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INTRODUCTION
Need for alternative fuel
Industrial growth over the past century has seen an ever-increasing demand
on the earth’s fossil fuel resources such as coal & oil .These fuels have been
favoured due to their ease of extraction & cost-effective conversion into usable
energy. However recent discussion into the effects of fossil fuels on the
environment have encouraged investigation into renewable energy sources as a
way of alleviating negative environmental effects & their existing nature.
Reasons for low utilization of biomass
Biomass is comparable to solar energy in one respect that it occurs in a
highly diffused form, scattered throughout the country. Just as concentrating solar
energy at one point is difficult to collect & transport the relatively light biomass
from its point of origin to a centrally located processing facility. Although it is
stated above that the biomass either in the form of agro-waste or in the form of dry
grass had no commercial value, it still needs to be collected & transported by
somebody to the processing facility just a few Km away, cost about Rs.1000/ton.
In addition to the above expense, one should not expect a farmer to surrender this
agro-waste to a processing plant without any payment.
Because villagers are seen to use biomass, as a free of cost material for a
variety of purposes, even the so-called experts make the mistake of considering
plant biomass as a ‘no-cost’ raw material for industrial scale processing operations
too. This lack of awareness of the ground realities has done more harm than good
to cause of biomass energy.
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What is BAGASSE?
Sugarcane is a seasonally-grown food and feed crop, the processing of
which creates bagasse, a low-cost biomass material, as its by-product. Bagasse is a
commodity that is readily available for use—in 1992, 610 million tons of bagasse
was produced worldwide. It is suitable for production of energy, ethanol, animal
feeds, paper products, composite board, and building materials; and it is a feed
stock for fluidized-bed production of a range of chemicals.
Selection of ‘Bagasse as an alternative fuel’
Biomass is a readily available renewable resource that has been used
throughout the past as a source of heat energy by means of combustion. In recent
there has been increased research into the feasibility of converting biomass such as
bagasse into other form of usable energy.
Bagasse is comprised of lingo cellulosic residues & is a by-product of many
agricultural activities. Bagasse is essentially the fibrous waste left after the sugar-
cane has been extracted for crystallizing into sugar. The fraction of bagasse
obtained from raw cane crushed is approximately 20% - 30%.
Previously, bagasse was burned as a means of solid waste disposal.
However, as the cost of fuel oil, natural gas & electricity increased after the energy
crisis in 1970, special attention was paid to alternative fuels in an efficient way.
Consequently, conception of bagasse combustion changed & it has come to be
regarded as biomass fuel rather than refuse. The actual tendency is to use bagasse
as fuel, especially for cogeneration of electric power & steam, to increase its
contribution to the country’s energy supply.
This report will investigate that food waste recycling rate just was 1% &
how the sugar industry is using the principles of cleaner production to minimize
waste from the cane milling process by using the energy stored in bagasse to
power the process & in many cases add green energy to the main electricity grid. It
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will also investigate the economical & environmental effects of other options
available to process the bagasse into usable products such as FUEL.
Renewable energy program Govt. of INDIA
1. The Indian Renewable Energy Development Agency (IREDA) was set up by
the Ministry of Energy in 1987 to provide assistance to manufacturers &users
of renewable energy system.
2. In 1992, the Ministry of Non-conventional Energy Sources (MNES) was
established as a department in the Ministry of Energy.
3. The Ministry implements the Integrated Rural Energy Program transferred to
it from the Planning Commission.
4. A National Program on Biogas Development is a major Program of Ministry.
Waste collected (% by weight)
Industry waste
51%food waste
40%
others
9%
Industry waste food waste others
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PROPERTIES OF ‘BAGASSE’
Physical properties
1. White & light green.
2. It is odorless.
3. The typical specific weight is 250 Kg/m3.
4. The main content: - 45% moisture, 50% cellulose - (27.9% hemicellulose,
9.8% lignin & 11.3% cell contents) & 6% others.
5. Energy content: - 19400 KJ/Kg dry ash free.
Chemical properties
The percentage distribution by dry wt. of major elements composing the
bagasse is present in the below table.
Components C H O N S Ash
% by wt (dry basis)
49 6.5 42.7 0.2 0.1 1.5
Chemical formula
Estimation of the chemical formula of bagasse:-
1. The percentage distribution of the elements with & without the water
contained.
Give: - !00 Kg bagasse based on 45% of moisture content.
Component C H O N S Ash
Weight in Kg
(without water)
27
3.6 23.5 0.11 0.055 0.735
Weight in Kg
(with water) 27 8.5 63.5 0.11 0.055 0.735
5
2. Computering the molar composition of the elements neglecting the ash
component.
Components C H O N S
Atomic weight 12 1 16 14 32
Moles (without water) 2.25 3.6 1.47 0.008 0.002
Moles (with water) 2.25 8.5 3.97 0.008 0.002
3. Setting up the computation table to determine the normalized mole ratio.
Components C H O N S
Moles ratio (without water) 1125 1800 735 4 1
Moles ratio (with water) 1125 4250 1985 4 1
4. Approximate chemical formula of bagasse.
Without water: - C1125H1800O735N4S
With water : - C1125H4250O1985N4S
Bagasses are the fibrous residue of the cane stalks after crushing & consist
mainly of cellulose, pentosans & lignin. Its final composition after milling
depends on method of harvesting as well as age & type of cane. On average it is
assumed to have 50% moisture, 47.7% fiber & 2.3% soluble solids. The Gross
Calorific Value (GCV) of dry ash free bagasse is 19400 KJ/ Kg while bagasse
with 50% moisture content has GCV of 9600 KJ/Kg & Net Calorific Value (NCV)
of 7600 KJ/Kg.
GCV is also know as the Higher Heat Value(HHV) & the NCV as the
Lower Heat Value(LHV) it assumes that water formed by combustion & water of
the fuel constitution remains in vapour form. In industrial practice it is not
practicable to reduce the temperature of the combustion products below dew point
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to condense the moisture present & recover its latent heat, thus the latent heat of
the vapour is not available for heating purposes & must be subtracted from the
HCV.
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3
OVERVIEW OF SUGAR INDUSTRIES
Dry matter productivity of some selected agricultural crop (1)
CROP DRY MATTER YEILD
Wheat 4.4
Rice 8.4
Corn 8.4
Barley 4.2
Oats 4.0
Rye 3.4
Soybean 5.7
Sugarcane 49.4
Sugarcane bagasse has been reported to contain 48% cellulose. It thus
implies that the total world production of 233.942 million tons of bagasse from
15,895 hectares would yield 112.29 million tons of cellulose. These data indicate
that based on per unit of land area sugarcane is the most productive cellulose
producing crop. The majority of bagasse produced in small or large scale factories
is generally used as fuel in the same factory where it is produced. It has been
estimated that 10-15% of the bagasse from any sugar mill could be made available
for feeding animals, using the rest as fuel for factory furnaces. Most of the sugar
mills burn bagasse to generate steam & electricity. One ton of bagasse when
burned is equal in fuel value to one barrel of fuel oil.
8
Sugar industries scenario in INDIA
Sugar industry is the second largest industry after textile in India. India also
stands among the first five countries of sugar production in the world. The annual
turnover of the sugar industry is around 5500 crores of rupees and the total
investment is around 3500 crores of rupees. It also employs directly or indirectly
of about 1.75 crores people in India.
Crushing capacity of sugar mills vary from about 1500-5000 tonnes per
day, while that of the 'Khandasari' Mills vary from 20-200 tonnes per day.
The process of sugar production involves following steps.
1. Cleaning of canes.
2. Milling of canes.
3. Extraction of juice
4. Concentration of juice
5. Removal of impurities by addition of calcium phosphate, lime and double
carbonization or double sulphination.
6. Concentration of juice to syrup by evaporation.
7. Crystallization.
8. Separation of sugar through centrifuge i.e. by centrifugation of crystals.
9. Final packaging and handling of sugar.
Sugar cane synthesis the maximum solar residue and energy into biomass like
sugar, cellulose, lignin and pentosans.
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Profile of Indian sugar Industry
Year No of Mills Sugar production
in lakh tons
Consumption of Sugar
in lakh tons
1950-51 138 11.34 10.98
1955-56 143 18.92 19.73
1960-61 143 30.28 21.13
1970-71 216 37.46 40.27
1975-76 253 42.62 36.87
1980-81 314 51.16 49.70
1985-86 341 70.16 70.59
1991-92 392 134.64 106.26
The sugar companies comes under the Board of Industrial & Financial
Reconstruction (BIFR)(6).
As per the information provided by the BIFR as on 30-06-2003 , 44
companies involving 76 sugar mills are registered with BIFR . The state-wise
break up is as follows
Sr. no. STATE No. of Sugar Mills
1. Andhra Pradesh 3
2. Bihar 4
3. Kerala 1
4. Karnataka 5
5. Madhya Pradesh 3
6 Maharashtra 4
7. Orissa 1
8. Punjab 2
9. Rajasthan 1
10. Tamil Nadu 8
11. Uttar Pradesh 43
12. West Bengal 1
Total 76
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4
TREATMENT OF BAGASSE
A.) Improvement of the calorific value of bagasse (7)
(1.) Using flue gas drying
The existing sugar surplus and an acute energy shortage in India have
prompted the examination of co-generation possibility using available surplus
bagasse, and ways of saving more bagasse for off-season co-generation.
Out of 20% condensation loss in the boiler efficiency, 14% of the loss is
due to the moisture in bagasse, which is around 50%. Reducing the bagasse
moisture would help to increase the calorific value of bagasse, resulting in an
increase in the quantity of bagasse saved. This reduction in bagasse moisture
could be achieved in two ways:
1. by mechanically increasing the pressure on the mill rollers.
2. by using various sources of energy and equipment to dry the bagasse.
An energy efficient way to reduce the bagasse moisture is to use the heat
from the flue gas. Over the years, rotary dryers, flash dryers, swirl burner system
and other means of drying bagasse were tried with varying degree of success.
After looking at the various options available, the Andhra Sugars Ltd.
installed an induced draft flash dryer at the Sugar Unit - I during the 1999 / 2000
season and had achieved substantial bagasse moisture reduction, bagasse saving
and an increase in boiler efficiency. Based on the data and experience gathered, a
forced draft flash dryer was commissioned during the 2000/2001 season at the
Sugar Unit - II. The operation of this dryer had been smooth since its
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commissioning and had resulted in substantial bagasse moisture reduction, bagasse
saving and boiler efficiency increase.
The installation of bagasse dryers at the two Sugar Units resulted in bagasse
saving that had enabled co-generation for four months during the off-season and
increased the energy generated from 12.5 to 16.4 million units per year.
(2.)Through the implementation of modified Java method mill setting
The “Java method” of mill settings was known since before the World War
II and it experienced good results for mill performances.
To obtain good overall extraction several conditions are specified by the
“Java method”, e.g.:
1. Operate the mills at a very low rotation (2.5–1.5 rpm, gradually decreased
from the ultimate to the ensuing mills).
2. The actual fibre loadings should be as low as possible (gradually increased
to the ensuing mills).
3. The top roller lifts should be limited precisely as calculated.
With steam turbines the mill rotations will be higher and approximately the
same throughout the tandem; and consequently also the fibre loadings.
When MILL MATERIAL BALANCE is used as base for the mill settings
calculation, which modifies the method with the inclusion of the same average
value of fibre loadings throughout the tandem; the main objective of the Java
method could be achieved although steam turbines are used as the drives.
The said MMB calculation defined the flow of material (mass and volumes)
to and from each of the mill in the tandem individually; therefore the result could
be used as base and reference for individual setting of the mills.
Bases for calculation are quantity and quality of cane to be crushed,
quantity of mixed juice and imbibition water, and the dimensions of all the rollers
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in use, the average analysis of the extracted juices and the last mill bagasse
analysis.
Numerous calculations are required to complete a comprehensive MMB,
but with the use of a computer simulation it becomes simple and easy. Beside the
extractions it could also simulate the last mill bagasse for low moisture content
even when a higher imbibition rate is applied. This means an improvement of the
net caloric value of bagasse as fuel for the boilers could be projected (by formula:
NCV = 4250 – 10 s – 48 w).
An example of last mill bagasse produced by a tandem of 5 mills with
steam turbine drives is compared here.
Average bagasse analysis during campaign (mills were set without MMB):
Moisture = 50.23%
Caloric value = 1818 kcal/kg.
Average bagasse analysis when mills were set based
Moisture = 48.30%
Caloric value = 1920 kcal/kg.
It is concluded that with the proper mill settings based strictly on MMB
(modified Java method) the caloric value of bagasse will certainly increase and the
total bagasse consumed as fuel for the boilers for process requirement will be
reduced and additional excess bagasse will be obtained that could be used for
other purposes.
The potential for power generation from bagasse alone is estimated about
4,000 MW.
In view of its tremendous potential in power production, the Government
launched a National Program on Bagasse-based co-generation in 1994. At that
time, only 3 sugar mills in Tamil Nadu had the capacity to export about 5 MW of
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electrical power to the grid. As of now, 16 sugar mills based on bagasse have been
exporting surplus power to the extent of 56 MW.
A sugar mill can produce surplus power of 150 kWh per MT cane, under
ideal conditions. Study of a co-generation plant in the south of India shows that
price realization varies from US Cents 5 to 6 per kWh. Also, during the crushing
season, they utilize bagasse and lignite in the ratio of 90:10 whilst during off-
season the ratio works out to 40:60.
Some of the concerns of bagasse-based co-generation plants are related to:
1. Optimum boiler / turbine configuration
2. Round-the-year operation offering firm tie-up for power
3. Alternate fuels
4. Energy efficient sugar processing
5. Efficient bagasse storage, handling and retrieval system
6. Effective grid interface systems
B.)Biological treatment and storage method for wet bagasse for year round
biomass supply (10)
.
Successful industrial operations need year-round supplies of the seasonally-
produced and harvested bagasse. The harvesting season for sugarcane is
approximately six months. Industries operate a 300 tpd bagasse particleboard
plant, a year-round operation that requires over 54,000 dry tons (equivalent to over
120,000 tons wet) of bagasse stockpiled on an ongoing basis. The collection,
transportation, and storage of this seasonal product for year-round use present a
difficult problem in bagasse utilization.
A biological treatment, “FERLAB” has been developed which quickly
ferments the soluble residual sugars and other low molecular weight extractable,
while it maintains low bale temperatures and reduces long-term fermentation
losses in storage. The FERLAB treatment was combined with a self-ventilating
14
piling method of wet-baled bagasse to reduce fermentation losses and bagasse
moisture contents. To test the treatment, six 100-ton bagasse bale piles were
constructed. Bale temperatures, fermentation losses, and moisture contents were
monitored for four and one-half months.
Ventilation of baled bagasse piles reduced moisture content from 55% to
25%. The FERLAB treatment reduced dry matter losses during storage. The
FERLAB process consists of a proprietary mixture of thermophilic microbes. A
schematic of the process is shown in Fig below.
Chemical analyses of the wet and conventionally pre-dried stored bagasse
showed that the FERLAB-treated bagasse had 66% lower extractive contents and
significantly higher relative alpha cellulose and lignin contents, which suggests
reduced acid hydrolysis of higher molecular weight cellulose and lignin during
storage.
The FERLAB/wet self-ventilating bagasse storage provided easier handling
and lower transportation costs (drying reduced the weights). Reduced moisture
content increased the Btu content of mill-run bagasse from 3,130 Btu/lb to 5,586
Btu/lb. The stored bagasse at 25% moisture content can provide not only year-
round boiler fuel, but also raw material for an air-fluidized-bed reactor system to
produce low Btu gasses and chars, tar/oils which can be used as a substitute for
phenol in adhesives and other chemicals. Bagasse also can be stored for year-
round methanol production. These bio energy products reduce the storage
problems and transportation costs unique to biomass fuels. The FERLAB/wet self
ventilating storage is an alternate to current wet storage.
15
Schematic of the FERLAB treatment for biological drying of bagasse (10)
.
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5
A COMPARATIVE STUDY Bagasse as an Alternative Fuel to Coal for Industrial Uses
Introduction
The usage of coal as a fuel in industries results in the generation of gaseous
emissions. The total coal reserves (as on 1-1-1998) have been assessed by
Ministry of Coal (MOC) at 206.24 billion tonnes but bulk of these (87%) reserves
are non-coking coals of inferior grade. The Ministry of Environment & Forests
(MOEF) has imposed restrictions on the usage of non-coking coal with high ash
contents in view of huge fly ash generation. The need of the hour in today’s world
is to globally phase-out coal as a fuel source. Since the deficit of coal
consumption over its availability is expected to continue and much of the available
coal is of inferior grade and likely to find restricted use in the future due to the
MOEF regulations, it would be desirable to choose a suitable fuel to substitute
coal. The alternate fuel should have advantages in terms of utility, financial
savings, less pollution loads etc. The various substitute materials currently being
utilized are fuel oil, locally available cheaper agro-residues such as bagasse, husk,
briquettes, wood etc.
Bagasse is a by-product/waste in the sugar industry. Its use as a fuel is
restricted because of its low Calorific Value (CV). Theoretically the thermal
efficiency of bagasse can be improved when mixed with waste oil in the optimum
ratio of 4:1. This optimum ratio was arrived at such that, the prescribed minimum
stack height (30m) attached to the boiler is not altered.
The present study is oriented towards:
• Exploring the feasibility of using bagasse as an alternate fuel to coal and its
comparison vis-à-vis coal with respect to both technical and economical
aspects
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• Incorporating provision for any future changes in the inputs used for
cost analysis
Problem Formulation and Results
Ministry of Coal (MOC) has classified the non-coking coal into A, B, C, D,
E and F grades based on its useful CV. The useful CV in Kcal/kg according to the
MOC classification is defined by the formula:
8900 – 138(% ash content +% moisture content)
In the case of coal having a moisture less than 2% and a volatile content
less than 19%, the useful CV shall be the value arrived at as above reduced by
150 Kcal/kg for each reduction in volatile content below 19% fraction pro-rata .
Pollutant emissions
The emission rates of coal and bagasse are found out for direct firing
of these fuels such that, the stack height for the emission do not exceed the
stipulated minimum stack height 30 m . The assumptions made in the calculation
of pollutant emissions are:
1. No pollution control equipment is needed for the minimum stack height of
30m.
2. Fly ash generation from bagasse is 0.081 kg per each kg of bagasse fired
while that of the coal is 75% of coal ash content.
3. The only emissions generated and discharged through the stack are due to
the fuel firing.
With the above assumptions, the minimum fuel inputs are back calculated
and are given in Table below. The three types of coal selected from literature with
ash and moisture contents 43.12% & 3.84%, 34.38% & 6.04% and 25.19% & 6.0
% respectively are designated as C1, C2, & C3 types for the purpose of present
study. As per the MOC classification of non-coking coals, the above-designated
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C1 & C2 types of coals come under the category of F grade while C3 type coal
comes under D grade.
Table: Particulate emissions from coal and bagasse
Fuel QPE (KPH) Bagasse used (KPH) Coal used (KPH)
Bagasse 35 432 --
C1 type Coal 35 -- 108
C2 type Coal 35 -- 135
C3 type Coal 35 -- 185
QPE = Quantity of Particulate Emissions; KPH = Kilograms per hour
Steam generation
The characteristics and the CV of bagasse and optimum BWO mix are
collected from literature. The expected steam generation is found out using the
net CV of optimum BWO mix, bagasse alone and for coal respectively and
given in Table below. This data is utilized in understanding the economics of the
options considered in the present study.
Table: Steam generation from fuels
Fuel Calorific Value (kJ/kg) Steam generation (kg/kg)
Bagasse 8,021 3.12
Optimum BWO mix (4:1)
14,924 5.80
C1 type Coal 18,426 7.18
C2 type Coal 20,970 8.16
C3 type Coal 23,669 9.21
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Economics
The economic returns involved in the alternative usage of optimum BWO
as fuel for coal and/or bagasse alone are compared. The utilization cost of steam
from the fuels attempted for the study is calculated @ 2.5 US $ or Rs. 108.60 per
1000 kg of steam and is shown in Table-3. The purchase costs of bagasse, D-,
& F-grade coals are taken as Rs. 150 per 1000 kg, Rs. 594 and Rs. 415
respectively. Using this information, the Net Utility Value (NUV) of the fuels
attempted is calculated as the difference of its steam utility value (SUV) and
purchase costs.
Table: Net utility value of various fuels
Fuel QOF
(KPH)
PC
(Rs.)
SUV
(Rs.)
NUV
Rs.(SUV-PC)
Bagasse 432 64.80 338.83 274.03
Optimum BWO mix (4:1)
346 51.90 629.88 577.98
C1 type Coal 108 44.82 779.75 734.93
C2 type Coal 135 56.03 886.18 830.15
C3 type Coal 185 110.00 1000.21 890.21
QOF: Quantity of Fuel; PC: Purchase Cost; SUV: Steam Utility value.
Analysis Results & Discussion
Considering the above results, the monthly saving/expenditure on the
attempted fuel for the user is calculated with the assumption of 21 working hours a
day and 30 working days per month. The results are:
1. The usage of optimum BWO mix as fuel in place of bagasse result in a
net saving of Rs. 1.915 lakhs per month or Rs. 22.98 lakhs per annum.
2. The financial loss to the user due to the non-usage of bagasse as fuel is Rs.
1.726 lakhs per month or Rs. 20.716 lakhs per annum.
20
3. If the user purchases C1 grade coal despite the availability of bagasse, the
net expenditure incurred is Rs. 2.904 lakhs per month or Rs. 34.85 lakhs
per annum.
This is under the assumption that, the available bagasse is not used as a
fuel. Though the BWO mix is slightly expensive when compared even with C1
type coal, its usage is strongly recommended as an alternative to coal in view of
increasing demand for coal and its limited availability of its limited resources. In
addition, the need of the hour is to globally phase out the usage of coal as a fuel
resource.
21
6
BIOMASS GASIFICATION FACILITY
USING BAGASSE
The reactor operates at a pressure of 2 MPa and feed rate of 90 dry tonne
bagasse per day. It consists of a refractory-lined fluidized bed which utilizes
alumina beads as the inert bed media. Air and steam are fed into the reactor
through separate distributors in order to control temperature and gas composition.
Airflow is typically maintained at approximately 30% of that required for
complete combustion. Steam for the process is supplied by the sugar mill’s
bagasse-fired boiler. A plug-screw feeder is used to sufficiently increase the
bagasse density so as to seal the feed system against the pressure of the gasifier.
The plug exiting the screw feeder falls onto a water-cooled shredding auger which
breaks up the plug and conveys the bagasse into the reactor. Raw gas exiting the
top of the reactor passes through a refractory-lined cyclone to remove entrained,
unreacted char and ash particles. In the operations reported here, the process
stream is dropped from reactor pressure to near ambient conditions before entering
a product gas flare where it is combusted and vented to atmosphere. The process
operated in an air-blown mode, with no steam added, at a nominal reactor
temperature and pressure of 840°C and 300 KPa, respectively. Bagasse from the
factory at ~50% moisture was dried to ~25% wet basis. Wet fuel feed rate to the
reactor was approximately 1.1 t/ hr. An average gas composition determined over
the period of operation was 4% H2, 10% CO, 18% CO2, 3.3% CH4, ~1% C2’s and
higher hydrocarbons, and the balance N2. The higher heating value of the gas was
3.7 MJ m3. Carbon conversion efficiency was estimated to be ~96%. Resulting in
an additional 60 hours of operation. Testing using air and steam was carried out at
a reactor temperature and pressure of 860°C and 500 KPa. Wet fuel feed rate was
22
1.6 t/hr with a reduced moisture content of 17%. Steam addition resulted in
improved gas quality and higher heating value. Composition was determined as
8.5% H2, 12% CO, 18% CO2, 7% CH4, ~1% C2s and higher hydrocarbons, and the
balance N2 with a higher heating value of 5.8 MJ m3(10).
Process schematic of bagasse gasifier (10)
.
Fouling Of Boiler (10)
Boilers fired with fuels having high levels of potassium or sodium (alkali
metals), particularly in the presence of chlorine and sulfur, are susceptible to
fouling and slagging. Sugarcane leaves and tops are expected to contain much
higher levels of alkali compounds than bagasse; therefore using such fuels might
cause significant problems in bagasse boilers.
23
A rough gauge of fouling potential is the amount of potassium and sodium
compounds per unit of fuel energy; e.g., kilograms of Na2O and K2O per GJ. The
figure below plots concentrations of total alkali compounds (expressed as K2O
and Na2O), sulfur (as SO3), and chlorine per unit fuel energy, for four different
biomass feedstock sugarcane bagasse produced by milling; diffusion; unprocessed
banagrass (a cultivar of elephantgrass); and processed (chopped and leached)
banagrass, along with an estimate of each fuel’s potential to cause fouling.
Whereas bagasse contains relatively low- to- moderate levels of alkali
compounds (~0.05 to 0.15 kg per GJ), sulfur, and chlorine, and normally does not
promote excessive fouling in boilers, unprocessed banagrass contains very high
levels of alkali compounds (~0.7 kg per GJ), sulfur, and chlorine, and is
anticipated to readily foul boilers.
Concentration of total alkali, sulfur, & chlorine for four different biomass fuels &
fuel fouling/slagging potential (10)
.
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7
OPTIMIZATION OF BOILER EFFICIENCY USING BAGASSE AS FUEL
Introduction (8)
The designs referred to as ‘fuel cell’ and ‘horseshoe’ boilers were those
typically used for bagasse combustion. In these boilers, bagasse is gravity-fed
through chutes and burned as a pile. Nowadays, bagasse is burned in spreader
stoker boilers, replacing the combustors that use pile type approaches, and
improving combustion efficiency. Furthermore, the use of additional heat transfer
surfaces, as air heaters, economizers, etc. allows for a reduction of the stack
temperature below 200oC. With these improvements, efficiency of the boilers can
be increased up to 70%. Special attention has been paid to the optimization of
stoichiometric ratio as well as the stack gases temperature, for their influence on
the principal heat losses and, consequently, on the overall efficiency of the boiler.
Boiler and fuel characteristics
The experiments were carried out in three RETAL boilers shows a detailed
sketch of the main thermal surfaces of these facilities. The total height and depth
of the boiler are 10.6 and 10.92 m, respectively, and the width (not shown in the
figure) is 8 m. Summarizing the main characteristics, a nominal steam power of 45
t/h is achieved for an approximate bagasse consumption of 22 t/h; with a pressure
and temperature of the superheated steam of 1.9 MPa and 320 8C, respectively.
Bagasse fed to these boilers enters the furnace through five fuel chutes and is
spread mechanically. The major part of the bagasse characterized by small and
light pieces, burns in suspension. Simultaneously, large pieces of fuel are spread in
a thin even bed on a stationary grate. An average ultimate (dry) analysis of the fuel
used in the tests gave a 46.27% (in weight) of carbon, 6.4% of hydrogen,
25
Sketch of a RETAL bagasse-boiler (8)
26
43.33% of oxygen, 0% of nitrogen, 0% of sulfur, and 4% of ash. The moisture
content of the bagasse ranged from 48 to 52% for all the analyzed samples.
Operational test procedure
More than 60 tests were performed, attending the ASME recommendations
for solid and liquid fuels. Each test comprised three stages, namely: preparation,
measurements, and laboratory analysis. According to the standard procedures, one
should wait at least 24 h after startup of the boiler and 2 h after cleaning of the
bottom ash, the ash hopper located in the U-turn of the flue gas duct and the heat
transfer surfaces before starting a test. The boiler should reach, and maintain, a
steady state for at least 8 h before starting the test. The fuel chute and the
stationary grate must also be cleaned 1 h before starting, and the speed of rotation
of the spreader stokers fixed. Some trays have to be placed in the proper locations
for refuse collection, and the fly ash wet scrubber has to be cleaned as well.
Boiler measurements and laboratory analyses are performed along the
following 9 hours. The first four hours are dedicated to measure all the boiler
parameters every 15 minutes. Every half hour, stack gas composition (O2, CO, and
CO2) is determined and bagasse samples collected for the determination of their
moisture and ash contents. The furnace temperature is also measured every 15 min
using water-cooled suction pyrometers. The sampling and measurement locations
are shown in fig.
Laboratory work begins with refuse collection from all the different
locations (ash bottom, ash hopper and web scrubber). In the following five hours,
moisture and ash contents of the bagasse and solid samples from the fly ash
hoppers, wet scrubber, and bottom ash hopper are analyzed. When all the data are
assembled, a statistical analysis determines the mean and standard deviation for
each parameter. If a steady state has not been achieved in the boiler, the test must
be rejected.
27
As it is well-known, the overall efficiency of a boiler can be calculated
using both direct and indirect methodologies. The direct measurement of the
bagasse consumption is always subjected to many error sources. For this reason, in
the present study, efficiency has been calculated using the indirect methodology.
In general, this method relates the efficiency (η) of the boiler with the different
heat losses through the equation
η(%) = 100 - ∑ qi
where ∑qi = q2 + q3 + q4 + q5: In this equation, q2 represents the exhaust gases heat
loss, q3 and q4 are the chemical and fixed carbon loss, respectively, and q5 the
conduction heat loss from the external walls of the boiler. To quantify the heat
losses, the following equations are used [2]:
Here, Ieg and Iea are the exhaust gases and external air enthalpy,
respectively, αb the stoichiometric ratio at the exit of the boiler, QP l the bagasse
heating value (as received), ∆HCC the carbon heat of combustion, ∆HCO
C the CO
heat of combustion, AP the ash contents of bagasse from ultimate analysis (as
received) and RCO/f is the rate of kilograms of CO produced during the combustion
of one kilogram of fuel.
The stoichiometric ratio, α = ma/f/moa/f is defined as the ratio of the actual
air-to-fuel mole number ratio (ma/f) to the theoretical one (moa/f) for the same
28
experimental conditions. In turn, the actual air-to-fuel mole number ratio (ma/f ) is
defined as the theoretical number of moles of air plus the extra moles due to
excess air needed to achieve the complete combustion of one mole of bagasse (in
moleair/molefuel). In order to more accurately reproduce the physical influence of
the different parameters and heat losses in the statistical models, two
stoichiometric ratios have been defined, namely: stoichiometric ratio at the
furnace, αf; and stoichiometric ratio at the exit of the boiler, αb. It should be
pointed out that in the case of αb; the amount of surrounding air in-leakage into the
boiler due to non-air tightness, ∆α; is also included in the total air mole number.
Ai refers to Afa; Aah and Aba; which correspond to ash percentages in the fly
ash, ash hopper and bottom ash, respectively, obtained through laboratory analysis
combusting and weighting the different samples of refuse collected in a special
oven following the methodology of ASME. In the same way, ai refers to the ratios
of ash in the fly ash, afa; ash hoppers, aah; and bottom ash, aba with respect to the
total ash in the fuel, in kgash in refuse/kgash in fuel. From a mass balance of ash in the
boiler, considering Gi as the refuse collected per time unit in the different locations
in fly ash, Gfa; ash hopper, Gah; and bottom ash, Gba; respectively, in kgrefuse/s, the
following equation can be written
the different ash ratios ai; in fly ash, afa; ash hoppers, aah; and bottom ash, aba; are
defined by
and hence
To calculate the conduction heat loss, q5; from the external wall to the
surrounding area, the total heat lost (including radiation) has to be considered. In
29
Eq. (5),λT is the total heat transfer coefficient in kW/(m2°C), F is the total heat
transfer area of the external wall in m2, ti and tea are the external wall and external
air temperature (K), respectively. It has to be noted that in Eq. (5), the fuel
consumption, B; has to be included for dimensional homogeneity. The unknown
fuel consumption in Eqs. (4) and (5) is calculated by an iterative procedure.
Waste heat recovery scheme
Over the years, the boiler has been redesigned, mostly by modifying its
combustion systems according to the changes in fuel type. Attention has been paid
to heat losses respect to the exhaust gas, because they can reach up to 30% of the
total energy in the fuel. To obtain the optimal value for the exhaust gases
temperature, it is necessary to use additional heat transfer surfaces such as an
economizer, air heater, bagasse dryer, or some combination of them. However, the
addition of new elements increases the investment and operating costs of the boiler
and hence, the importance of establishing the optimal stack temperature. To solve
this problem, a minimum total cost (Z) has to be found through the equation
where i is the type of recuperative heat transfer surface (furnace water-walls, super
heater, generating tubes, air heater, economizer, and bagasse dryer); Pi the annual
cost of 1 m2 of the surface i ($/(m2 yr)); Fi the heat transfer area of surface i (m2),
Pef the equivalent fuel-oil cost ($ s/(yr kg)) and Bef the equivalent fuel-oil
consumption (kg/s). This fuel-oil equivalence means the amount of commercial
fuel oil with an average heating power (QP ef ) of 41,868 kJ/kg (and its price at the
oil market), needed to yield the same energy as the total bagasse consumed to
produce a given steam power. Once the efficiency is determined using the indirect
method, the total bagasse consumption, B; is calculated by
30
where Dsh is the measured steam power in t/h and Ish and Ie ec are the superheated
steam and the fed water enthalpy, respectively. The equivalent fuel-oil
consumption Bef is determined by
Using the common methodology to calculate the minimum value of a function, the
equation obtained to determine the optimal stack temperature, considering all the
heat transfer surfaces is
In this equation, T is the stack temperature; P and F define, respectively, the cost
and area for all thermal surfaces considered. Subscript w indicates furnace water-
walls; sh super heater, gt generating tubes; AH the air heater; ec the economizer,
and bd the bagasse dryer.
RESULTS AND DISCUSSION
Bagasse heating value determination
In general, bagasse has a broad range of heating values, extending from
6500 to 9150 kJ/kg (as received). Due to the importance of this parameter in the
determination of the efficiency of a boiler, it was carefully determined using a
calorimeter on more than 1000 samples collected during the tests. Results yielded
an average heating value for the bagasse of 7738 +/- 100 kJ/kg, as received.
However, in most sugar mills, it is not possible to carry out such a determination
in their laboratories. An alternative method has been considered, calculating the
heating value using the well-known equation [2]
31
taking into account the general chemical composition, but considering bagasse
moisture and ash contents from the samples measured in the laboratory tests. The
above equation provides a means for quickly determining the heating value of
bagasse (as received) with a high confidence level, good accuracy, and avoiding
the more difficult and time consuming experimental measurements. In contrast to
the ultimate fuel analysis, bagasse moisture and ash contents are relatively easy to
measure and are accessible to every sugar mill.
Optimization of boiler operation
The determination of the bagasse moisture and ash contents need to be
performed only once during the test. The analysis of exhaust gas composition is
measured at the beginning and at the end of the test. To carry out the boiler
optimization for different operational regimes, experimental measurements have
been obtained from the full tests according to the ASME procedure. As a
consequence of the experimental results, some important simplifications on both
the fixed carbon loss, q4; and conduction heat loss, q5; are considered.
From the experimental tests, it is concluded that q5 shows only a strong
dependence on steam power. For this reason, a simplified equation relating q5
with the steam power commonly used in this type of boilers was considered,
namely
Considering the physical influence of the fixed carbon loss (q4) on the
remaining heat losses (q2 and q3), it must be the first of all the heat losses to be
evaluated in the efficiency calculation. For the terms inside the brackets in Eq. (4),
32
experimental measurements during the tests performed demonstrated the validity
of the following inequality
which means that the terms corresponding to ash hopper and bottom ash can be
neglected when compared to the fly ash one.
The terms [100 – Ai] in Eqs. (4) and (15) are, by definition, the unburned
fuel (carbon) for the refuse collected in the different locations. Having in mind that
q4 is expressed as an unburned loss, it is convenient at this moment to introduce
the relation Cuf = [100 - Afa] as the unburned carbon in the fly ash. For this reason,
Eq. (4) can be rewritten as
the last parameter, Teg; is needed to calculate the exhaust gases enthalpy in Eq. (2).
The influence of the stoichiometric ratio in the furnace (αf) and steam power (Dsh)
on the unburned carbon in the fly ash, Cuf; has been plotted. As can be seen, the
unburned carbon increases with increasing steam power and decreases with
increasing stoichiometric ratio in the furnace. As steam power is raised at a
constant stoichiometric ratio, both the amount of bagasse fed and the combustion
air flow rate increase, since the air volumetric flow rate per unit weight of bagasse
is fixed. This, in turn, increases the average gas velocity in the furnace and the
fraction of fuel that burns in suspension, rather than in the bed on the stationary
grate. The shorter residence time available for combustion in suspension results in
an increased unburned carbon carryover and poorer combustion performance.
Therefore, when steam power and bagasse consumption are increased, a higher
stoichiometric ratio in the furnace is needed to achieve the same carbon
conversion (Cuf): Taking into account all the experimental data, a statistical model
is fitted
33
reproducing the experimental dependence of Cuf on both the stoichiometric ratio at
the furnace, αf ; and the steam power, Dsh: In this equation Dsh has units of t/h.
Once q4 is calculated, q3 and q2 can also be evaluated. To determine the chemical
carbon heat loss (q3); the parameter RCO/f needs to be calculated by the equation
µCO/C being the CO-to-C molecular weight ratio. CP and SP are the carbon and
sulfur contents of bagasse from ultimate analysis.
Performance of the unburned carbon in fly ash vs. stoichiometric ratio at the exit
of the furnace (αf) for different values of steam power.
34
The statistical model relating carbon monoxide, expressed as a fraction of
the total carbon oxides, to both αf and Dsh; is
It is important to note that the physical parameters measured in the tests are the O2,
CO and CO2 concentration in the exhaust gases, which are used to calculate the
stoichiometric ratio, αb; at the exit of the boiler applying the equation [2]
where O2; CO and CO2 are the stack gases composition analysis. This equation is
obtained using the combustion reactions as a function of the mole number,
However, Eqs. (17) and (18) are correlated to the stoichiometric ratio at the
furnace exit,αf ; because of the physical dependence of both Cuf and CO on αf
rather than on αb: These two stoichiometric ratios are closely related throughout
the air in-leakage, ∆α; by
Air in-leakage, ∆α; represents the leakage of surrounding air, due to non-air
tightness, into the boiler and can be calculated by
where ∆αnom is the air in-leakage at the nominal steam power (Dnom sh = 45 t/h).
This parameter, ∆αnom; was previously determined for all boilers tested and
yielded a roughly constant value of 0.2.
Finally, to calculate the exhaust gases heat loss, q2; the exhaust gas
enthalpy, Ib; needs to be known. This enthalpy depends on the stack temperature,
Teg. Relating Teg to αf and Dsh; the following equation is obtained.
35
Even when the dependence of Teg on Dsh is weak, it is noteworthy that the
stack temperature is raised as the steam power decrease. When the steam power is
decreased for a constant stoichiometric ratio at the furnace, combustion air flow
rate also decreases, reducing the average gas velocity in the furnace as well as the
heat transfer in the waste heat recovering scheme. As a result, a higher exhaust
gases temperature is measured at the exit of the boiler.
If the set of Eqs. (17), (18) and (22) is introduced into the methodology to
calculate the heat losses q4; q3 and q2; their individual contributions to the overall
boiler efficiency can be analyzed. This influence is shown in Fig. 5(a), (b) and (c)
for three different levels of stoichiometric ratio at the exit of the boiler, αb : 1.45,
1.5 and 1.8, respectively, considering a fixed constant bagasse moisture of 50%
and an ash content of 4% (dry). In all these figures, the stoichiometric ratio at the
furnace exit, αf ; has also being included. The evolution of αf as a function of
steam power derived from Eqs. (20) and (21) and its dependence on the level of
the stoichiometric ratio at the boiler exit, αb; can be easily verified. Comparing the
three figures, it is evidenced that, as the stoichiometric ratio at the exit of the
boiler increases, the heat losses have different behavior; the exhaust gases heat
loss, q2; undergoes a significant rise, while q3 and q4 decrease. Even though Eqs.
(17), (18) and (22) are valid for Dsh of 20 t/h, it can be seen that for low
stoichiometric ratios, Fig. 5(a), (αb =1:45); experimental data is only available for
steam flows above 30 t/h.
On the other hand, as can be observed in this figure, at higher steam
powers, q5 decreases, as predicted by Eq. (14). As the total heat transfer area is a
fixed value (for each boiler) and the external wall temperature is roughly constant
irrespective of the steam power, then the total heat lost to the surroundings (in
kW) is nearly constant as well. However, as an increase in the steam power is
36
related to higher fuel consumption, a reduction in the conduction heat loss is
finally achieved, according to the behavior also predicted by Eq. (5).
All these features are summarized, where the overall efficiency, η; is
plotted as a function of the stoichiometric ratio at the exit of the boiler and the
steam power, demonstrating the global effect of heat losses on boiler efficiency
(Eq. (1)). Data for only four values of the stoichiometric ratio are displayed, for
clarity. It is concluded that the highest efficiency is reached for a steam power
value in the vicinity of the nominal one, 45 t/h and for low values of αb (1.45).
This result is supported by the fact that the largest heat loss in these boilers is that
corresponding to the exhaust gases, q2. However, again for this value of αb; a
decrease of the steam power below 30 t/h, causes the unstable combustion regime
described before, which finally results in flame extinction. On the contrary, for αb
of 1.5 and 1.6, a nearly flat behavior of the efficiency with respect to the steam
power is reached, for the whole range, with values quite close to those achieved
for the lowest stoichiometric ratio, αb. It is for this reason that, including in the
analysis the results obtained for all the boilers tested, the optimal value of the
stoichiometric ratio at the exit of the boiler, αb; has been determined to range from
1.5 to 1.55, which allows for a full coverage of the whole range of steam powers.
It should also be noted that, prior to this experimental research, engineers and
boiler operators used to run the boilers at higher stoichiometric ratio values at the
exit of the boiler, even exceeding 1.8, loosing a large amount of thermal energy
resulting in a lower efficiency.
37
Calculated heat losses (q2, q3, q4, q5) vs. steam power for three levels of
stoichiometric ratio at the exit of boiler (αb): a) 1.45, b) 1.5, & c) 1.8.
38
Optimization of heat recovery scheme
Boiler design optimization is a very complex problem, requiring some
assumptions to simplify its mathematical treatment. In this study, to optimize the
waste heat recovery scheme, the speed of the flue gas, steam, water and air flow
are supposed to have their optimal values for all the heat transfer surfaces studied.
At the same time, the furnace exit gas temperature is kept constant at 900 8C and
the steam power is fixed at the nominal value of 45 t/h. It is also necessary to
consider the specific heat at the exit of the boiler and its average value at the
different heat transfer surfaces to be independent of the stack temperature. The
heat transfer coefficients should also be considered as independent of the
optimized temperature. To solve Eqs. (9) and (12), a thermal analysis of all the
heat transfer surfaces in the boiler was first performed followed by a coupled
mathematical and graphical analysis.
where subscript 1 means hot gas and 2 refers to the cold fluid (water, steam or air).
A summary of the heat transfer coefficients for the different surfaces studied is
presented in Table 1. Note that for the bagasse dryer, the volumetric heat transfer
coefficient is given in kW/ (m3 K).
Heat transfer coefficients used for the different surfaces studied
Generating tubes
Air heater Economizer Super heater
Bagasse dryer
k(kW/(m2K) 0.041 0.014 0.057 0.054 0.1014
A typical value for the stack temperature for boilers that burn common fuel
oils and coal ranges between 150 and 300 °C to avoid acid corrosion. However,
sulfur contents in bagasse are negligible, yielding a low dew point temperature for
the exhaust gases, around 60 °C, based on experimental determinations. Therefore,
39
keeping the external pipe temperature in the last equipment over 70 °C (or the
stack temperature over 80 °C) the problem of acid deposition is avoided Teg; varies
between 80 and 100 °C for all the cases analyzed, except when a bagasse dryer is
taken into account. The optimal stack temperature is slightly higher than 60°C;
which is permissible when the last recuperative piece of equipment is the bagasse
dryer, because acid deposition on the previous heat transfer surface (in this case
the economizer) will never occur. The optimal value for bagasse moisture, W; is
near 41% when a bagasse dryer is taken into account.
40
8
FAST PYROLYSIS OF BAGASSE TO PRODUCE BIO OIL FUEL FOR POWER GENERATION
Introduction
It is a well-established fact that combustion of fossil fuels such as coal, oil
and natural gas for power generation is a significant contributor to global
warming. On the other hand biomass has long been identified as an alternate
sustainable source of renewable energy. However, power generation using a solid
fuel has had significant limitations with respect to materials handling requirements
and efficient energy conversion. Converting biomass fuel into a liquid addresses
these issues and makes possible the use of higher efficiency combined cycle
systems for power generation. ‘Fast pyrolysis’ technology is a unique process that
converts solid biomass waste materials, such as bagasse, into a relatively clean
burning liquid fuel called BioOil that is also carbon dioxide and greenhouse gas
(GHG) neutral. The nearest term commercial application for BioOil is as clean
fuel for generating power and heat from small stationary diesel engines, gas
turbines and boilers.
Fast Pyrolysis of biomass
Fast Pyrolysis (more accurately defined as thermolysis) is a process in
which biomass material, such as bagasse, is rapidly heated to high temperatures in
the absence of air (specifically oxygen). The bagasse decomposes into a
combination of solid char, gas, vapors and aerosols. When cooled, most volatiles
condense to a liquid referred to as ‘BioOil’. The remaining gases comprise a
medium calorific value non condensable gas.
BioOil is a liquid mixture of oxygenated compounds containing various
chemical functional groups, such as carbonyl, carboxyl and phenolic. BioOil is
41
made up of the following constituents: 20-25% water, 25-30% water insoluble
pyrolytic lignin, 5-12% organic acids, 5-10% non-polar hydrocarbons, 5-10%
anhydrosugars and 10-25% other oxygenated compounds.
In this particular fast pyrolysis process, biomass feedstock is introduced
into a thermolysis reactor having a bed of inert material, such as sand, with a
height to width ratio greater than one. The biomass is shredded to sufficiently
small dimensions so that its size does not limit significantly the production of the
liquid product fraction. Simultaneous introduction of pre-heated, non-oxidizing
gas at sufficient linear velocity performs two principal functions: firstly, as a
medium for fluidizing the hot sand bed and secondly, to cause automatic
elutriation of the product char from the fluidized bed reactor. The process includes
removing the elutriated char particles from the effluent reactor stream and rapidly
quenching the gas, aerosols and vapors to produce a high conversion yield of
liquid BioOil. For maximum yield of liquid, the thermolysis reaction must take
place within a period of a few seconds at temperatures ranging from 450°C to
500°C. The products must then be quenched as soon as possible to prevent
cracking of the newly produced BioOil.
BIO THERM Flow sheet
42
Fast Pyrolysis heat and mass balance
Feedstock for the fast Pyrolysis process can be any biomass waste material
i.e. bagasse. To maximize yield and minimize process development risk,
DynaMotive has focused near term BioOil production from sugarcane bagasse.
Preparation includes drying the feedstock to less than 10% moisture content
to minimize the water content in the BioOil and then grinding the feed to small
particles to ensure rapid heat transfer rates in the reactor.
When processing bagasse feed stocks, the conversion yield to liquid BioOil,
solid char and non-condensable gas is approximately 62%, 26% and 12% by
weight, respectively, on an as fed basis. The heat required for thermolysis is the
total heat that must be delivered to the reactor to provide all the sensible, radiation
and reaction heat for the process to proceed to completion.
The heat of reaction for the fast Pyrolysis process is marginally
endothermic. When operating the pilot plant using prepared feedstock, the total
heat requirement to produce BioOil at a 62% yield rate (including radiation and
exhaust gas losses) is approximately 2.5 MJ per kilogram of BioOil produced. The
net heat required from an external fuel source, such as natural gas, is only 1.0 MJ
per kilogram of BioOil produced. This applies when the non-condensable gases
produced in the process are directly injected into the reactor burner. This
represents approximately 5% of the total calorific value of the BioOil being
produced.
Bio Oil Analysis
BioOil is a dark brown liquid that is free flowing. It has a pungent smoky
odor. BioOil contains several hundred different chemicals with a wide-ranging
molecular weight distribution. The following Table below lists the properties of
BioOil produced by the BioThermä pilot plant, derived from bagasse biomass feed
stocks.
43
Table: DynaMotive BioOil Properties
Biomass Feedstock Bagasse
Moisture wt% 2.1
Ash Content wt% 2.9
BioOil Properties
pH 2.6
Water Content wt% 20.8
Lignin Content wt% 23.5
Solids Content wt% < 0.10
Ash Content wt% < 0.02
Density kg/L 1.20
Calorific Value MJ/kg 15.4
Kinematic Viscosity cSt @20°C
57
cSt @80°C 4.0
The density of BioOil is high, approximately 1.2 kg/liter. On a volumetric
basis BioOil has 55% of the energy content of diesel oil and 40% on a weight
basis.
The solids entrained in the BioOil principally contain fine char particles
that are not removed by the cyclones. As can be seen, the solids in the BioOil have
been reduced significantly to levels of approximately 0.1% by weight. The ash
content in these solids ranges from 2% to 20%, depending on the ash content in
the feedstock.
44
Table: BioOil Composition
Biomass Feedstock Bagasse
BioOil Concentrations wt%
Water 20.8
Lignin 23.5
Glyoxal 2.2
Hydroxyacetaldehyde 10.2
Levoglucosan 3.0
Formaldehyde 3.4
Formic acid 5.7
Acetic acid 6.6
Acetol 5.8
Market Competitiveness
In the proprietary DynaMotive process, BioOil and char are commercial
products and the non-condensable gases are recycled back into the process. No
waste is produced in the DynaMotive process. The overall simplicity of the
technology gives DynaMotive significant competitive advantages over other
Pyrolysis technologies in terms of capital and operating costs. DynaMotive
process also produces higher quality fuel and higher yields of BioOil compared to
competing technologies. DynaMotive’s target cost for BioOil production is $5 per
gigajoule (GJ) based on a 400 tonne per day (tpd) commercial plant. The earliest
and most appropriate market application for BioOil is as a clean fuel to produce
“green power” and heat in boilers, kilns, gas turbines and diesel engines.
DynaMotive is also developing higher value fuel and chemicals
applications for BioOil including ethanol blended fuels, diesel/BioOil emulsions
and catalytic upgrading of BioOil to synthesis gas, which can be converted to
synthetic transport fuels and bio methanol for use in hydrogen fuel cells.
45
Bio Oil Application as a Fuel in Gas Turbine Engines
As a clean fuel, BioOil has a number of advantages over fossil fuels:
1. CO2 / Greenhouse Gas Neutral – because BioOil is derived from biomass
(organic waste), it is considered to be greenhouse gas neutral and can
generate carbon dioxide credits.
2. No SOx Emissions – As biomass does not contain sulfur, BioOil produces
virtually no SOx emissions and, therefore, would not be subjected to SOx
taxes.
3. Low NOx – BioOil fuels generate more than 50% lower NOx emissions
than diesel oil in gas turbines.
4. Renewable and Locally Produced – BioOil can be produced in countries
where there are large volumes of organic waste.
As BioOil has unique properties as a fuel, it requires special consideration and
design modifications. Some of these properties are presented and are compared to
those of diesel fuel.
Table: Typical Properties of BioOil Compared to Diesel Fuel
BioOil Diesel
Calorific Value MJ/kg 15-20 42.0
Kinematic Viscosity 3 – 9
@ 80°C
2 – 4
@ 20°C
Acidity pH 2.3 - 3.3 5
Water wt% 20 – 25 0.05 v%
Solids wt% <0.1 0.05 v%
Ash wt% <0.02 0.01
Alkali (Na + K) ppm 5 - 100 <1
A first generation fuel system and combustion system were designed and
tested, demonstrating the capability to operate a 2.5 MW industrial gas turbine on
46
BioOil. These tests not only revealed the feasibility of operation but also
demonstrated that similar performance could be achieved for BioOil and diesel.
Although CO and particulate emissions were higher than diesel, testing revealed
that NOx emissions were about half that from diesel fuel and the SO2 emissions
levels were so low as to be undetectable by the instrumentation.
Pyrolysis Oil NOx Emission Reduction
The engine being utilized for this program is the 2.5MW OGT2500
industrial gas turbine engine. The OGT2500 offers distinct technical advantages
over other engines. Unlike aero-derivative engines, it has been designed as an
industrial engine with durability being one of the main design criteria and not
weight. In addition to its ruggedness, the distinct “silo” type combustion system
allows for easy access and modifications to the entire combustion system, which is
one of the critical systems for the adaptation of the engine to BioOil.
Application of Pyrolysis Oil to Gas Turbine Operation.
47
BioOil has an energy density approximately half that of diesel fuel.
Therefore, to meet the same energy input requirement, the flow rate must be
double. This requires design changes to the fuel system to be able to control higher
flow rates and also modify the fuel nozzle to accommodate this larger flow. This
lower energy density also can affect combustion since physically there must be
twice as much fuel in the combustion chamber as with diesel.
Higher viscosity of the fuel reduces the efficiency of atomization, which is
critical to complete combustion. Large droplets take too long to burn. Proper
atomization is addressed in three ways. Firstly, the fuel system is designed to
deliver a high-pressure flow since atomization is improved with larger pressure
drops across the fuel nozzle. Secondly, the fuel is pre-heated to lower the viscosity
to acceptable levels. Thirdly and most importantly, the fuel nozzle has been
redesigned to improve spray characteristics. These design improvements are
important for complete combustion and effectively reducing CO emissions.
Due to its relatively low pH, material selection is also critical for all
components wetted by BioOil. This does not require the use of exotic materials;
however, it does eliminate some standard fuel system materials. Typically, 300
series stainless steels are acceptable metallic materials and high-density
polyethylene (HDPE) or fluorinated HDPE for polymers.
Although looked at as a contaminant for diesel fuel, the water content in
BioOil has some advantages. Firstly, it is helpful in reducing the viscosity, since it
is a relatively low viscosity fluid. As well, it is a factor in lowering thermal NOx
emissions.
The solids content is a combination of ash and char fines which have
carried-over to the liquid part of the BioOil. The effect of these solids is to cause
sticking of close tolerance surfaces. They can result in particulate emissions
because of the long residence time required to fully combust. It is important that
the solids level in the BioOil is controlled to be less than 0.1 wt%.
48
The ash content in the fuel represents the material that cannot be
combusted. Depending on the elements in the ash, it can result as a deposit on the
hot gas path components that will reduce the turbine efficiency. This operational
problem is a familiar one with the use of low-grade fuel oils that also have high
ash content. The solution is a turbine wash system. This typically consists of two
separate systems in which an abrasive medium is injected during operation to
physically ‘scrub’ off the deposits. This allows turbine cleaning without any
downtime. The second system is an offline process,, which injects a cleaning fluid
and allows a soak period to loosen the deposits that are then removed when the
engine is started.
Due to the poor ignition characteristics of BioOil, one other key design
requirement is a BioOil specific ignition system or process. To overcome this, the
OGT2500 system starts on diesel fuel flowing through the primary channel in the
fuel nozzle. Following a warm -up period, BioOil is fed into the secondary
channel at an increasing rate while the diesel fuel flow is reduced until 100%
BioOil flow is achieved.
Converting biomass wastes produced from agriculture and forestry
operations to a liquid BioOil, using DynaMotive’s fast Pyrolysis technology, has
been demonstrated at the pilot plant level as a reliable and repeatable process. Test
programs to demonstrate BioOil application as a fuel in gas turbine engines, diesel
engines and boilers are underway with a host of engine manufacturers. To-date the
results have been most encouraging.
49
9
APPLICATIONS
Locomotives
Compared to oil , coal or even wood , the calorific value of bagasse
being very low & most locos have large auxillary tenders to carry it . Since
it is so fibrous , much of the fuel passes from the fire box straight through
the boiler tubes unburned & burns up in the air .
Electricity generation(9)
India, which accounts for around 85% of South Asian electricity
generation, is facing serious power problems with current generation is about 30%
below demand. New options have to meet the challenge and need to invest heavily
in new electric generating capacity. Overall, Indian power demand is projected to
increase to 1,192 billion-kilowatt hours (BkWh) in 2020, around three times the
378 BkWh consumed in 1996.
Net Electric Output Calculations for Combustion of Bagasse:
Combining of all thermal efficiencies of equipment involved equates the
total thermal efficiency for the total system:
ηoverall= ηcomb * ηboil * ηgen
Overall thermal efficiencies for the standard combustion steam cycle and
integrated gas combined cycle technologies reported in were used in the following
calculations:
Net electric output =ηoverall * φfuel * LHVfuel * Pparasitic
Where
fuel = mass flow of entering fuel and
Pparasitic = amount of electricity consumed by plant equipment
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Parasitic electricity consumption was neglected due to lack of information on
exact equipment parameters and for ease of calculation. However, for a true
account of net electricity these should be included in the calculations.
φ Fuel = 100 tonne bagasse per hour; LHV = 7600 kJ/kg
Net electricity output = 7600 kJ/kg *0.35 *100 000 kg/hr *1 hr/3600 s
= 73.89 MW.
Ethanol production (9)
Ethanol is produced through the fermentation of sugars by yeast. The most
commonly used yeasts come from the Saccharomyces and Zymomonas genii and
use the Embden-Meyerhof and Entner-Doudoroff pathway respectively. Glucose
is bio-chemically converted to the intermediate pyruvic acid, the next step is non
oxidative decarboxylation and acetaldehyde formation catalyzed by
decarboxylase. This is followed by acetaldehyde reduction catalyzed by
dehydrogenase to form ethanol. The net chemical reaction is that of the following:
C6H12 O6 � 2CH3CH2OH + 2CO2
The fermentation is carried out non-aseptically at 23 to 32°C. Antibiotics
may be added to control possible contaminants. Because the overall reaction is
exothermic, cooling is required. The fermentation can take on average 40 to 50
hours. Carbon dioxide is normally vented; if it is to be recovered the vent gas is
scrubbed with water to remove entrained ethanol and then purified using activated
carbon. The out-gassing of the carbon dioxide from the fermentor provides
sufficient agitation for small tanks. Mechanical agitation may be added for large
fermentation tanks. The Theoretical yield of ethanol from bagasse is stated as
111.5 US gallons per US ton of bagasse.
Essentially, there are three basic steps in ethanol production from bagasse:
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1. Feed preparation – involves the separation of cellulose and hemicellulose
(sugar containing components) from the unwanted solid waste (lignin);
2. Fermentation – xylose and glucose from the hemicellulose and cellulose
respectively are fermented by yeast (generally a species such as
Saccharomyces cerevisiae) to generate ethanol.
3. Product finishing – centrifuging and distillation operations are used to
separate the solid waste and broth from the ethanol product. The resultant
product stream is usually a minimum of 95% ethanol.
There are two specific technologies used in the conversion of bagasse to
ethanol: (1) two-stage dilute-acid process, and (2) enzyme-based process. Figure
below shows the two-stage dilute-acid process flow diagram; this consists of four
basic unit operations: first stage hydrolysis; second stage hydrolysis; ethanol
fermentation; and product purification.
Two-stage Dilute-acid Process Flow Diagram
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Ethanol is currently the most commonly used fuel alternative throughout
the world, used primarily as an octane booster to prevent early ignition and to
extend gasoline. It also helps to prevent air pollution by acting as an oxygenate,
reducing carbon monoxide and ozone. . Moreover, ethanol from product of the
sugar industry by its use in the transport sector can also play a critical role in
reducing GHGs. Ethanol can be used as a 10% gasoline blend in the automobiles
without any modification to the engines. It can also be used as a diesel blend in
stationery engines and automobiles along with an additive. India consumes nearly
6000 billion liters of gasoline and 42000 billion liters of diesel. Ethanol, which is
currently, produced from molasses has the capacity to substitute more than 1000
million liters of gasoline per annum. To meet additional demand of ethanol other
methods such as direct production from cane juice and bagasse can be explored.
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10
CONCLUSION
India one of the leading sugarcane producers in the world realizing the
potential of bagasse, a by-product of the sugar industry, for power generation, has
come up with various programs and incentives to boost the sector.
India produces nearly 40 million metric tonne (MMT) of bagasse, which is
mostly used as a captive boiler fuel other than its minor use as a raw material in
the paper industry. Sugar mills in the country especially in the private sector have
invested in advanced cogeneration systems by employing high-pressure boilers
and condensing cum extraction turbines. These sugar mills have been able to
export power in the season as well as in the off-season by using bagasse or any
other locally available biomass and to some extent coal. Off-season operation has
been more lucrative by exporting power which otherwise earlier was non-existent
except some operation and maintenance work. High technology has made these
sugar mills efficient by improving the economic viability of the mills in terms of
higher production of units of electricity per unit of bagasse.
Environmental benefits
The sugar mills showing interest in cogeneration projects, it has benefited
the environment by reducing the greenhouse gases (GHGs) in the atmosphere in
terms of the usage of biomass as fuel. Bagasse and other biomass, which are
renewable, can play a major role in substituting fossil fuel for future power
generation. There is a potential of 3500 MW bagasse based cogeneration potential
and 16500 MW other biomass power potential in the country. A typical 2500 TCD
sugar mill having a cogeneration potential of 22 MW exports nearly 0.3 million
units of electricity in the season with a gross generating capacity of more than 150
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million KWhs in a year and thus can offset nearly 0.166 million tonne of carbon
dioxide. The Clean Development Mechanism (CDM) can be an effective tool in
the sugar sector creating a major impact by the way of technological and financial
transfer between India and developed countries and can help start the transition
towards truly environmentally, economically and socially sustainable energy
systems.
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11
REFERENCES
1. D.S Chahal. “Food, Feed & Fuel from Biomass”, reprint 1991, pp.23.
2. N.H.Ravindranath, K.Usha Rao, Bhaskar Natranjan. “Renewable Energy &
Environment”, 2nd reprint 2001 pp.106, 242.
3. L.A Ekal, S.H Pawar. “Advances in renewable energy technologies”, 1st reprint
pp.35, 194.
4. S.Rao, Dr. B.B.Parulekar. “Energy Technology”, 2nd edition, 1997.
5. G.D Rai. “Energy Resources”, 3rd edition, 1999.
6. http://164.100.24.208/Is/committeeR/Food/27.pdf
7. M.Narendranath & G.V.S. Prasada Rao. “Improvement of the calorific value of
bagasse using flue gas drying”, & T.Sumohandoyo ,“by JAVA method mill
setting”, www.greenbusinesscentre.com/casestudies/sugar/sugar-case%20study
8. Jorge Barroso, Felix Barreras, Hippolyte Amaveda, “On the optimization of boiler
efficiency using bagasse as fuel”, www.fuelfirst.com
9. http://bioproducts-bioenergy.gov/pdfs/bcota/abstracts/30/z130.pdf
10. G.A.Grozdits. “Biological treatment & storage method for wet bagasse”,
S.Turn. “Demonstration-Scale biomass gasification facility operates on bagasse”.
International Cane Energy news, July 1997
Newsletter of the International Cane Energy Network.
