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