Droplet Combustion. Reza Fakhrai. Heat and Power Technology, Stockholm, Sweden

Transcription

1 Droplet Combustion Reza Fakhrai Heat and Power Technology, Stockholm, Sweden 1

2 Overview 85% of all energy consumed in the United States is derived from the combustion of fossil fuels. In 1994, statistics show that the combustion of liquid petroleum-based fossil fuels accounted for 39% of all energy consumption, and an astounding 97% of energy consumption in the transportation sector 2

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4 Droplet combustion is: Liquid Fuels or solid (black liquor) Oil, Alcohol etc Mineral Oil is the fuel that is used extensively during the last century It is easy to handle Easy to atomized High heat value 4

5 Mineral Oil The most common mineral oil are: Gasoline Diesel oil EO1-EO5 all produced by crud oil It consists mostly of (CxHy)z and are product of distillation process and some other more complicated processes 5

6 Typical analysis of Oil Eo1 Purpose to be used in small and meddle sized power plan equipped with low and high pressure burner Typical analysis Density kg/m 3 Total Sulfur 0.29 Wt % Cock salts (Rams bottom) <0,1 Wt % Viscosity (20 C) 1,9 mm 2 /s Low flow Temperature -50 C Flame point min 60 C Effective Heat value 42,9 Mj/kg C % 85,9 H 2 % 13,2 S % 0,8 O 2 % 0,1 Ash % - 6

7 Physical Properties Flame point (flammability): It the temperature which at, that the vaporization rate is high enough to get a stable combustion Ex gasoline has a flammability under 21 C Explosion (rapid Combustion, => rapid increase of the pressure) Explosion demand some certain conditions Relation between the amount of the fuel and air should be in a certain range upper (vol%) Gasoline 1,4 7,0 Ethanol 2,6 8,0 Lower (vol%) 7

8 Combustion Devices Oil consists mostly of (CxHy)z At some hundred C It dissociates to H 2 and C above 500 C carbon goes to solid Coal: Difficult to combust, Only way to combust that is by high temperature and intensive mixing Wish: That combustion happens at high temperature and well mixed condition => Cocks particles will burn as fast as they have been produced 8

9 O 2 and Combustion Process Low O 2 concentration Cocks will not combust => Soot High amount of O 2 vaporization completes but combustion will not take place (short residence time) (white smoke) How to fix this? Amount of fuel and the needed air should be carefully measured and the combustion chamber should be divided in different zones. Combustion air is delivered by fans (often Axial type) in many cases 9

10 Oil Burner What dose it do? -To deliverer oil and mix it with combustion air -Fast and complete combustion a drop should not be bigger that 200 µm in diameter. 10

11 Oil Burner 1. Air inlet to the Burner 3. Flame holder 7. Burner lance 11

12 Oil burner Example of separate air distribution in a bigger boiler 12

13 Swirl and Pressure Distribution in flame 13

14 Flow Field Distribution and the Flame shape 14

15 Oil fog Pressurized oil burner 7 bar to 60 bar Injection with 30 to 80 15

16 Boiler 16

17 Combustion Pre-mixed Flames pre-mixed lean/rich combustion Non-premixed Flames Diffusion Flame= Chemical reaction rate is very high compared to the rate of flow, diffusion and mixing Infinitesimally thin exothermic reaction zone Solid and Liquid Fuels 17

18 Solid and Liquid Fuels Liquid and solid fuels are often considered to be member of a single family The gasification (or vaporization) temperature of theses fuels is usually lower than the self-ignition temperature of fuel vapor/air mixture <=> Combustion occurs mainly in the gas phase Gasification of light weight liquid fuels is purely physical process whereas that of heavily liquid and solids involves a chemical break-up Surface and boiling temperature 18

19 Flash and Fire point temperature Flash Point The least value of the liquid temperature T 0, at which the mixture is marginally capable of flashing with a pilot flame Fire Point (ignition) The lowest Liquid temperature at which the mixture, once ignited by a pilot flame 19

20 Atomization of Liquids The process of manufacture and dispersion of the droplets is known as atomization The process involves stretching of the liquid into sheets and filaments by acceleration in through a nozzle orifice, formation of ripples and disturbances in the filaments due to air drag and turbulence, breakage of filament which collapse into droplets 20

21 Atomization of liquids Pressure Atomization The fuel is forced with high velocity into a relatively quiescent air Air Atomization High velocity air stream flows by a relatively low velocity fuel jet Impinging Jets Two Jets of liquid are made to collide headon to form an unstable sheet at stagnation 21

22 Efficiency of atomization Break up of a bulk of liquid into droplets requires work in order to increase the surface area against the surface tension resistance. This is known as the efficiency of the atomization The efficiency of atomization depends upon the viscosity and surface tension of the liquid and method of atomization It is usually between 0.1 and 1% Air and impinging jet atomization are known to be more efficient than pressure atomization 22

23 Assumption into d 2 -law 1. Spherical symmetry 2. Isolated droplet in infinite medium 3. Isobaric process 4. Chemical reaction infinitely fast with respect to diffusion 5. Constant gas phase transport properties and heat capacity 6. Gas phase quasi-steadiness 7. Constant, uniform droplet temperature (No droplet heating) 8. Unity Lewis number for all gaseous species 9. Negligible buoyancy 10.Negligible radiation. 23

24 Droplet evaporation rate Gas-phase 24

25 Mass fraction around an evaporating spherical drop 25

26 Droplet mass conservation 26

27 Mass conservation in the gaseous phase 27

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30 d 2 -law based on heat transfer B q = c pg ( T h fg T boil ). m d 2 d 2 = in 4πk r = g s ln( Bq C Kt pg + K 1) 8kg = ln( B + 1) ρ C L pg 30

31 Example A particular droplet takes 1ms to evaporate in a pure air environment and Y fu,0 =0.8 i) Calculate the evaporation time if the mass fraction of fuel vapor far from the droplet becomes Y fu,0 /2 with everything else staying the same. ii) Calculate the evaporation time of a droplet with twice the diameter, all other quantities being unaffected. 31

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33 Black liquor Fuel of the future 33

34 Mechanical Pulping In the traditional method of mechanical pulping, debarked roundwood is pressed against a rotating grindstone in a water bath tearing the fibers and producing an aqueous slurry. This brute-force shredding of the wood results in short fibers yielding paper with poor physical properties. This pulp is blended with higher quality chemical pulp to make newsprint, catalogs, container board, and tissue. About 95 percent of the roundwood weight fed to the grinder ends up as pulp. In addition, the manufacture of this pulp requires no chemicals, thus eliminating their initial cost as well as the costs associated with reprocessing spent liquor and treating noxious wastes. The advantages of mechanical pulping high yield and low environmental impact provide strong incentive to improve the quality of mechanical pulps so that they can be more widely used. 34

35 Chemical Pulping Chemical pulping achieves fiber separation by dissolving the lignin that cement the fibers together. There are two major chemical processes: Sulfate and Sulfite pulping. Sulfate Pulping The makeup chemical for sulfate pulping is essentially sodium sulfate. This process is usually referred to as the Kraft or alkaline process. It produces a pulp of high physical strength and bulk, but relatively poor sheet formation. The process has a recovery system that not only recycles the pulping chemicals, but also is a source of about one-half of the steam generation. The yield of pulp is about 45 percent. 35

36 Chemical Pulping Sulfite Pulping Sulfite pulping uses sulfurous acid and an alkali to produce pulps of lower physical strength and bulk, but exhibits better sheet formation properties. The yield on the basis of chipped wood is again about 45 percent. These pulps are blended with ground wood for newsprint and are used in printing, bond papers, and tissue. Sulfite pulping was originally designed with a recovery system similar to the older soda process still used in some plants. Environmental pressures have often forced these plants to develop a recovery process 36

37 Chemi-Mechanical Pulping Chemi-mechanical pulping combines mechanical and various chemical processes for defibration. The most important of these is the neutral sulfite semi-chemical process, known as NSSC. Using this process, a mixture of pulps is produced, the principal use of which has been the manufacture of corrugated medium. The lignin content is too high for most other applications. Yields are in the range of 70 to 85 percent. 37

38 Chemical Pulping Process Diagram Kraft pulping is a closed loop process in which the chemicals used to make pulp are, recycled and reused in a digester. The recovery boiler is crucial to the process. It serves three critical function in this process. Firstly it makes use of chemical energy in the organic portion of the liquor to generate steam for the mill, secondly, it plays a major part in the sulphate process as a chemical reactor, and thirdly it destroys the dissolved organic matter and thus eliminates an environmental discharge 38

39 Black Liquor Composition 39

40 Black liquor Composition The composition of black liquor depends on the alkali charge (and white liquor composition), pulp yield, and the wood species used. Considerable differences exist between liquors from different species, especially between hardwood and softwood liquors. Alkali lignin 30-45% Hydroxy acids 25-35% Extractives 3-5% Acetic acid 5% Formic acid 3% Methanol 1% Sulphur 3-5% Sodium 17-20% Table 2 shows typical ranges for Kraft liquor. 40

41 Lignin and its Properties There is general agreement, that the term lignin refers to a group of phenolic polymers that confer strength and rigidity to the woody cell wall of plants. These compounds make up the second most abundant class of chemicals found in wood. Cellulose is first. A small section of a lignin polymer is presented below illustrating some typical chemical linkages seen in Lignin. 41

42 Recovery Boiler A black liquor recovery boiler functions as both a steam boiler and chemical reactor It generates steam from the energy liberated during combustion of the organic constituents of black liquor, while chemical from pulp digesting, such as sulfur and sodium, are recovered as smelt 42

43 Recovery Boiler The development of the recovery boiler by Tomlinson in cooperation with Babcock and Wilcox in the early 1930 s led to the predominance of the kraft process. 43

44 These stages occur in three different zones in a furnace The upper section is the oxidising zone. Some example reactions in this zone are: CO + 1/2 O 2 > CO 2 H 2 + 1/2 O 2 > H 2 O Na 2 S + 2 O 2 >Na 2 SO 4 H 2 S + 3/2 O 2 > SO 2 + H 2 O The middle section (where the black liquor is injected) is the drying zone. Some example reactions in this zone are: Organics > C + CO + H 2 2NaOH + CO 2 >Na 2 CO 3 + H 2 O 44

45 These stages occur in three different zones in a furnace The bottom section is the reducing zone where, the sulphur compounds are converted to Na2S in a bottom bed of char. Some example reactions in this zone are: Organics > C + CO + H 2 2C + O 2 > 2CO Na 2 SO 4 + 4C > Na 2 S + 4CO C + H 2 O > CO + H 2 In addition to combustion the overall chemical reactions in the recovery boiler are: Conversion of sodium salts: 2NaOH + CO 2 >Na 2 CO 3 + H 2 O Reduction of make-up chemicals: Na 2 SO 4 + 4C < > NaS + 4CO 45

46 Some Information The maximum combustion temperature occurs between the plane of black liquor entry and the plane of secondary air entry. The firing of black liquor at 65% solid leads to a maximum combustion temperature of C while combustion at 70% solids leads to combustion temperatures greater than 1400 C. The steam pressure in the recovery boiler, varies from 40 bar to 100 bar. Temperature parallels the pressure, but 480 C is the maximum attained. A study and demonstration are attempting to reach higher temperature (500 C)[1] [1] 30 Years Recovery Boiler Co-operation

47 Main Features/Design Recovery boilers have two main sections: the furnace section and the convective heat transfer section. In the furnace, mixing and combustion of the fuel and air should be completed. About 40% of the heat are transferred in the furnace. Heat transfer to the boiler water to form high-pressure steam is then completed in the convective heat transfer section. The equipment arrangement of the convective part (upper portion and back end) of the recovery boiler is generally similar to other types of boilers. The superheater, boiler banks economiser, electrostatic precipitator and the induced draft and forced draft fan have some feature in common with other boilers. The nose arch or bullnose separates these two sections. 47

48 Furnace The unique characteristic of a recovery boiler is in the lower furnace in which at least four process results are accomplished: 1. High solid black liquor (e.g %) is dried to remove water. 2. Organics are burned to recover heat. 3. Sulphur is reduced and sodium is recovered to recycle sodium sulphide and sodium carbonate to recausticizing and kraft pulping. 4. Sodium or sodium compounds are released in vapour form to recapture sulphur oxides by forming sodium sulphate containing dust. In doing so sulphur (and sodium) are recycled back to the boiler and sulphur emissions in the stack gases are minimised[i]. [i] Bousfield D,W. Research on Droplet Formation for Application to Kraft Black liquors 48

49 Lower Furnace Sketch of the lower furnace of a flat bottomed recovery furnace Seconndary air Nozzles Char bed Smelt Spot 49

50 Water Walls 50

51 Combustion Air System Primary Air The primary : The primary air is located approximately 1 m above the furnace floor. At this level About 40-50% of the air required for combustion is added by forced-draft fans. The primary air supply is preheated to 150 C and burns the organic compounds and the pyrolysis gas. It is used to control the shape and position of the perimeter of the char bed and to provide a portion of the air for char combustion The carbon gasification in the region above the bed produces heat. This heat melts the inorganic compounds and reduces Na 2 SO 4 to Na 2 S, resulting in a smelt, but maintaining reducing conditions. Primary airports are always placed on all four walls of the furnace. They are always the smallest and most numerous airports. 51

52 Combustion Air System Secondary Air Secondary air Secondary air ports are located approximately 2 m above the furnace floor. About 35% of the air required for combustion is added at this level. The secondary air ports are generally large and less numerous than the primary ports. There are 4 to 16 ports on each wall of the furnace with a rang in size from 5 by 25 cm to 12.5 by 67.5 cm. A wind box pressure of 1 to 3 kpa is typical for the secondary air. The secondary air controls the position of the top of the bed. It burns the pyrolysis gas and the CO from the bed. 52

53 Combustion Air System Tertiary Air Tertiary air Tertiary air is located above the liquor guns approximately 8 m above the floor of the furnace. Tertiary air ports are the largest and usually located on two opposing walls. They are normally located on the front and rear walls. Typically, 3 to 8 tertiary ports between 10 by 45 cm and 15 by 75 cm in size are used on both walls. An air pressure between 1.25 and 5 kpa is typical for the tertiary air, which completes the combustion process and finally mixes the combustion gases. The number and size of ports at each level is determined by the desired liquor burning capacity, the available air pressure, and the air distribution between the levels selected by the manufacturer. 53

54 Processes during black liquor combustion 54

55 Processes during black liquor combustion Graphical presentation of black liquor combustion in a recovery boiler 55

56 Processes during black liquor combustion Quantitative evaluation of black liquor combustion 56

57 The droplet swells during combustion 57

58 Physicochemical process In the recovery boiler furnace a nimeber of entirely separate physicochemical process take place simultaneously: Air injection and mixing with the furnace gases Black liquor spraying and droplet formation Drying of black liquor droplets Pyrolysis of black liquor and combustion of the pyrolysis gases Gasification and combustion of the char residue Reduction of sulfur compounds to sulfide 58

59 Liquor Composition % in dry soilds C 38,2 % H 3,4% O 31,1% N 0,1% S 5,2% Na 19,8% K 1,9% Cl 0,1% Others(Ca, Si, Fe, Mg, Al,Mn) S/Na=0,38 mol/mol 0,2% 59

60 Liquor Composition An example of distribution of sulfur compounds in unoxidized kraft black liquor Idea Recovery furnace chemistry in unite of g/1000g BLS 60

61 Sulfur and Sodium Balance Sulfur and sodium balance in atypical recovery furnace (kg/bls) 61

62 Recovery Boiler 62

63 Black Liquor Spray Nozzles Spray droplet size and roplets size distribustion are the key variables in controlling black liquor combustion, smelt reductio, entrainment, and carryover A drop should be smal enough to dry and partially pyrolyze before reaching the char bed but large enough to avoid being entrained in the furnace gas flow Droplet about 3 mm in diameter are usually considered to be the proper size 63

64 Black Liquor Spray Nozzles Two basic types of black liquor nozzles are: - Babcock & Wilcox Co (B&W) splash-plate nozzle. A splash-plate nozzle consists simply of a flat plate or rounded cross-section attached at an angle to the end of the pipe. Black liquor exits from the end of the pipe it strikes the flat face of the plate at an angle of approximately 45º. - The swirl-cone nozzle. Black liquor exits from the end of the pipe. Just ahead of the nozzle is a block or swirl plate with spirial grooves which cause the flow to rotate as it flows along the walls of the nozzle cap. 64

65 Splash- plate Spray Nozzles 65

66 Splash- plate Spray Nozzles 66

67 Swirl-cone Nozzle 67