Numerous factors must be considered when selecting control technologies as well as the interactions between them, writes Jon Norman

The EU and other international authorities are now implementing or considering new regulatory requirements for air emissions from coal-fired boilers. Pollutant emissions to be reduced include sulphur dioxide (SO2), hydrochloric acid (HCl), hydrofluoric acid (HF), and mercury (Hg).

Dry Sorbent Injection (DSI) systems for SO2 and other acid gas removal, along with Activated Carbon Injection (ACI) systems for Hg removal, are two proven technologies used to meet stringent compliance requirements.

New rules on coal-fired boiler emissions are on the way
New rules on coal-fired boiler emissions are on the way
Credit: IEA Clean Coal Centre

SO2 emission limits in the EU vary depending on the size and type of the affected boilers and annual hours of operation. Most coal-fired boilers will need to meet SO2 emission limits between 100 and 220 mg/Nm3. Mercury emission limits in the EU are still being developed, but are expected to be approximately 4 to 7 μg/Nm3.

Boiler owners are actively evaluating available technologies and strategies for compliance.

Removal efficiency, capital, operating cost, plant life, outage requirements, scalability and impact on existing air pollution control (APC) equipment are all considerations when assessing viable technologies for air emissions compliance.

Multi-pollutant controls needed

There is not a one-size-fits-all solution for multi-pollutant control to meet the new emissions requirements. Industry testing has shown that although some coal units already meet the expected Hg limits without additional controls, most will require further controls for at least SO2 emissions.

Several variations of wet and dry scrubber technologies can provide high levels of SO2 and HCl/HF removal. However, high capital cost will likely deter new scrubber purchases, except in cases when a very high-level of SO2 removal is required (>90 per cent) and the unit will continue to use an ESP for particulate capture.

The majority of boilers without existing scrubbers will likely install lower cost DSI technology to meet emission requirements. Economical and flexible DSI systems can achieve the medium to high SO2 and HCl/HF removal needed for bituminous and lignite coals.

Figure 1. Simultaneous HCL and SO2 removal
Figure 1. Simultaneous HCL and SO2 removal

Depending on mercury levels in the coal, plants may require ACI systems or other Hg removal technologies, and ESP upgrades or the addition of a fabric filter. While some units may use novel technologies, such as Hg removal filters, powdered activated carbon (PAC) injection in some form has become the industry standard.

Emissions compliance appraisal

Multiple factors, such as coal type, combustion conditions, ductwork, temperature and other APC equipment impact emission levels. DSI system design and sorbent selection require comprehensive experience in complex chemistry, fluid dynamics, injection location and interaction with existing APC equipment.

DSI demonstration testing can not only validate a plant’s compliance strategy, but optimize total system performance. With over 25,000 hours of data from more than 100 tests, United Conveyor Corporation has developed a robust database of test results that can be used to predict emissions reduction performance for a wide variety of scenarios. This testing also forms the basis for developing strategies for compliance around different coal types and plant configurations.

Hydrated lime and sodium bicarbonate are both effective sorbents that can be injected to simultaneously achieve SO2 and HCl/HF removal. Although there can be exceptions, generally HCl emissions reductions will be 30-50 per cent higher than SO2 reduction at a given hydrated lime or SBC injection rate.

The reason is that HCl is a stronger acid gas. HF emissions reductions can vary significantly depending on baseline levels of this acid gas, which are often very low.

SBC particles at 60o
SBC particles at 60°

Both hydrated lime and SBC have been demonstrated to achieve high removals of SO2 and HCl/HF. However, the best sorbent for a given boiler must take into account several factors. In general, SBC is capable of higher acid gas reduction levels compared to hydrated lime, particularly when a unit has an ESP for particulate control.

Not only is SBC more reactive with SO2, the sodium will often enhance, or at least not degrade, ESP performance as can be experienced with hydrated lime. Therefore, if higher SO2 reductions are needed, hydrated lime is generally not used unless a fabric filter is installed. Hydrated lime has an advantage in that it can often allow for the resale of ash, while this is not usually the case for SBC injection.

SBC particles at 345o
SBC particles at 345°

Flue gas temperature at the injection location is of critical importance to achieve the required SO2 reduction performance. SBC needs temperatures above 135°C in order to rapidly decompose to sodium carbonate and form sufficient porosity for the subsequent reaction with SO2. Higher temperatures then increase the reaction kinetics, but only up to a temperature of 345°C, when the SBC particles start to soften and lose porosity. This loss is illustrated in the scanning electron microscope pictures below of SBC particles subjected to different flue gas temperatures.

Hydrated lime can be used over a range of temperatures for SO2 and HCl/HF reduction, but has been shown to be most effective at temperatures above about 300°C.

Hg reduction with ACI

A variety of PAC and other mercury removal sorbents have been used on coal-fired boilers with success. These can be grouped into the following broad categories, along with their common uses:

• Non-halogenated PAC

• High Cl Bituminous coals when a high percentage of the Hg is oxidized;

• Combination with CaBr2 fuel additive for low sulphur/low Cl coals.

• Brominated PAC

• Low-Cl coals when majority of the Hg is non-oxidized;

• Injecting alkali sorbent at the air heater inlet and therefore can’t use CaBr2 fuel additive (which will adversely react with the alkali sorbent).

• Non-brominated oxidizing PAC

• Low-Cl coals when majority of the Hg is non-oxidized and want to avoid the corrosion concerns with bromine.

Numerous ACI tests have shown that 90-95 per cent or more mercury removal is readily achievable when the correct sorbent is injected in the proper location on a given unit.

Although a customized strategy must be implemented for each unit, taking into account a number of factors, high performance can often be grouped into the following for low Cl/low sulphur and high Cl/high sulphur coals.

Low sulphur/low Cl coals – a combination of a CaBr2 fuel additive and non-halogenated PAC, or a brominated PAC, or a non-brominated oxidizing product injected at the air heater inlet, has been shown to give the highest performance. High Cl/high sulphur coals – non-halogenated PAC, or brominated PAC, or a SO3-tolerant PAC injected at the air heater outlet can be used for the best performance. If there are significant levels of SO3 present in the flue gas (>5 ppm), then either hydrated lime or trona is often injected at the air heater inlet to lower the SO3 levels that interfere with Hg adsorption.

Note that mercury removal strategies for lignite coals will depend on the specific coal and its sulphur level, but will often be more similar to high sulphur bituminous coals.

Interactions between DSI and ACI

Results from dozens of United Conveyor Corporation combination DSI/ACI tests has found that the interactions between these two technologies can be quite complex. Hydrated lime or sodium injection can have substantial effects on Hg removal and corresponding ACI injection rates. These effects must be understood before choosing a final compliance strategy.

For any given unit and fuel, it is best to conduct testing to quantify the exact injection rates needed for both DSI and ACI. It is also helpful to understand the interactions between DSI and ACI before designing the test plan. If testing cannot be conducted, understanding the interactions is critical to avoid unforeseen issues that could lead to undersized or underperforming DSI/ACI systems.

Although the interactions are complex and unit/fuel specific, common observations from testing are summarized in Table 1 in an attempt to give simplified guidelines.

Due to the multitude of variables and unique characteristics at each plant, no single solution can be applied to all units to achieve compliance for multiple air pollutants.

Numerous factors must be considered when selecting control technologies, as well as the interactions between the technologies.

ACI, DSI, or combinations of both, will enable the majority of coal boilers in operation to achieve a cost-effective compliance strategy.

Jon Norman is Global Sales and Technology Manager at United Conveyor Corporation. www.unitedconveyor.com