7.2 NOx Control Systems & Combustion Modification

$NO_x$ & $SO_x$ Combustion Control Engineering

Nitrogen oxides ($NO_x$, primarily nitric oxide $NO$ and nitrogen dioxide $NO_2$) and sulfur oxides ($SO_x$, primarily $SO_2$) are major criteria air pollutants regulated under National Ambient Air Quality Standards (NAAQS). $NO_x$ is a mandatory precursor to ground-level ozone (smog) and nitric acid rain, while $SO_2$ drives particulate matter ($PM_{2.5}$) and sulfuric acid deposition. Environmental engineers must select and design pre-combustion, combustion modification, and post-combustion control technologies to meet stringent EPA Title V and Prevention of Significant Deterioration (PSD) permit limits.

Mechanisms of $NO_x$ Formation

$NO_x$ is generated during fossil fuel combustion via three distinct chemical reaction pathways:

1. Thermal $NO_x$ (The Zeldovich Mechanism)

Thermal $NO_x$ is formed by the high-temperature oxidation of atmospheric nitrogen ($N_2$) present in combustion air. It dominates in clean gas and distillate oil combustion above $1300^\circ C$ ($2370^\circ F$). The fundamental chemical kinetics are described by the Extended Zeldovich Mechanism:

$$N_2 + O \rightleftharpoons NO + N \quad (E_a = 318 \ kJ/mol)$$ $$N + O_2 \rightleftharpoons NO + O$$ $$N + OH \rightleftharpoons NO + H$$

Because the initial step breaking the $N \equiv N$ triple bond has an exceptionally high activation energy ($318 \ kJ/mol$), the thermal $NO_x$ reaction rate ($d[NO]/dt$) exhibits an extreme exponential dependence on temperature ($T$) and linear dependence on excess oxygen ($[O_2]$):

$$\frac{d[NO]}{dt} = 2 k_1 [O][N_2] = \frac{2 k_1 A_0 \sqrt{[O_2]} [N_2]}{T^{1/2}} \exp\left(-\frac{318,000}{R T}\right)$$

2. Fuel $NO_x$

Fuel $NO_x$ results from the oxidation of nitrogen atoms organically bound within the fuel matrix (e.g., coal containing $0.5 - 2\% \ N$, heavy residual oils). Fuel-bound nitrogen compounds ($R-NH_2, R-CN$) volatilize in the flame zone, forming intermediate radicals ($NH_i, HCN$) that oxidize into $NO$. Fuel $NO_x$ accounts for $50 - 80\%$ of total $NO_x$ in coal combustion and is strongly sensitive to local stoichiometry (air-fuel ratio) rather than temperature alone.

3. Prompt $NO_x$ (The Fenimore Mechanism)

Prompt $NO_x$ forms rapidly in the immediate flame front under fuel-rich conditions where hydrocarbon radicals ($CH, CH_2$) react with atmospheric nitrogen:

$$CH + N_2 \rightleftharpoons HCN + N$$ $$HCN + O_2 \rightarrow \dots \rightarrow NO$$

Prompt $NO_x$ contributes a minor fraction of overall emissions but establishes a baseline floor in low-temperature combustors.

Combustion Modification Technologies

Combustion controls aim to suppress $NO_x$ formation at the source by lowering peak flame temperatures, reducing excess oxygen in the high-temperature zone, and extending combustion residence time.

• Low-$NO_x$ Burners (LNB): Stage the introduction of fuel and air. Air-staged LNBs create a primary fuel-rich, oxygen-deficient combustion zone (inhibiting $NO$ formation) followed by a secondary burnout zone where remaining air completes combustion at lower peak temperatures ($30 - 50\%$ $NO_x$ reduction).
• Overfire Air (OFA): Diverts $15 - 30\%$ of total combustion air from the main burners to ports located higher up in the boiler furnace, ensuring sub-stoichiometric burning in the primary flame zone ($30 - 40\%$ $NO_x$ reduction).
• Flue Gas Recirculation (FGR): Recirculates $10 - 20\%$ of cool, inert exhaust gas back into the primary combustion air stream. The recirculated $CO_2$ and $N_2$ act as thermal heat sinks, lowering peak flame temperatures below $1300^\circ C$ ($40 - 70\%$ reduction of Thermal $NO_x$).

Post-Combustion Flue Gas Treatment: SCR vs. SNCR

When combustion modifications cannot satisfy permit limits, post-combustion catalytic or non-catalytic chemical reduction systems are installed.

Selective Catalytic Reduction (SCR)

SCR is the most effective $NO_x$ removal technology, capable of $> 90\% \ NO_x$ reduction. Ammonia gas ($NH_3$) or vaporized urea is injected into the flue gas and passed over a ceramic honeycomb catalyst bed composed of Vanadium Pentoxide ($V_2O_5$) and Tungsten Trioxide ($WO_3$) supported on Titanium Dioxide ($TiO_2$).

Stoichiometric Reduction Reactions:

$$4 NO + 4 NH_3 + O_2 \xrightarrow{cat} 4 N_2 + 6 H_2O$$ $$2 NO_2 + 4 NH_3 + O_2 \xrightarrow{cat} 3 N_2 + 6 H_2O$$

• Operating Temperature Window: Strict window of $300 - 400^\circ C$ ($570 - 750^\circ F$). Below $300^\circ C$, catalyst activity drops and unreacted ammonia forms ammonium bisulfate ($NH_4HSO_4$), plugging downstream air preheaters. Above $400^\circ C$, $NH_3$ oxidizes directly into additional $NO_x$.
• Ammonia Slip: Unreacted $NH_3$ exiting the reactor. Must be controlled to $< 2 - 5 \ ppmv$ to prevent secondary particulate formation.

Selective Non-Catalytic Reduction (SNCR)

SNCR injects ammonia or aqueous urea directly into the high-temperature upper furnace without a catalyst:

$$2 (NH_2)_2CO \text{ (urea)} + 4 NO + O_2 \xrightarrow{850-1100^\circ C} 4 N_2 + 2 CO_2 + 4 H_2O$$

• Operating Temperature Window: Narrow window of $850 - 1100^\circ C$ ($1560 - 2000^\circ F$). Below $850^\circ C$, reaction kinetics are too slow, causing excessive $NH_3$ slip. Above $1100^\circ C$, $NH_3$ oxidizes to $NO$.
• Removal Efficiency: Typically $30 - 50\%$, but with drastically lower capital cost than SCR.

Flue Gas Desulfurization (FGD) Systems for $SO_2$

Sulfur dioxide ($SO_2$) emissions are controlled post-combustion using alkaline sorbent scrubbers.

Wet Limestone Forced Oxidation Scrubbing

Flue gas passes through a counter-current spray tower contacting a limestone ($CaCO_3$) slurry:

1. Absorption & Neutralization: $$SO_2(g) + H_2O \rightleftharpoons H_2SO_3$$ $$CaCO_3(s) + H_2SO_3 \rightarrow CaSO_3(s) + CO_2(g) + H_2O$$
2. Forced Air Oxidation: Air is bubbled into the reaction tank to convert calcium sulfite into commercial-grade gypsum ($CaSO_4 \cdot 2 H_2O$): $$CaSO_3 + \frac{1}{2} O_2 + 2 H_2O \rightarrow CaSO_4 \cdot 2 H_2O \text{ (Gypsum)}$$

Wet FGD achieves $> 95 - 98\% \ SO_2$ removal efficiency.

Dry & Semi-Dry Systems (SDA & DSI)

• Spray Dryer Absorber (SDA): Atomizes a lime slurry ($Ca(OH)_2$) into the hot flue gas. Water evaporates, producing dry calcium sulfite/sulfate powder captured in a fabric filter baghouse ($85 - 92\%$ efficiency).
• Dry Sorbent Injection (DSI): Injects dry hydrated lime or sodium bicarbonate (Trona) directly into ductwork ($50 - 80\%$ efficiency).