backpass 34. This backpass contains in its upper part a separator such as a secondary inverted cyclone 35, which is designed for separating the major part of the fly ash containing the calcined raw mix. This feature is needed to avoid back pass recarbonation and to decrease energy need in the kiln clinkering step.

The separated gases are further treated before disposal. They are cooled down, thereby superheating and reheating steam and evaporating and preheating water in subsequent tube bundles integrated in the water/steam cycle of the steam system and turbine island 42. The gas is supposed to leave the boiler backpass with a temperature of about 120-150°C. Downstream the back pass a dust filter 37 is provided in the line 38 to remove from the gas all the remaining solids. This filter 37 could be a fabric filter or an electrostatic precipitator. A fan 40 is installed in the gas line exiting the filter on the clean side of the filter 37. Its purpose is to control the pressure in the system close to atmospheric conditions. The cleaned gas leaves the system via the stack 41.

The solids separated in the filter 37 are fed via line 39 back to the clinkering kiln. The hot calcined raw mix of the correct particle size and composition for cement clinker making separated in the inverted cyclone 34 is then forwarded to the kiln via line 54. It might be that the precalcined raw material produced in the power plant contains some carbon. In this case either the coal amount to the kiln has to be reduced appropriately to achieve the desired temperature range for kiln operation.

Obviously, numerous modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other- wise than as specifically described herein.

List of Designations 1 circulating fluid bed boiler furnace 2 carbon rich material feed line 3 raw cement mix/limestone feed line 4 secondary air line to 1 5 primary air supply 6 cyclone downcomer 7 cooled solids return line 8 cyclone 9 fluidized bed heat exchanger, with solids mixing chamber 10 solids return line to 9 11 hot bed material discharge 12 main solids return line 14 additive supply to 78 15 primary air 16 kiln 18 fuel supply to 16, coal 19 kiln clinker discharge line, nodules 20 clinker cooler 21 line to clinker grinder 22 fan 23 air supply to 20 25 air supply to 16 27 combustion air fan 28 air heater 29 air supply to air heater 30 hot air discharge from air heater, air supply to 1 32 flue gas and fly ash duct 33 tube bundles in 9 34 boiler backpass 35 inverted cyclone 36 evaporator walls 37 dust filter, electrostatic precipitator 38 gas exhaust line 39 lime discharge line from 37 40 fan 41 stack 42 steam system and steam turbine island 43 inlet end of 16 44 discharge end of 16 45 off gas from kiln 16 46 off gas cooler 47 off gas filter 48 off gas fan 49 scrubber 54 calcined material line to 16 55 coal supply to kiln 67 clinker cooler air line 68 de-duster, cyclone 69 clinker cooler air fan 70 solids exit port 71 inclined solids duct 72 fluidized bed classifier 73 classifier air supply 75 return duct 76 valve 77 duct 78 mill 79 return line 80 line