Hey guys my thermodynamics 2 professor sent out a practice e

Hey guys my thermodynamics 2 professor sent out a practice exam in preperation for our final next week. This exam is supposed to help us prepare for it. Any help is appreciated

Write all the states of the cycles and all processes. Draw diagrams for each problem very clearly and pay attention Problem i: (Grade: 10/100) Consider two biomass plants each producing 20 MWe. The first plant uses a conventional steam-cycle plant burning woodwaste. The second plant is a combined cycle plant using a gasifier feeding a gas turbine with the heat from the gas turbine driving a steam-cycle to produce additional power. The combined cycle plant uses willow trees grown on a dedicated short rotation energy plantation. Assume the steam plant is 25% efficient and the combined cycle plant is 45% efficient. You can assume the biomass can be represented in all cases by C6Hi206 with a heat content of 16 MJ/kg a) Under ideal conditions, burning biomass produces water vapor, N2, CO2 and heat. If this is the case for both plants, estimate how much Co2 is released per kWhr of electricity produced by each plant. Typical combustion is not ideal and Co, gaseous hydrocarbons, particulate matter and ash are also produced b) In the conventional boiler, 10% of the fuel by mass is unburned and remains in the flue gas in the form of co and unburned fix carbon called Particulate Matter (PM). This translates into 40 g-Carbon as CO and 1 g-Carbon as PM emitted for every kg of biomass burned in the boiler. Estimate the emissions in g-Carbon per kWhr of electricity for CO2, CO and PM

Solution

PART a

20 Mwe plant produces 20000 KW-hrs of energy every hour

To generate 20 MW power at 100% efficiency, 1.25 Kg per sec of biomass is required

(Heat content of biomass = 16 MJ/Kg, (20MJ/Sec)/16MJ/Kg = 1.25 Kg/sec)

Steam cycle plant at 25% efficiency will need 5 Kg/sec of biomass

Hence to produce 20000 KW-hrs of power, biomass required is 5 x 60 x 60 = 18,000 Kg

To produce 1 KW-hr, 0.9 Kg (900 gms) of biomass is required.

Under ideal condition, burning of biomass can be represented as

C6H12O6 (mol mass 180 gm) + 6O2 = 6CO2 (6x44 gm) + 6H2O

i.e on burnign 180 gm of biomass, 44x6 =254 gm of CO2 is produced

Hence on burning 900 gms (180 x 5) of biomass, 1320 gms (264 x 5) of CO2 is produced.

i.e 1320 gms of CO2 is released per KWhr of electricity produced by Steam cycle plant

Combined cycle plant at 45% efficiency will need 2.78 Kg/sec of biomass

Hence to produce 20000 KW-hrs of power, biomass required is 2.78 x 60 x 60 = 10,000 Kg

To produce 1 KW-hr, 0.5 Kg (500 gms) of biomass is required.

Hence on burning 500 gms (180 x 2.78) of biomass, 734 gms (264 x 2.78) of CO2 is produced.

i.e 734 gms of CO2 is released per KWhr of electricity produced by combined cycle plant

Part b

Given that for every Kg of biomass burned in the boiler, 40g CO & 1g PM is emitted.

Steam cycle plant burns 0.90 Kg of biomass per KWhr electricity,

hence, this unit will emit 36 g CO & 0.9 g PM per KWhr of electricity produced.

Combined cycle plant burns 0.50 Kg of biomass per KWhr electricity,

hence, this unit will emit 20 g CO & 0.5 g PM per KWhr of electricity produced.

Part c

The following gases will combust with oxygen

2CO + O2 = 2CO2

2H2 + O2 = 2H2O

CH4 + 2 O2 = CO2 + 2H20

Steam cycle plant burns 900 gm of biomass per KWhr.

Out of which 36 gms is CO, 99% of which is burnt & 1% escapes up the stack

Hence emission of g-C per KWhr is 0.36 gm

Combined cycle plant burns 500 gm of biomass per KWhr.

Out of which 20 gms is CO, 99% of which is burnt & 1% escapes up the stack

Hence emission of g-C per KWhr is 0.20 gm

Part d

Heat from turbine in the gasification process can be productively used for

heating within the process gas/water etc.