Services to Technology Providers Training and Capacity building activities with HP boiler manufacturers Session 1: the water and steam system Trainer: Frans Baltussen Date: first half 2015
Sugar processing flow chart See next sheet
Sugar processing flow chart
The co-generation plant in sugar mill
25 bara, 350 C The co-generation plant in existing Sugar mill configuration with 25 bara system Surplus power in public grid Sugar mill own consumption P T De-superheater VSD Bagasse Boiler Power house Crushing rolls P T 2,2 bara 130 C Sugar proces s house VSD P T Bagasse deaerat or 1,2 bara 105 C Boiler Mill house Steam bypass shredder P T T T Drying steam 4 bara,170 C Clean condensate 85% of input steam, 95 C RO and softeni ng plant Condensa te vessel Contamin ated condensat e and steam
The co-generation plant in existing Sugar mill configuration with 25 bara system The boiler is used for generating steam at 25 bara and 350 C. fficiency of boiler is low with approx. 72% on LHV 2. The steam is used in mill house to drive the rollers and shredder. 3. The rest of the steam is used for generating electric power. A part will be used in sugar mill and surplus will be supplied in public grid. 4. Steam from all back pressure steam turbines is fed to the evaporators in the sugar process house. This steam should be slightly overheated. However not too much. 5. The sugar cane has a certain amount of cellulose. It is called bagasse. Can be used in boilers to generate the steam required. This part will be changed into a HP cogeneration system Bagasse in carrier
Indicators use in sugar industry: 1. TCD: This is the installed capacity of the sugar mill for crushing cane specified in Tons Cane per Day ( TCD) 2. Duration season: The total season of crushing is depending on the area. It is 110 till 150 days. 3. Utility factor: How much period of the total season the sugar mill have been in operation. 4. Bagasse amount: the cellulose in the sugar cane which will be used for firing in the boilers. This is mostly approx. 29 till 30,5% of the crushed cane weight. 5. Process steam demand. This is the process steam amount necessary in the process house for making the sugar. This amount depends on area and system used. The process steam amount in Sindh area is in example higher then in other parts of Pakistan. It varies from 40 to 55% of crushed cane. 6. Own electricity demand mill house plus sugar process: The drives of boiler are mostly electric, in mill house it is mostly all with steam turbine. When all are electric drive then the electric consumption is 30 till 35 KW per crushed ton of cane.
Boiler used in existing sugar mills for producing 25 bara steam at 350 C Steam outlet superheater fficiency approx. 72% on LCV Steam drum Space in which sometimes a small economizer is placed furnace Bagasse supply from feeder Bagasse spreader Dumping grate Air suppl y Flue gas air preheater To ID fan and stack 230 C Fly ash arrestor water drum Bi- drum design
22500 Boiler types self supporting In HP cogeneration system are nowadays 2 systems used in sugar mills: Bi-drum suspended design fficiency approx. 87% on LCV Single drum self supporting design single-drum HP superheater Desuperheating station conomiser 2 evaporator furnace Over firing air Bagasse spreader Dumping grate conomiser 1 To ID fan and stack 150 till 160 C cyclones conomiser 3 Flue gas Air preheater plus steam air preheater HP bagasse fired boiler 140 tph 65,7 barg 485 C, single drum and self supporting
Boiler types - suspended HP superheaters furnace Bi-drum conomiser Flue gas Air preheater Dumping grate cyclones HP bagasse fired boiler 140 tph 65,7 barg 485 C, bi-drum drum and suspended
Water and steam as medium in co-generation plants Steam is favored for: Power production Heat transfer It is a unique combination of: High thermal capacity High critical temperature Wide availability Non toxic nature It has a high thermal capacity and high expansion capabilities which makes equipment small and is a clean product. The properties of water and steam. 1. nthalpy in KJ/kg: the internally stored energy as per unit mass of flowing stream 2. Specific entropy in KJ/kgK: is a measure of thermodynamic potential of a system in the units of energy per unit mass and absolute pressure 3. Specific volume in m3/kg: is the volume per unit of mass
The properties of water and steam Pressure In bara The properties of water and steam are tabulated in steam tables by International Formulation Committee (IFC) which has representative of following countries: Czech France Japan Russia UK USA temperature In C Specific volume saturated water in m3/kg Specific volume saturated steam in m3/kg Density of saturated steam kg/m3 nthalpy saturated water in KJ/kg nthalpy saturated steam in KJ/kg vaporation heat of water into steam in KJ/kg ntropy of saturated water in KJ/kgK ntropy of saturated steam in KJ/kgK
The properties of water and steam Steam tables with indication for particulate pressure for each temperature level the: Specific volume nthalpy entropy Saturation line
What is impact of pressure on steam condition? At what temperature water boils? Means at higher pressure the steam volume flow is much less and pipe sizes can be less. How steam volume decreases with increased pressure? Note: Steam is not an ideal gas so the formula P*V/T = C does not fully apply. There will be deviation of 20% higher then the actual value. i.e.: at 1 bar v is 1,694 m 3 /kg and Tsat is 100 C. At 40 bara Tsat is 250,3 C and with PV/T the v is 0,059 m 3 /kg. The real figure is 0,050. means approx 18% too high.
The nthalpy: nthalpy is the total energy in each kg of water and/or steam economizer evaporator Temperature versus nthalpy ( T H diagram) superheater
More details given in T-H diagram 374 C 221,2 bara 318 C 282 C 110 bara 80 bar 66 bara 225 C 25 bara At 110 bara r=1258,7 KJ/kg At 25 bara r=1839 KJ/kg 46% more then at 110 bara Comparison of evaporation heat 1 bara Dryness of steam
Heat load heating surfaces at different pressures HAT LOAD HATING SURFACS eco evap sup 328 613 690 759 1838 1531 1406 1255 962 1247 1345 1451 25 BARA, 350 C 66 BARA, 490 C 86 BARA, 520 C 110 BARA, 540 C
The QT diagram To review the heat absorbed by the heating surfaces we show the total heat versus the temperature in water/steam side and also flue gas side 25 bara, 350 C 66 bara, 485 C Heat absorbed by heating surface vaporator 47,2 MW Flue gas air-preheater: 13,8 MW vaporator 14,7 MW Flue gas air-preheater: 6 MW
Dryness of steam 318 C 225 C 110 bara 25 bara x dryness fraction 1 bara h= 800 KJ/kg h = 417,51 KJ/kg h = 2675,4 KJ/kg Dryness fraction steam: =weight of steam/total weight Or =heat supplied from boiler heating surface into water/total required evaporation heat (all per kg of water) X = (800-417,51)/2675,4 417,51) = 0,1694 Means 16,9 wt% is saturated steam and 83,1 wt% is water In case the dryness is 10% the specific volumes of the steam water mixture are: v = 0,9 * v + 0,1 * v Boiler pressure bara 10 25 110 Specific volume saturated water : v Specific volume saturated steam: v Specific volume water steam mixture with 10% dryness fraction: v M3/kg 0,00011 0,00120 0,00149 M3/kg 0,1943 0,07991 0,01601 M3/kg 0,0195 0,0097 0,0029
Dryness fraction steam: void fraction correlation With low pressure large volume of steam at already low dryness fraction steam How higher the pressure how lower the volume
The ntropy: H in KJ /kg ntropy is a measure of the degree of disorder within a system. The greater the disorder the higher the temperature. You cannot measure it. It is difficult to imagine this figure. It can be only calculated out of measured data. The heat input per kg and then divided by the average temperature change. In small temperature change steps In steam turbines it is used to indicate the efficiency. A perfect expansion process as in a steam turbine is with a constant entropy. See line A B in This is a isentropic process (ideal) However this is not possible as all kind of losses will occur like leakages between wheels and friction losses. The isotropic efficiency is the ht = 100 * (ha hf) / (ha hb) % The power supplied to the shaft is m * (ha hf) KW m= mass of steam per kg The power at generator terminal is minus the losses in gearbox and generator h4 h3 h2 h1 A B h1= friction losses in diffusor, heat into steam h2= friction losses in wheels, heat into steam h3= friction losses at outlet, heat into steam H4= losses through seals of wheels C D F S in KJ /kgk
The enthalpy versus the entropy (H- S diagram) Also called Mollier diagram Isotropic efficiencies different pressure levels: At 110 bara: Isotropic eff= (3464 2350)/(3464 2090) * 100%= 80,2% Power per kg/s of steam: 1114 KW At 62 bara: Isotropic eff= (3370 2360)/(3370 2150) * 100%= 82,7% Power per kg/s of steam: 1020 KW 110 bara 540 C 64 bara, 480 C 25 bara, 350 C xtraction point in sugar mill at 2,5 bara Steam temperature should be slightly superheated 5 C. Other wise the heating surface in evaporators are not working properly.
Why the existing steam turbine in sugar mill for driving the shredder and crushing rollers are replaced by electric drives?? Isotropic fficiency existing steam turbine with 66% is lower then the 80 to 83% for the HP steam turbine. Means high losses 110 bara 540 C 64 bara, 480 C 25 bara, 350 C The flows of 15 to 30% are too high for extracting from High pressure steam turbine. Will reduce the isotropic efficiency. The steam temperature is too high at extraction point which requires de-superheating. This results in less steam over HP steam turbine.
Steam turbine types in co-generation plants There are 2 types of turbines: The impulse type (curtis wheel) is used in the higher pressure zones in order to minimize the leakage around the wheels The reaction turbine (parson wheel) is used in the lower pressure zones as in condensors.
Why the process steam should be only slightly superheated in? 1. Poor heat transfer when it is superheated 2. To minimize the condensate in steam lines which causes high energy and water losses
What happen in case of throttling the steam? With 64 bar to 15 bar wet steam In case we extract steam from steam drum for burner or sootblowers or deaerator. With 40 bar to 15 bar dry steam The enthalpy will remain at inlet and outlet approx. the same. The valve needs to have special internals to avoid corrosion due to wet steam at smallest opening in valve With 110 bar to 15 bar wet steam
Rankine Cycle The Rankine cycle is used to predict the efficiency of steam turbine systems. We take the T-S diagram for reviewing the complete cycle in steam and water system of power plant Heat input P turbi ne fficiency rankine cycle is: (Pturbine Ppump)/ Qin * 100% Ppu mp Rejected heat ntropy in KJ/kgK TS diagram of a rankine cycle operating between pressure of 0,06 bar and 50 bar Limit the wet steam amount in wheel
System with one boiler and one condensing /extraction steam turbine Process steam 2,2 bara, 135 C Demi water Condensate return 85% at 98 C It is a simple Having low investment Advantage Disadvantage Condensing wheel has low efficiency during season The steam turbine will have low efficiency as it will run at 60 to70% of the steam load during off season. When one main equipment trip then sugar mill will shut down
System with two parallel systems each 50% capacity and consisting each of one boiler and one condensing/extraction steam turbine Process steam 2,2 bara, 135 C Demi water Advantage High redundancy in case same quality will be taken for equipment as of option. a When one unit trips then other unit can run at 50% load Condensate return 85% at 98 C Disadvantage Condensing wheel has low efficiency during season High investment costs of approx. 24% of total investment cost because of additional boiler and steam turbine.
System with one boiler and one back pressure steam turbine for season operation and one condensing steam turbine for off season Demi water Advantage In case back pressure trips then condensing/extraction steam turbine can run and plant has an output of 70% load High efficiency during season and off season The size of boiler and steam turbine can be 12 to 14% smaller Process steam 2,2 bara, 135 C Disadvantage Condensate return 85% at 98 C 2 nd steam turbine which result in higher investment cost High storage of bagasse
System with two parallel systems each 50% capacity and consisting each of one boiler and one back pressure steam turbine for operation during season, and one condensing/extraction steam turbine for off season Process steam 2,2 bara, 135 C Demi water Advantage In case back pressure steam turbine trips the plant can run still at 100% capacity. High efficiency during season and off season In case one boiler trips the plant can run still at 50% capacity Condensate return 85% at 98 C Disadvantage 3 rd steam turbine and 2 nd boiler which result in higher investment cost. High storage of bagasse
What is efficiency during season and off season Off-season season High losses in condensor Minimum steam flow over condensing wheel into condensor More process steam required in relation to electric power result in higher plant efficiency
What is effect of higher pressure system for sugar mill with its specific demands Description Unit 66 bar (a), 485 o C 110 bar (a), 540 o C Season/Off season - Season Off season Season Off season Steam production boiler tph 149.7 103 154 108.5 Bagasse consumption tph 63.4 43.4 65.5 46.1 Bagasse consumption ton 205,416 40,387 212,220 36,167 Total hours power generation hours 3,240 982 3,240 785 Power output generator kw 26,139 24,569 30,021 28,325 Power consumption of the sugar mill, shredder with electric drive kw 7,574-7,574 - Power plant own consumption kw 2,091 2,580 2,425 2,718 Power supply to the grid kw 16,474 21,989 20,022 25,607 Total power generation MWh 53,376 21,593 64,871 20,101 13,3 % more Total power supply to the grid annually MWh 74,969 84,972 Annual Revenues (Based on 10,500 PKR per MWh) PKR million 787.2 892.2 Power plant size MW 26.1 30
System with one boiler and one condensing /extraction steam turbine and air steam preheater Boile r F G e c o stack For additional steam air pre heaters Steam air preheater air revents corrosion in flue gas air reheater and generate more power ver full season for fixed bagasse mount pprox. 5,2% in 66 and 110 bara ystem To water storage Demi water Process steam 2,2 bara, 135 C Condensate return 85% at 98 C
The HP co-generation plant with steam air preheaters 66 bara More steam over steam turbine and extracted to heat up the air. No increased losses in condenser while more energy generated 9 barg xtraction pressure control 2,5 barg
What is effect on output of additional air preheating with 1 additional steam air preheater. For 66 bara Description Unit 66 bar (a), 485 o C at 45% steam demand Steam Air Preheater With steam preheaters from 2.2 and 9.0 bar (a) extraction With steam preheaters from 2.2 bar (a) extraction only Season/Off season - Season Off Season Season Off Season Steam production boiler tph 149.7 103 147 101.5 Bagasse consumption tph 63.4 43.4 63.2 43.6 Bagasse consumption ton 205,416 42,630 204,768 43,246 Total hours power generation hours 3,240 982 3,240 992 Power output generator kw 26,139 24,569 25,855 24,458 Power consumption mill with electric drive shredder kw 7,574-7,574 - Power plant own consumption kw 2,091 2,580 2,080 2,542 Power supply to the grid kw 16,474 21,989 16,201 21,916 Total power generation MWh 53,376 21,593 52,491 21,741 Total power supply to the grid annually MWh 74,969 1% more 74,232 Annual revenue (Based on 105,00 PKR per MWh) PKR million 787.2 7,8 million 779.4 PKR more
The HP co-generation plant with steam air preheaters 110 bara Flue gas air preheater eliminated
What is effect on output of additional air preheating with 2 additional steam air preheater. Total 3. For 110 bara Description Unit 110 bar (a), 540 o C at 45% steam demand Steam air preheaters With steam preheaters from 2.2 and 9 bar (a) and 21 bar (a) extractions With steam preheaters from 2.2 bar (a) extraction only Season/off season - Season Off Season Season Off Season Steam production boiler tph 154 108.5 146 102 Bagasse consumption tph 65.5 46.1 64.8 44.7 Bagasse consumption ton 212,220 36,167 209,952 38,321 Total hours power generation hours 3,240 785 3,240 857 Power output generator kw 30,021 28,325 29,032 27,206 Power consumption mill with electric drive shredder kw 7,574-7,574 - Power plant own consumption kw 2,425 2,718 2,310 2,555 Power supply into grid kw 20,022 25,603 19,148 24,651 Total power generation MWh 64,871 20,101 62,040 21,126 Total power supply into grid annually MWh 84,972 2,2% more 83,166 Annual revenue (Based on 19 million PKR PKR million 892.2 10,500 PKR per MWh) more 873.2
The HP co-generation plant with BFW preheaters for 66 and 110 bara Take care for corrosion at cold end More steam over steam turbine and extracted to heat up the air. No increased losses in condenser while more energy generated Boil er BFW preheaters e c o F G air stack xtraction pressure control Process steam 2,2 bara, 135 C Demi water Condensate return 85% at 98 C
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