Question

Combined heat and power (CHP) is a technology that simultaneously produces electricity and heat from a

single fuel source. Here we consider a CHP system that consumes 72 GJ of coal per hour, while producing

6000kWh electricity and 32.4 GJ of heat. Assume that coal has a heating value of 25 MJ=kg.

(a) Using energy-based allocation, determine the carbon dioxide emission per kWh for this CHP system.

(b) How does it compare with a coal-fired power plant that produces electricity only? For the coal-fired

power plant, it is assumed the thermal efficiency is 35 %. Assume complete combustion and the coal

used has a CO2 emission factor of 2.5 kg=kg.

(c) It is assumed that the heat generated by the CHP system replaces heat from a coal-fired boiler with 90%

efficiency. If this is so, what is the carbon dioxide emission per kWh based on substitution?

Answer 1

Ans) Given,

Heat consumed (H_T)= 72 GJ

Electricity produced (P)= 6000 kWh or 21.6 x 10⁹ J or 21.6 GJ

Heat produced (H)= 32.4 GJ

Heating value of coal = 25 MJ /kg

Coal consumed = 72000/25

Coal consumed = 2880 kg

Total direct emission(E) = Coal consumed x emission rate

E = 2880 x 2.5

E = 7200 kg CO2

Assuming efficiencies of system for electricity production(N_p) = 35%

and, Effeciency for heat production(N_h) = 80%

(a) We know, CO2 produced for heat production, E_h,

E_h = H / N_h / [(H /N_h) + (P/N_p)] x E

E_h = (32.4/0.80) / [(32.4/0.80) + (21.6/0.35)] x 7200

E_h = 0.40 x 5335.2

E_h = 2880 kg CO2

E_p = E - E_h

E_p = 5335.2 - 2036.8

E_p = 4320 kg CO2

CO2 emitted per kWh = 4320/6000

= 0.72 kg CO2/ kWh

(b) For a plant producing only electricity,

Coal used = 21600/25

= 864 kg

Since, efficiency is 35%,

Actual coal required = 864 / 0.35

= 2469 kg

CO2 emitted = 2469 x 2.5

= 6172.5 kg CO2 or 1.03 kg CO2/kWh

Hence, this will emit more CO2

(c) again , using same formula as in part a , but with efficiency value as 90% for heat,

E_h = (32.4/0.9) / [(32.4/0.9)+21.6/0.35)] x 7200

E_h = 0.36 x 7200

E_h = 2592 kg CO2

Hence, E_p = 7200 - 2592

= 4608 kg CO2 or 0.768 kg CO2 / kWh