CEE 452/652. Week 14, Lecture 1 NOx control. Dr. Dave DuBois Division of Atmospheric Sciences, Desert Research Institute

Transcription

1 CEE 45/65 Week 14, Lecture 1 NOx control Dr. Dave DuBois Division of Atmospheric Sciences, Desert Research Institute

2 Today s topics Today s topic: NOx control Read chapter 16 Presentations on Nov 9 and Dec 4 schedule Noe Steven Heston Sarah Casey Kyra James 9-Nov 9-Nov 9-Nov 4-Dec 4-Dec 4-Dec 4-Dec Remember that your presentation topic must be related to air pollution control.

3 Grading based on the following Relevance to an air pollution control issue Was there a clear outline? Technical quality (details and facts to back up your discussion) Quality of graphics/slides Ability to answer questions List of references 3

4 Control of NOx Recall what is meant by NOx (nitrogen oxides) In air pollution work NOx = NO + NO However 95% of NOx is NO (nitric oxide) Why is it important to control? Ozone precursor Health standard Forms particulates (primary standard & visibility) Same sources that emit greenhouse gases Where does NOx come from? 58 percent from mobile sources 0 percent from power plants See next slide 4

5 Latest Estimate of NOx Sources #3 For whole US # #4 #1 5

6 NOx Formation Thermal NOx formed by reaction between N and O in the air; sensitive to temperature Fast formation rate at high temperature Fast cooling rate freezes formed NOx Can be about 5% of the NOx from coal fired EGUs Fuel NOx formed from combustion of fuels containing organic nitrogen in the fuel (coal or heavy oil) dependent on local combustion conditions (O ) and nitrogen content in the fuel Can be about 75% of the NOx from coal fired EGUs 6

7 Thermal NOx Basic reactions to generate NO: Principle source of O atoms is dissociation of O, although O can be formed from hydrocarbon reaction products Highly temp dependent, occurs mainly at temps above 300F; produces N for following reactions Produces same amount of NO as reaction above Less important than above rxns 7

8 Thermal NOx Major factors that influence NO production include: Temperature O concentration Residence time Changing these parameters can reduce NO concentrations called quenching 8

9 Experiments to look at NO formation in natural gas fired power plant boilers Reduce residence time NO vs equivalence ratio and time Stoichiometric A/F = 16.3, air preheated to 650 F Equivalence Ratio φ = ( F / A) actual ( F / A) stoich 9

10 Fuel NOx Fuel contains organically bound nitrogen and release upon combustion Heterocyclic-ring nitrogen compounds such as pyridine (C 5 H 5 N), piperidine (C 5 H 11 N) and quinoline (C 9 H 7 N) are found in fuel oil Chain and ring nitrogen compounds are also found in coal Sensitive to air to fuel ratios and mixing Not sensitive to small changes in temperature 10

11 Control Types Alter combustion conditions to emit less NOx Reduce peak temperatures of the flame zone Reduce gas residence time in the flame zone Reduce O concentrations in the flame zone Treat the flue gas Catalytic reduction 11

12 Altering Operating Conditions Strategies Off-stoichiometric combustion (OSC) Combusting the fuel in two or more steps Often called staged combustion Fuel rich then fuel lean Flue Gas Recirculation (FGR) Reroute some of the flue gas back to the furnace; Lower O and allow NOx to proceed the frozen reactions Water Injection reduce flame temperature Energy penalty 1

13 Gas Reburning Injection of natural gas into the boiler above the main burner to create a fuel-rich reburn zone Natural gas is used since it is essentially nitrogen free Hydrocarbon radicals react with NOx to reduce NOx to N Often used with recirculated flue gas to enhance mixing 13

14 Gas Reburning Evaluation of Gas Reburning and Low-NOx Burners on a Wall-Fired Boiler A DOE Assessment. DOE/NETL-001/

15 Low NOx Burner (LNB) Inhibits NOx formation by controlling the mixing of fuel and air Lean excess air and offstoichiometric combustion Example of low NOx burner from pulverized coal burner 15

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17 Flue Gas Treatment Techniques Achieves highest removal efficiencies compared to combustion controls Examples are: Selective Catalytic Reduction (SCR) Selective Noncatalytic Reduction (SNR) SCONOx Dry Sorption Wet Absorption 17

18 Selective Catalytic Reduction (SCR) Most effective method of post-combustion NOx reduction Ammonia reacts with NOx on the surface of a catalyst to produce N and water 4NO NO + 4NH 3 + 4NH O O TiO or VO 5 supported catalyst 4N TiO or VO 5 supported catalyst 3N At optimum temperatures between C for catalyst activity + 6H + 6H O O Achieves about an 80% NOx reduction 18

19 Schematic of SCR Catalyst in form of pellets for gas turbine or honeycomb in oil fired units 19

20 SCR: Ammonia Slip A common problem with all SCR systems is ammonia slip, unwanted emissions of ammonia that occur when exhaust gas temperatures are too low for the SCR reaction to take place. the injection device feeds more reductant into the exhaust gas stream than there is NOx. A variety of strategies have been developed to deal with ammonia slip, including the use of extra catalysts after the SCR catalyst 0

21 Selective Noncatalytic Reduction (SNR or SNCR) Less desirable since NOx consumed as well as O Not compatible with natural gas turbines 4NH 4NH NO + 5O + O 4NO 4N Temperature ~ o C Above 1000 o C + 6H + 6H O O 1

22 SCONOx Involves a two-step process on a single potassium nitrate-impregnated catalyst In the first step, the catalyst oxidizes NO to NO and CO to CO NO is then absorbed onto the treated surface of the SCONOx catalyst Catalyst is potassium carbonate (K CO 3 ) The absorbed NO is periodically removed from the catalyst with dilute hydrogen As the hydrogen contacts the catalyst in the presence of oxygen, the absorbed NO is converted to N and water

23 SCONOx Pros No ammonia used EPA considers it LAER technology for NOx control Cons High cost Higher pressure drop than SCR Cannot be used with fuels that contain sulfur 3

24 Dry Sorption: Activated Carbon Use activated carbon with NH 3 injection to reduce NOx and N and oxidize SO to H SO 4 The carbon is operted in the 0 to 30 C range and thermally regenerated to remove concentrated H SO 4 4

25 Dry Sorption: CuO Catalyst Called Shell Flue Gas Treating System Copper oxide (CuO) and copper sulfate (CuSO 4 ) are good catalysts for selective reduction of NOx with NH 3 CuO 4NO CuSO Cu + 0.5O + 4NH 4 + H + 0.5O 3 + SO + O Cu CuO CuSO CuO or CuSO4 as catalysts 4N + SO 4 + H O + 6H O 70% NOx reduction (90% SO reduction) 5

26 Wet Absorption Generally good for only NO because NO is insoluble Use hydrogen peroxide (H O ) H O sprayed into hot flue gas (500 C) About 90% NOx control H OH OH OH O HO HO + NO + H O + OH OH + NO + NO HONO HNO HO H 3 HNO 3 + H O + O O 6

27 Natural Gas Fired Turbines Natural Gas turbines are combustion systems that generate hot combustion gases that are used for driving a turbine to produce electrical power Gas turbines are stationary power and energy generating systems that were originally developed from aircraft jet engines Commonly being built across the nation for power generation Thermal NOx formation is responsible for the majority of NOx emissions from gas turbines The nitrogen levels in the fuels appropriate for gas turbines are too low to contribute substantially to fuel NOx formation 7

28 Elements of a Gas Turbine Compressor Combustor Power turbine 8

29 Categories of Gas Turbines Simple cycle Exhaust gases are released with no heat recovery Combined cycle Heat is recovered to produce steam for a steam turbine Rankine cycle Regenerative Heat is recovered by a heat exchanger to preheat the combustion air. Cogenerative Heat is recovered in a heat recovery steam generator to produce steam for the process 9

30 Simple Cycle Turbine Air is adiabatically compressed to approximately 30 times ambient conditions in the compressor prior to ignition in the combustor The air and fuel are burned in the combustors at the elevated pressure After exiting the combustion zone, the exhaust gas expands and cools, transferring energy to a rotating shaft in the form of mechanical work 30

31 Combined Cycle Turbine the hot exhaust stream from the gas turbine is used in a heat recovery steam generator (HRSG) The steam is used to generate additional electrical power 31

32 Controls for Gas Turbines 3

33 Gas Turbine with SCR 80 to 90% reduction in NOx Best used with combined cycle units: temperature compability 33