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Power Plants, Combustion, Heat Engines

Power Plants, Combustion, Heat Engines - Energy Society and...

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Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines
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Outline Combustion: Energy Generation and Pollutants 1 st Law of Thermodynamics 2 nd Law of Thermodynamics Efficiency
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Combustion What do we do with fossil fuels: burn them Combustion: impacts Fuels Balancing combustion chemical equations Combustion products
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What happens in combustion? Fuel + oxidizer -> Products + light + heat Combustion, in its simplest form, e.g: methane CH 4 + 2 O 2 CO 2 + 2H 2 0 A clean reaction, except for the issue of carbon dioxide and the global climate This idealized reaction takes place in an ‘atmosphere’ (oxygen) free of impurities
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How much CO 2 is produced when 1 ton of cellulose (C 6 H 12 O 6 ) is burned? (Cellulose is the primary component of plant matter, wood etc) Write the equation: C 6 H 12 O 6 + O 2 CO 2 + H 2 O Balance first the carbon : C 6 H 12 O 6 + O 2 6 CO 2 + H 2 O Then the hydrogen : C 6 H 12 O 6 + O 2 6CO 2 + 6 H 2 O Last, the oxygen (because you can change the oxygen without altering other elements): C 6 H 12 O 6 + O 2 6 CO 2 + 6 H 2 O, So, 6 + x = 18 x = 12, or 6 O 2 The balanced equation is: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O
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How much CO 2 is produced when 1 metric ton of wood is burned, continued ... How much do these weigh?
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Remember your atoms C = 6 p + 6 n = 12 O = 8 p + 8 n = 16 (atomic mass unit, OR 1 mole of C is 12 grams)
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How much CO 2 is produced when 1 metric ton of wood is burned, continued ... So, from C 6 H 12 O 6 6CO 2 + 6H 2 O, we see that: = 264g x 10 6 g 180g = 1 .47 x 10 6 g = 1 .47 tons of CO 2 How much do these weigh? C 6 H 12 O 6 = (6 x 12) + (12 x 1) + (6 x 16) = 180 grams (in 1 mole) 6 CO 2 = 6 x [(1x 12) + (2 x 16)] = 264 grams (from 1 mole of cellulose) 180 grams cellulose 264 grams CO 2 10 6 grams cellulose ?? grams CO 2 ??
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Simple combustion equation, but put it in air Example: methane reacts with air CH 4 + (O 2 + 3. 76N 2 ) -> CO 2 + H 2 O + N 2 (this is termed the ‘unbalanced’ version) CH 4 + 2(O 2 + 3. 76N 2 ) -> CO 2 + 2H 2 O + 7.52N 2 (balanced: exactly the correct amount of oxidizer to convert all C to CO 2 , and all H to H 2 O) Note: Air is 78% nitrogen, 21% oxygen, + other stuff
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Real Combustion If combustion occurs without complete oxidation , we get instead: CH 4 + O 2 + N 2 mostly (CO 2 + 2H 2 0 +N 2 ) + traces (CO + HC + NO...) This can occur when: temperature too low insufficient O combustion too rapid poor mixing of fuel and air, etc. ...
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Real, Real Combustion At higher temperatures, N reacts with O: air(N 2 +O 2 ) + heat NO x (nitrogen oxides, x can be 1 or 2) So much for pure fuels, now add impurities : enter N, S, metals and ash (non-combustibles) What we really get : Fuel (C, H, N, S, ash) + air (N 2 +O 2 ) (CO 2 , H 2 O, CO, NO x , SO x , VOCS, particulates) + ash V olatile O rganic C ompounds: VOCs
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Real, Real Combustion and Emissions NO x + VOCs = SMOG CO 2 , NO x , SO x + oxidants + water = ACID RAIN Particulates (especially ultrafine): • Create inflammatory response • Affect heart rate variability • Inducing cellular damage • May be associated with premature death
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