notes on involved energy in cane sugar processing dr carlos de armas dr oscar almazan
TRANSCRIPT
Notes on Involved Energy in Cane Sugar Processing
Dr Carlos de ArmasDr Oscar Almazan
Cane Sugar Processing
Extraction Separation of the sugared juice
from the bagasse (fiber+water+ )
Purification Separation of non desirable
substances from juice; colloidal +
Evaporation Separation of most of the water
Cristallization Separation of sucrose from
different classes of molasses
Centrifugation Separation of sugar crystals
Steam and Power Generation
EXTRACTION (MILLING)
bagasse
Cane Pre-paration
Mill No 1
MillNo. 2
MillNo. N
Exhaust Section Counter-current Extraction 3 to 5 Mills
Mixing
Mixed juice to purification
First extraction juicewaterJuice
Mixed juiceBrix 13 to15Purity 80 to 90
COMMON NOMENCLATURE INEXTRACTION
Cane; Raw material fed to the milling station
Imbibition water; Water added in theexhaust section for washing out andrecovering most of the sucrose in cane.Common numbers are 20 to 35 % on cane.
Absolute juice; total weight of cane minusthe weight of present fibre. A commonrelation between both is 86 to 14 % on cane.
Fibre; The lignocellulosic structure givingstrength to the cane to keep itself erected.Common values are 12 to 14 % on cane.
Mixed juice; Juice coming off the milling station andgoing into the purification station. The weight ofmixed juice produced per unit time, is quite similar tothat of cane ground per same unit time, in manyhealthy installations .
Bagasse; Is the lignocellulosic residue left frrom caneafter the juice extraction in the milling station. Most ofits components are fibre, between 45 and 47 % onwet bagasse, and moisture , between 49 and 51 % onwet bagasse. From 2 % to 4 % may be soluble solids,mainly sucrose .
Fundamental Equationof milling is:
Cane + Imbibition Water = Mixed Juice + Bagasse.
CANE SUGAR; AN ENERGY INTENSIVE INDUSTRY
Cane sugar industry is an insdustry with strong involvements with energy.
~The raw material, sugar cane, bring its own fuel for processing, and even more.
~It shows high thermal (steam) demand for processing , while its demand of mechanical energy is low, allowing high cogeneration.
SUGAR AND ETHANOL PRODUCTION
• 9 ton of cane 1.0 ton sugar
2.5 ton bagasse
2.0 ton cane wastes
300 kg final molasses
• 15 ton of cane 1.0 m3 ethanol
4.0 ton bagasse
15 m3 liquid wastes
Energy in Processing (Main Elements)
~Steam generation efficiency
~Efficient use of steam
~Efficiency in the conversion of
thermal energy into mechanical
Bagasse It is the natural fuel in processes of production of sugar and etha-nol. Enough for fulfilling whole demands. Reaching in practice, in addition, a balance between produced and burned bagasse, through control of boilers effi-ciency. Surplus bagasse without a goal, is as bad as not enough bagasse.
Bagasse In Cuba, when producing in a campaign, 6 million ton of sugar, there are ground 50 mil-lion ton of cane, with a bagasse production of 15 million ton, out of which, 95 % is burned, going the difference to derivatives. This 15 million ton bagasse, are equivalent to 3 million ton fuel oil.
Bagasse ..and the most interesting fact ..!!
While in producing cane sugar, it is spent the whole energy freed by the 2.5 kg of bagasse coming along with 1.0 kg of sugar , i.e. 4500 kcal , in beet sugar proces-sing, there are spent per kg produ-ced not more than 2000, that is, potentially, there exists about 50 % surplus bagasse.
Why it is not so in practice?
~ Up to the seventies there were
no possibilities, 1.0 bb of “fuel”
costed less than US $ 400
~Current policy ; to avoid surplus
without goal. They cost money.
~Seasonal fashion of sugar pro-
duction
~Different kinds of bussiness,
laws and regulations.
Bagasse
Generation and use of energy
Sales to the grid
32-36 kW-h /tc for fulfilling
whole demand of the factory. For 3000-3500 tc per day, 150 (ton/hour), power generation is of the order of 5000 kw (inclu-ding the mills). Energy reser-ves due to co-generation plus surplus bagasse may grow up to 10000 kw (70 kw-h/tc) as per Mauritius Island experience
Generation and Use of Energy
Sales to the Grid
Through changes in steam generation parameters, and with efficient use of steam in process, which in general mean investments, there are reached surplus of the order of 70-80 kw-h per ton of cane, i.e. for a factory grinding 150 ton per hour, it is not impossible to deliver to the grid 12000 kw with proved technologies (Mauricio Island and Hawaii).
Generation and Use of Energy
DifferentApproaches
In Operation Today 1) BackPressure Turbines
To the Grid 10/15 kw/tc-h
2) Cond.-Extr. Turbines-
Mauricius Island 70 kw/tc-h
In development at present
3) Combined Cycle, GT +
gasifying 240 kw/tc-h
Extraction-CondensingTurbines
A main drawback is the sea-sonal character of cane sugar processing all over the world and the scale economy of Ran-kine cycle. Possible sizes are not enough efficient, and veryexpensive per kw to operate 60to 70 per cent time with fossilfuels. It is possible only in very small countries and where very efficient cane harvest wastes useare reached or with energy canes
CombinedCycle
Present status
-Following bagasse gasification; It is almost ripe the technology.After this, semi or commercial tests. It will be ready in a few years.
Through bagasse hydrolysis, thefuel can be fed directly to the combustor. It is now at bench scale level, then semi or commertial tests. May be ready in ten years.
CombinedCycleEconomy
Operation plus maintennance costof a hydroelectric plant in Brazil is of the order of US $0.001/kw-h,while capital cost US$ 0.06/kw-h
In a conventional fossil fuel plantthese costs are 0.005 and 0.025 respectively and that of fuel 0.02 for a total of US $ 0.05 per kw-h
CombinedCycleEconomy
Gasification; operation plus maintennance costs 0.005,capital cost 0.025, fuel 0.02for a total of US $ 0.05 per kw-h.
SUMMARISING
STEAM AND POWER GENERATIONBase: 1000 kg of cane
Sugar; 80 to 140 kg Bagasse; 260 to 320 kg 2.0 and 4.0 kg/kg sugar Steam; 380 kg to 600 kg 2.7 to 7.0 kg/kg sugar Energy; 3700 to 7400 kcal/kg sugar 15.5 to 31.0 MJ /kg sugar common value 4500 kcal/kg sugar 18.8 MJ/kg sugar
MAIN ASPECTS IN THE EFFICIENT USE OF ENERGY IN CANE SUGAR PROCESSING
Steam Generation Configuration
Engineering Design of Process Steam Layout
Engineering Design in the Transformation of Thermal Energy into Mechanical Energy
STEAM GENERATIONCharacterizing SG Efficiency, specification
of Gross Calorific Value, or Nett Calorific Value
as a function of % moisture(W) .
metric units
NCV = 4250-4850*W/100 kcal/kg (Hugot)
english units 1.8*(kcal/kg) = Btu/lb
NCV = 7650-8730*W/100 Btu/lb (Hugot)
1.0 kW-h = 3.6*106 watt-seg (joule) = 860 kcal;
1.0 kcal = 4.186 kj
BOILER EFFICIENCY FOR GCV AND NCV
Bagasse with 50 % moisture
NCV = 1825 kcal/kg GCV = 2300 kcal/kg
Eff. defined as the % of freed heat from the bagas-se, leaving with the steam (enthalpy of steam less enthalpy of fed water, times steam rate, divided by the Caloric Value of one mass unit of bagasse.
GCV Efficiency of best bagasse boilers 67.5 %
NCV Efficiency of these units,
(2300/1825)*67.5 = 85 %
GENERAL BOILER CONFIGURATION
Furnace
Water walls
Screen
Superheater
Water Evaporation Bundle
Economizer
Air Pre-heater
MAIN ENERGY LOSSES IN STEAM GENERATION
Sensible heat carried by gases leaving, 12-30 %
Non complete combustion, 2-12%
Excess air over the minimum necessary,
including air infiltration
Conduction and convection through walls 2%
Water Extractions
FURNACES; DIFFERENT TYPES
Burning in pile; Horse shoe
Cell
Spreader stoker (grate) oscillating
travelling
Suspension firing
COMBUSTION / STOICHIOMETRYBagasse (dry) analysis, changed to ashes free
Carbon 47.0/0.975 = 48.2 %
Hydrogen 6.5/0.975 = 6.7
Oxygen 44.0/0.975 = 45.1
Ashes 2.5 -----
Dividing by the MW of each element it is reached a pseudo-
structural formula, with which it is easier to do the combustion calculations using the moles approach.
C4.02 H 6.7 O 2.82
Stoichiometry Equations
(/100)C4.02H6.7O2.82 ; Excess air %
bagasse ; Base of Calc.
+
4.285(1.0 + /100)*(/100) O2
oxygen in air
+
16.12 (1.0 + /100)*(/100) N2
nitrógen coming with air
COMBUSTION PRODUCTS
4.02*(/100) CO2 + (3.35*( /100)+ BC*(hum/100)/18)H2O
Carbon anhydride + water from water due to
combustion moisture of fuel.
+ 4.285(/100)*(/100)O2
non-used oxygen in gases
+ 16.12 (1.0 + /100)*( /100) N2
nitrogen in gases
……..LAST COMMENTARIES
AFTER STOICHIOMETRY, IT IS POSSIBLE TO BUILD MOLAR AND ENERGY BALANCES, ANDAFTR THIS , ADDING DETAILS OF CONFIGURATION, TO BUILD THE WHOLE MODEL OF STEAM GENERATION
AFTER THE ADDEQUATE PROCEDURESTHE REST OF THE WHOLE PROCESS ENGINEERING MAY BE MODELED, REACHINGTHE WHOLE PROFILE OF ENERGY TRANSFORMATIONS.
Liquids transportation in the factory;
mixed and clarified juice to their tanks,
syrup and molasses to their tanks,
injection water to condensers and from
batches (barometric leg seal) to spray
pond. General purpose water from source
to tank. Imbibition and recirculation of
juices in mill, etc.
Mixed juice to tank; head 15 m, flow, one ton
of juice (1000 kg), 100 % mixed juice extract.
1000(2.204 lb/kg))15 (3.28 ft /m) =
=108437 ft-lb / ton/hour, for 300 ton / hour
= 108437*300 = 32531040 ft-lb /hour
= 32531040/3600 = 9036.4 ft-lb / sec
as one hp = 550 ft-lb/sec, power for pumping
9036.4 /550 = 16.4 hp, i e 12.3 kW
Another example; pumping cooling water to vacuum pans condensers. Evaporation in pans 18% cane = 180 kg / ton cane, need of cooling water 60 times, head 20 m, taking to English system
=180*60 *20 *2.204 *3.28 *300/3600/550 =
237 hp or 176 kW. 176/300 = 0.6 kW-h/tc
Efficiencies has not been taken in consideration nor densities in pumping of
fluids other than water
Total Mechanical Energy Demand (different of installed power) is of the order of 32 to 36 kW-h( 115 to 130 mJ)per ton (metric) of cane
Irrelevant of type of prime mover; steam or electric, it is a number slightly different
Note: metric ton may be identified also byTonne.
With a total, general distribution, just for
giving an approximate idea as follows
Cutting knives, including leveling blades
1.3 – 1.7 kW-h per ton cane (one machine)
Shredders 1.5 – 2.5 kW-h per ton cane,
depending on design
Milling, (only for energy demands
estimations, Hugot )
For three roller mills ; T= 0.134PnD / tc
T; kW- h per ton cane for each mill
P; total hydraulic load, tons, n; speed,
rpm, D; diameter of rollers, m
tc; ton cane coming in per hour.
Change coefficient 0.134 by 0.1 for crusher (two rolls) For mills with pressure feeders (Walker), multiply power demand by 1.1 For losses in gearing use 2.0 % in closed reducers with oil bath, and 8 % in open gearing. In combined gearing
eff. in transmision=(1-0.02)*(0.92)=0.90
Energy demand at exit prime movers =
= energy demand at exit of speed red./ eff.
Energy demand in reception-transportation and elevation of cane
0.19 kW- h per ton cane
Energy demand in intermediate carriers
0.12 times number of intermediate
carriers kW- h per ton cane
Energy demand in carrier to steam boilers
0.03 kW-h for each 50 m length, / ton cane