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In
early 1930's when sugar factories were introduced in India, most of the working
units were operated by steam engines of low steam pressure and temperature. But
after development in technology and considering the economy in sugar-
industries, high pressure and high temperature boilers were introduced and it
made the low pressure and temperature steam engine obsolete. During the last
decade or so, concept of electrically operated units came into existence and
were introduced because of higher efficiency and lower maintenance cost.
After
implementation of steam economy measures at various stages, the requirement of
steam reduced to a great extent from 65% on cane to 45% on cane, which resulted
in large bagasse (fuel) savings. However sale of bagasse is not very lucrative
and its price fluctuates widely from year to year.
In
recent past, to meet increasing power demand and with a view to efficiently
utilise renewable energy sources into power generation, Govt.of India is
encouraging sugar industries to supply surplus power to grid and having realized
the scope of value additions by co- generation, many sugar industries are
looking at this concept seriously.
In sugar factories, requirement of steam in process
which is 50% on cane is fulfilled by generating steam in boiler and passing it
through Back pressure Turbo-Alternator set, Fibrizer turbine, other steam driven
prime movers and balance steam demand is meet by passing high pressure and
temperature steam to PRDS to get 1.1 Kg./Cm2 steam for process. Around 20-30
Ton/Hr. steam are passed to PRDS for its pressure reducing and desuper heating.
But to utilise this ,otherwise waste heat in
PRDS, to generate surplus power instead of wasting energy in PRDS and
utilise exhaust steam in process double extraction cum condensing turbine is required.
By using Extraction cum condensing turbine co-generation plant can
run in season as well as in off-season
High
pressure boilers are recommended for co-generation plants.For better efficiency
following points are recommended for operation
of boiler .Use of high pressure boiler ,
- Bunker System: An effective bunker system for the storage of baggase of
the requirement for 10-15 minutes for boiler MCR above the feeder for
effective combustion control , is required. But for this system bagasse
should be well prepared and not more then 50% moisture to avoid the choking
of the system.
- Traveling Grate is successful for bagasse as well as lignite.
- Bagasse Handling: During crushing season surplus bagasse is taken to
storage yard and this saved baggase could be used for cleaning day or
off-season in cooperation of co-generation plant. But as unit size is
larger, back feeding of larger quantum of baggase may be non-uniform by
bulldozers or pushers resulting in overloading and damage of conveyors. to
overcome th9s difficulties stackers reclaimers have been designed. such
system are successful in use in Marutious and Reunion Island. but cost
of such system is prohibitidly high.
- Milling System :- To run the cogeneration plant effectively better milling
i.e. well preparation of bagasse and controlling of moisture by a value of
around 50% is essential otherwise problem in combustion control and handling
of bagasse will be faced and also by increase of 1% moisture in bagasse ,
boiler efficiency reduces by 0.6%. In addition optimum imbibition should be
adopted for steam economy in boiling house and also all drives should be
changed to electric or hydraulic drives from steam turbine. By changing this
drives exportable power could be increased by about 1 MW for 2500 TCD plant.
- ESP System:- Operation of Electrostatic Precipitator for bagasse firing
has been proved to be successful . It reduces the emission level
as low as 50mg per normal cubic meter, but the large unburnt in the
ash may create some hazardous situation.
- Feed Water Quality Management :- This is essential for H.P boiler
and smooth running of turbine. Following are the points of attention :
-
Exhaust condensate may be used how ever sincere and
disciplined monitoring to keep the condensate free from contamination, for
this plant should ensure that piping and equipment using this steam and the
condensate are maintained properly for no contamination of this condensate.
-
If contamination is suspected better to use D.M. Water.
-
As quality of vapour condensate varies, generally pH is low,
the TDS and silica are high and there could be traces of ammonia and
organic compounds. This condensatye can be discarded but its heat energy
should be recovered. Then the requirement of boiler watre increases.A system
of online monitoring &batching to ensure that good quality vapour
condensate is taken has been devised, and also it was reported in
"power magazne" that the sugar contamination could be treated with
activated carbon & hydrogen peroxide to eliminate contaminant. By using
these techniques vapour condensate may be used in some extent to save the
requirement of DM water.
-
Resin and regeneration in DM plant is to be checked
regularly.
TURBO GENERATOR:- For Cogeneration plant to ensure
more surplus power to grid double extraction condensing turbine is
recommended for utilization of max. heat energy to generate surplus power. In
double extraction turbine 1st extraction is of 7 kg/sq.cm with flow range 3-9
tph. ,which is being used for sulphur burner etc. 2nd extraction which is
controlled extraction is of 1.5 kg/sq.cm. with flow as required in process
and small quantity for derator & rest steam to condenser. During off season
extraction steam flow is minimized for use in H.P. heaters and derator &
rest all steam being passed through condenser. The steam consumption of this
turbine for condensing operation is about 4kg/kw, which is very much economical
for off season operation.
(ii) Secondly for co-generation Turbine should be rugged &
of slow speed.
(iii) Steam purity should maintained to avoid the deposition on
blades. Silica contaminant is very critical which gets carried over as vapour as
operating pressure of boiler increases.
(iv) For condensing turbine quality of cooling water in
condenser tube need to be maintained & also some time for this vibration
exhaust hood spray falling on the blades and causing vibration which may be due
to misdirected spray nozzle in the exhaust hood.
(v) Ventilation system of T.G. house should be provided.
(vi) Due to grid problem T.G. Islanding is taking place. Fast
response governor should act accordingly to avoid overspending etc.
4. CONTROL AND INSTRUMENTATION:- DCS system is preferred
for fast response.
5. PROJECT:- Better to down size the plant as per
requirement to ensure good plant load factor.
6. VIABILITY:-
TECHNICAL:-
(i) Process side steam economy :- 45% steam consumption in place
of 50% for 2500TCD plant may export 1200KW surplus power which improve the
viablity of cogeneration plant.
(ii) Replacement of steam turbine by electrical hydraulic
drives.
(iii) Use of variable frequency drives whenever possible.
(iv) Thermal cycle optimisation.
(v) Paralleling at higher voltage levels.
(vi) O&M needed trained and qualified staff.
COMMERCIAL:-
-
NO. OF DAYS OF OPERATION OF PLANT:- This is the tradition of
sugar plants to stop when recovery goes down to certain limit. But with
co-generation plant apart from recovering sugar , plant also produces
precious fuels and when recovery is low fiber content is high and hence
quality of bagasse is more. Then it is worth the financial model for total complex
including sugar and cogeneration plant and should decide about the cut-off
recovery.
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Off-season operation.
- Adequate soft loans
- An equitable power purchase agreement with SEB and attractive purchase
price based on avoided cost.
- Making power purchase from co-generation mandatory on the parts
of SEBS.
- Transmission line to be laid by SEBs.
- ElECTRICAL mAINTENANCE:- Monitoring preventive maintenance
and testing of following equippments in plants were suggested as per
schedule
- Alternator:-for monitoring safe value and process value of
voltage, current, power factor, frequency, excitation voltage and current
are to be monitored on hourly basis.
Preventive maintenance:
- Space heater should be put on during long stoppage to avoid moisture.
- Bearing and journals should be checked during long stoppages.
- Air cooler should be cleaned
- Termination should be checked
- Carbon bush and its assembly should be cleaned and tightened
- Grounding/ earthing connection should be tightened
Testing
- Insulation resistance of stator winding with 5 KV Megger should be
tested.
- AVR should be tested for proper functioning.
- Air cooler leak test should be carried out to ensure no leakage.
- Bearing pedestal insulation should be tested with 500V megger.
- Protective relay should be tested by secondary injection.
- IR value of exciter and ohmic value exciter shall be tested.
- Voltage withstand test should be done.
- Transformer:-
Monitoring :- for monitoring safe value and process value of
transformer oil temperature, transformer winding temperature, conservator
oil level, silica gel color, EHV & HV side voltage & current should
be noted down periodically on hourly or shift basis.
Maintenance:-
- Transformer oil dielectric strength improvement by stream line filtration.
- Minimising earth resistance in switch yard.
- Dehydration breather (silica gel regeneration).
- Realizing air from Buchholz relay.
- Grounding / Earthing connections and terminations be tightened and
insulator be cleaned.
- In switchyard breaker and isolator mechanism should be provided proper
lubrication.
Testing :- Oil testing
- BDV
test
: 50KV/ 1 min.
- Acidity
: 1 Mg KOH per 1 gm oil
- Crackle test
: No audible noise.
- Insulation Resistance value of following be tested:-
- Transformer HV to LV winding.
- Transformer HV winding to frame & LV to frame.
- Insulation to ground.
- Earth resistance to each pit and mat with earth tester.
- Functional testing of all protective relays with necessary primary/
secondary injection.
- Contact resistance testing of circuit breaker and isolators.
- Station Battery Bank:- As co-generation plant is running in
parallel with grid, and there may be some instability in grid of either
voltage or frequency then in this case to avoid the damage of T.G set,
trip circuit should be healthy for which maintenance of battery for its
healthiness is essential. other maintenance are as per normal routine.
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