Technical Requirements for Large Combustion Plants (LCPs) By Pat Swords
Content of the Presentation Goals and Principles Control Technologies for NOx Control Technologies for SO 2 Control Technologies for Particulates
Goal to reduce significantly emissions of NOx, SO 2 and Particulates Options available: Fuel switching, e.g. heavy fuel oil to natural gas Combustion control; reduce formation of thermal NOx by controlling flame / combustion conditions, i.e. avoid hot spots. Reduction of NOx with ammonia: NO 2 + NH 3 N 2 + H 2 0 Scrubbing of SO 2 with limestone: CaCO 3 + SO 2 CaSO 3 + CO 2 Scrubbing of SO 2 with the carbonates in sea water Particulate removal by cyclones, fabric filters or electrostatic precipitators
Plenty of guidance on all of this from European Integrated Pollution Prevention and Control Bureau Very detailed Best Available Reference Documents (BREFs): Final draft of updated BREF for LCPs
Primary NOx control design of burners and combustion chamber: Low excess air: NOx reduction rate 10 45% Air staging: NOx reduction rate 10 70% Flue gas recirculation: NOx reduction rate 20 50% Reduced air preheat: NOx reduction rate 20 30% Fuel staging: NOx reduction rate 50-60% Low NOx burners: NOx reduction rate 20 70% Water / steam injection in burners: NOx reduction rate 60 85%
Primary NOx control Low NOx burner design to reduce flame temperature
Primary NOx control Low NOx burner design to reduce flame temperature
Selective Non-Catalytic Reduction (SNCR): Injection of urea and ammonia into combustion chamber at circa. 900⁰C General reduction rate 30 50%; NH 3 slippage
Selective Catalytic Reduction (SCR) Injection of ammonia / urea at circa. 300⁰C over a catalyst General reduction rate 80-95%; catalysts can block and are expensive
Limestone based desulphurisation: Scrubbers (wet or dry) or injection into fluidised bed furnace Sorbent injection more efficient with Circulating Fluidised Bed (CFB) rather than Bubbling Fluidised Bed (BFB)
Limestone scrubbing is most common solution: SO 2 removal rate of 85 to 99% possible
Spray dry desulphurisation SO 2 removal rate 85 98%
Desulphurisation by sea water scrubbing not so common: SO 2 reduction rate of 85 98%; cross media effects
Particulate Removal Simple cyclones are crude pre-filters only Principal technique is Electrostatic precipitators (ESPs)- Particles are charged and separated under the influence of an electrical field. Fabric filters are less common - Electrostatic precipitator overall (mass) collection efficiencies can exceed 99.9%, and efficiencies in excess of 99.5% are common. - Excellent control of PM 10 and PM 2.5 will be achieved with welldesigned and operated electrostatic precipitators.
Electrostatic Precipitators:
Fabric Filter Fabric based bags or cartridges. 99% removal rates, but filters can clog and need to be replaced
Electrostatic Precipitators (ESP) and Fabric Filter performance for very fine particulates (sub 10 microns):
Submission of Irish semi-state Electricity Supply Board (ESB) on cost of NOx control. Moneypoint: 900 MWe coal fired LCP commissioned mid- 80s; 250 million desulphurisation and DeNox upgrade by 2008. CCGT- Combined Cycle Gas Turbine LNB Low NOx burners SCR Selective Catalytic Reduction Graph overleaf shows options for ESB to reduce NOx emissions from 38,000 tpa and cost per tonne which results at each LCP.
Base Load Plants which run nearly continuous, such as relatively modern coal plants like Moneypoint with their lower operational costs Mid Merit Plants which fill the gap between the peak load and base load. In some cases the Mid Merit power plants are older and less efficient base-load plants, not effective and cheap enough to run full time. Peaker Plants Plant which can ramp up fast for Peak Loads, but have high operational cost.
Circulating Fluidised Bed Power Plant Lough Ree Ireland
Typical Modern Coal Plant:
Gas turbines rely on primary NOx controls (burners), but Emission Limit Values are specified by EU at > 70% load: Data for LM2500+ GE aero-derived gas turbine
Questions?