Question

What percentage reduction in carbon emissions do the following natural gas-fired CHP systems provide compared to the specified separately generated electricity and heat systems? For carbon emissions, assume natural gas emits 53 g CO2/MJ, and for average grid emissions assume 0.6 kg CO 2 /kWh. The joule equivalent of 1 kWh of electricity is 3.6 MJ.

a. CHP with 36% electrical efficiency and 40% thermal efficiency versus an 85%-efficient gas boiler for heat and the above average grid for electricity.

b. CHP with 50% electrical efficiency and 20% thermal efficiency versus a 33%-efficient, 1.0 kg CO 2 /kWh, coal-fired power plant for electricity and an 80%-efficient gas-fired boiler for heat.

Answer 1

CHP stands for combined heat and power

(a) CHP

1kWh of electricity it will emit (53 g CO₂/MJ/.36) x 3.6 = 530 g of CO₂

At this time it will also produce heat which will be (530g/53g)x0.4 = 4 MJ

1kWh of electricity we wll emit 0.6 kg CO₂/kWh

85% efficient gas boiler 4 MJ of heat (53 g CO₂/MJ/.85 )x4= 249 g CO₂

Total CO₂ produced is 600g + 249 g = 849 g

Whereas CHP produces 530 g of CO₂

% saving (849-530)/849x 100 = 37.6% saving in CO₂ production/kWh of electricity + 4MJ of heat

(b) CHP with 50% electrical efficiency and 20% thermal efficiency

1kWh of electricity it will emit (53 g CO₂/MJ/.50) x 3.6 = 381.6 g of CO₂

At this time it will also produce heat which will be (381.6g/53g)x0.2 = 1.44 MJ

1.0 kg CO 2 /kWh, coal-fired power plant for electricity

80%-efficient gas-fired boiler for heat. 1.44 MJ will require (53 g CO₂/MJ/.80 )x1.44= 95.4 g CO₂

Total CO₂ produced is 1000g + 95.4 g = 1095.4 g

Whereas CHP produces 381.6 g of CO₂

% saving (1095.4-381.6)/1095.4x 100 = 65.1% saving in CO₂ production/kWh of electricity + 1.44MJ of heat