How to calculate the capacity of Sugar based Cogeneration plant??

Capacity of the Cogen plant is nothing but the power and heat generation capacity of the particular plant.

Cogen plant capacity calculation involves deciding the capacity of the Boiler, steam Turbine, WTP and other auxiliaries like cooling tower, fuel handling plant etc.

Inputs required for Cogen plant capacity calculation.

1-Sugar plant capacity

2-Sugar plant electrical power demand

3-Sugar plant thermal load (Steam) demand

4-Percentage of fuel (bagasse) generated from the Sugar plant

5-No. of operation days of the Sugar plant.

6-Price per unit for excess power export to the Grid.

7-Area available

8-Future expansions in sugar plant

Let us consider the Sugar plant capacity 5000 TCD for demonstration.

A-Steam demand

1-Steam for sugar plant.

Sugar plant capacity: 5000 TCD or TPD (Tonnes per day)

Usually sugar process needs low pressure saturated steam for juice heating.

Required Steam parameters are;

Pressure: 1.5 to 2 Kg/cm2

Temperature: 125 deg C to 140 deg C

Consider thermal load i.e steam demand for Boiling house (Juice heating) is 40% of Sugar cane crushing.

Then steam required to sugar process = 5000 X 40% = 2000 TPD

Steam flow per hour = 2000 / 24 = 83.34 = 84 TPH

Rounding the figure, steam flow to sugar Boiling house = 85 TPH

2-Steam for distillery plant.

Generally, molasses-based distillery plant consumes around 3 to 3.5 kg of steam to generate 1 litre of ethanol.

So, total steam required for distillery process plant =Distillery plant capacity X 3.5

= 60 KLPD X 3.5 = 210 TPD = 8.75 TPH

Therefore, total steam demand for sugar and distillery process = 85 + 8.75 = 93.75 TPH

B-Power demand

The auxiliary power consumed per hour by any sugar plant as a Thumb rule = Cane crushing capacity X 80%

i.e total power required for sugar plant process = 5000 X 80% = 4 MW/day

Total power required for sugar plant considering 10% extra margin = 4 MW X 110% = 4.4 MWH

Other power consumption units related to sugar process are;

Employee & labour Colony power consumption = Sugar plant power consumption X 4 to 5%

= 4400 X 4 to 5% = 170-220 KWH = 0.17 to 0.22 MW

Consider Distillery plant of capacity = 60 KLPD for 5000 TCD sugar process.

Power consumption of distillery plant = Sugar plant power consumption X 20-25%

Power consumption of distillery plant = 4400 X 20-25% = 0.88 to 1.1 MW

Therefore, total power consumption of Sugar process and related auxiliaries is = 4.4 + 0.22 +1.1=5.72 MWH

C-Bagasse or fuel generated from sugar mill

Generally, an average of 28 to 29% bagasse is generated from sugar mill at various cane crushing conditions.

Therefore, bagasse generated from Sugar mill =5000 X 29% = 1450 TPD = 60.42 TPH.

D-No. of operation days of the Sugar plant

Based on current availability of sugar crop, sugar plant runs around 120 days in a year.

Cogeneration plant Capacity calculation

Let us decide the capacity of the power plant major equipment / auxiliaries.

Major equipment auxiliaries in the power plant/cogeneration plant are;

1-Boiler

2-Steam Turbine

3-Alternator

4-Lube oil system

5-Export transformer

6-Auxilary Transformer

7-Boiler feed pumps

8-Boiler fans

8-Air compressor units

9-Bagasse handling plant

10-Water treatment plant

11-Cooling tower and cooling water pumps

12-ESP

13-Chimney/stack

Let us decide the Boiler operating pressure and temperature for this plant.

Now a days high pressure and high temperature Boilers are selected for economical point of view.

High pressure boilers are more efficient and consume less fuel and also steam Turbines operating on high pressure steam consume lesser steam to generate unit power.

1-Calculation of Boiler Capacity.

Let us consider the operating pressure be 110 Kg/cm2 and temperature 540 deg C.

In design let us add HP heaters to increase the cycle efficiency.

Assumptions:

Return condensate from Sugar and distillery process = 95%

Total steam load = Process steam + Deaerator steam + HP heater steam + Exhaust steam

Note: During cane crushing season exhaust steam to condenser is very minimum, it is around 12 to 15% of Turbine inlet steam & during off season operation i.e. during condensate mode operation, turbine exhaust flow will be around 70% of Turbine inlet steam.

During cane crushing operation, steam flow to the Deaerator is least and is around 10% of total steam extracted from Turbine.

As a thumb rule,

TG inlet steam = Extraction steam / 70%

Extraction steam flow = Process steam flow + Deaerator steam flow

Extraction steam flow= 85 + Process steam flow X 10% = 85 + 85 X 10% = 93.5 TPH

Therefore, TG inlet steam = 93.5 / 0.7 = 133.5 TPH

Boiler capacity = 135 TPH-------------------------------------------------------1