Question

Assuming capture and recovery efficiencies of 98% and 97% respectively, show that 1,000 ton/day of CO2 would correspond to capturing most of the carbon emissions from a 50 MWe plant fired with coal. State all your assumptions including fuel H/C ratio, and fuel-to-electricity conversion efficiency. How would your answer change if the electrical output of the plant remained the same but you switched from coal to natural gas as the fuel?

Answer 1

CO₂ captured and recovered per day = 1000*0.98*0.97 = 950.6 ton/day

CO₂ captured and recovered in kg per day = 950.6*907.185 = 8.62 x 10⁵ kg/day

Now, because for coal H/C ratio is found to be near 0.8 from literature.

So, 1 mol of coal approximately weighs = 12 g C + 0.8 g H = 12.8g/mol

Also, 1 g of CO₂ weighs to be 44 g/mol.

So, coal that must neen used to generate this much CO₂ is = (8.62 x 10⁵ ) * (12.8/44) kg/day

= 2.5 * 10⁵ kg/day

Now, for sake of simplicity, let us assume the calorific value for coal is around 3000kcal/kg and fuel to electricity conversion efficiency is 60%.

So, enegy generated per day= 2.5 * 10⁵ * 3000 * 4.184 = 3.14 * 10⁹ kJ/day

energy needed by plant per day = 50 * 10³ *24 * 3600 = 4.32 * 10⁹ kJ/day.

Thus, the energy needed is more than that can be generated from CO₂ . Thus, all the CO₂ produced can be utilized to generate the energy for the plant.

Now, if we change to natural gas, natural gas has a high H/C ratio of about to be 4.0 and calorific value is very high equal to 12500 kcal/kg(as compared to coal). So, there will be a high amount of excess CO₂ will be produced.