A carbon-friendly approach

Many utilities are underway with plans to reduce nitrogen oxide (NOx), sulfur dioxide (SO2) and now greenhouse gas (GHGs) emissions from their boilers, but have units in their fleet that do not justify the capital, added complexity and operating impact of the systems, such as selective catalytic reduction (SCR) for NOx control, alkali scrubbing for SO2 reduction and amine scrubbing for GHGs. Yet, control will be necessary. This field note outlines a multi-pollutant control technology that operates in-furnace, providing practical advances in emissions control without the extensive back-end equipment. It is applicable to virtually all types of solid fuel-fired boilers including wall-and tangential fired, stokers, circulating fluidised-beds, and allows boilers to convert to co- or completely firing of biomass. An important and fundamental feature of Nalco Mobotec’s ROFA® system is optimised combustion through improved in-furnace mixing, which lowers particulate emissions, improves boiler efficiency and can facilitate carbon-friendly biomass firing that provides credit for reduced greenhouse gas emissions.

The primary process
When combustion is staged to provide an initial zone at a slightly sub-stoichiometric ratio of oxygen to fuel, the reducing environment significantly inhibits NOx formation. Staging allows many of the fuel-bound nitrogen atoms to combine with each other to produce N2 rather than react with oxygen to produce NOx. This discovery led to the development of overfire air (OFA) techniques, where a portion of the combustion air is redirected from the burners, and is injected above the primary combustion zone. Conventional OFA can successfully reduce NOx concentrations by perhaps 30 percent, but results in lowered combustion efficiency in the furnace, higher unburned carbon in the fly ash and greater emissions of carbon monoxide (CO).  Rotating Opposed Fired Air (ROFA), as shown in Figure 1, is an advanced overfire air technique that, like conventional OFA, reduces NOx by providing a fuel-rich initial combustion zone via diversion of a portion of the inlet combustion air to points above the burner level. The diverted air is given a pressure boost by an auxiliary fan, and then is injected at locations determined through a design process that includes computational fluid dynamic (CFD) modelling. Individual CFD models are performed for every steam generator retrofitted with the ROFA system.

The technology has been applied to over 50 industrial and power boilers worldwide, including over 30 in the United States.  Many units have been in operation for five years or more. Results have shown that the ROFA system alone can reduce flue gas NOx emissions by 40 to 60 percent, much higher than conventional over-fire air. 

The enhanced mixing provided by the ROFA system also improves combustion efficiency. Typically, unburned carbon (UBC) in flyash is unchanged from baseline conditions.  Carbon monoxide concentrations at the stack are often below 20 ppm.  These reductions are possible while simultaneously reducing excess air, all of which favourably impact unit efficiency. An additional benefit of the enhanced combustion process is a reduction in furnace hotspots that otherwise can lead to premature waterwall tube failures or localised slag formation.

Supplemental NOx control
Additional NOx reduction is possible with the Rotamix® process, a patented process of ammonia or urea injection into the upper furnace. The Rotamix process is similar to conventional selective non-catalytic reduction (SNCR), where ammonia (NH3) or urea [CO(NH2)2] and NOx react to produce elemental nitrogen and water as outlined by the following equations:

4NO + 4NH3 + O2 > 4N2 + 6H2O         Eq. 1
2NO2 + 4NH3 + O2 > 3N2 + 6H2O       Eq. 2

However, the Rotamix system performs more efficiently than a standard SNCR system because of the enhanced mixing of the chemical into the flue gas through the use of high∞velocity air jets. The Rotamix process operates independently of the ROFA process, and has its own chemical feed tank and pumps.  As with the ROFA system, the injection point locations are determined through a design process that is strongly dependent upon CFD modelling to ensure that the chemicals are injected into the proper temperature region of the furnace. The Rotamix process has the potential to reduce NOx again in half as compared to the ROFA system alone1.In fact, in the example outlined in reference 1, the ROFA/Rotamix combination reduced full∞load NOx on an 82 MW (gross) power boiler from 700mg/Nm3 to 120mg/Nm3 at 6 percent O2, or 0.58lb/MMBtu to 0.10lb/MMBtu. The latter value is at the threshold of what more costly SCR systems can achieve.

Going after SO2
For decades, sulfur dioxide reduction technology in coal-fired boilers has been based upon reacting the SO2 with an alkaline material to produce a benign salt. For large units in particular, this involves installation and operation of a large, backend scrubber that typically employs limestone (CaCO3) or hydrated lime [Ca(OH)2] slurries to react with the SO2. Wet flue gas desulfurisation (WFGD) equipment is very large, expensive, and complicated to operate, although well∞designed and operated scrubbers can do a great job of removing sulfur dioxide.An extension of the ROFA system is the technique known as Furnace Sorbent Injection (FSI), which utilises the overfire air system for enhanced mixing of either limestone (principal component, calcium carbonate [CaCO3]) or hydrated lime [Ca(OH)2] in the furnace. These two products respectively calcine or dehydrate to quicklime (CaO), which then reacts with sulfur dioxide (SO2) to form a benign material of calcium sulfate (CaSO4).

CaO + SO2 + 1/2O2 > CaSO4    Eq. 3

Components of an FSI system include a materials storage silo, solid materials feeders, and a pneumatic conveying system to move the reagent to the furnace. FSI may require some back-end modifications. In many cases with simple coal combustion, an excellent device for removing flyash from the flue gas before it exits the stack is an electrostatic precipitator (ESP). However, sorbent injection to remove pollutants alters the quality of the flyash, sometimes adversely affecting ESP performance. The increasingly popular alternative to ESPs on units with sorbent injection, and particularly those utilising activated carbon for mercury removal, is the fabric filter device, commonly known as a baghouse. These utilise thousands of bags to mechanically filter ash particles.  An advantage of a baghouse with the FSI system is that partially reacted particles collect on the bags and continue to react with residual SO2 as it passes by. At plants equipped with FSI and a baghouse, over 80 percent SO2 removal has been achieved2.  Work continues on increasing removal efficiencies to 90 percent without excessive reagent consumption. 

Biomass conversion
Because of the lower calorific value and density, larger amounts of biomass must be burned to maintain unit capacity when replacing pulverised coal. A large fraction of the heat energy in biomass is released as volatiles (up to 95 percent of mass) therefore gas phase temperature distribution along the boiler height will be different than firing pulverised coal alone. In order to fully burn out, biomass∞fired boilers require better mixing and combustion optimisation. By using the ROFA system to control the oxygen distribution and the residence time in the boiler, NOx can be reduced while facilitating more complete combustion of the biomass. Units have been successfully converted with the ROFA system to co-fire 25-40 percent biomass while maintaining operating capacity and flexibility and earning credit for reducing greenhouse gas emissions.  One unit in Helsingborg Sweden has even been converted to 100 percent biomass firing, maintaining the required steam production and availability since 2006.