Common problem area for users of dry scrubbers

Posted on January 28, 2016

Dry scrubbing has become a commonly accepted technology for reducing acid gas emissions from stationary sources such as power boilers, industrial boilers, incinerators, steel and coking facilities, non-ferrous metals production, etc.  Common dry scrubbing processes include Dry Sorbent Injection (DSI), Spray Dryer Absorbers and Recirculating Dry Scrubbers.  Each has its own areas of concerns for owners and operators.

Dry Scrubbing Technology Overview

Dry Sorbent Injection, or DSI, typically involves the pneumatic injection of a powdered alkali reagent into the ductwork upstream of a particulate collector such as a Fabric Filter or Electrostatic Precipitator.  Frequently the particulate collector and ductwork are existing.  The DSI system is simple and relatively inexpensive, consisting of storage silo, pneumatic blowers, piping and a sorbent distribution system.  Common alkali reagents include hydrated lime, sodium bicarbonate and trona.  (Powdered activated carbon and other reagents are also used to reduce emissions of Hg and dioxin/furan compounds.)  DSI systems do not control the temperature of the reaction.  Rather they look to inject in the optimized location.

Spray Dryer Absorbers have been used to control SO2 emissions for almost forty years now.  They are especially common where Powder River Basin coal is burned, as the low sulfur content and ash chemistry of PRB make for a convenient mix.  Spray Dryers inject lime slurry (frequently combined with recycled ash) into the flue gas, where the slurry is atomized into fine droplets.  The atomization mechanism can be either a high speed rotating mechanical disk or a two-fluid nozzle.  Water evaporates from the slurry droplets, simultaneously cooling the gas and increasing its moisture content.  The moisturized lime particles react with the acid gases (SO2, SO3, HCl, HF, etc.) to form calcium salt particles, which are swept into the particulate collector for capture.  The cleaned flue gas is exhausted to atmosphere through the stack.  Since the reactive form of lime is calcium hydroxide (Ca(OH)2), it is frequently produced onsite by slaking quicklime (CaO).

As Spray Dryers have one combined slurry feed, the control loops for acid gas removal and absorber operating temperature are merged into one, which can constrain operations under certain conditions such as low gas inlet temperature and high sulfur contents in the incoming flue gas.

Recirculating Bed Scrubbers also use Calcium Hydroxide to react with the incoming acid gases.  However, the calcium hydroxide is pneumatically injected in powdered form.  Water for gas cooling and conditioning is injected separately through nozzles.  Recirculating Bed Scrubbers capture the solids entrained in the reactor off-gas and recirculate them back into the reactor.  This increases contact and turbulence in the reactor and also increases the internal lime to acid gas ratio.  The high internal stoichiometry drives the neutralization reaction further towards completion, allowing for high reduction efficiencies at low stoichiometric ratios. 

Recirculating Bed Scrubbers also use two separate feed loops for lime and water, decoupling the control loops and eliminating some operating constraints.  Since the calcium hydroxide is introduced as a powder, it must either be purchased as such or hydrated onsite.

Common Problem Areas with Dry Scrubbers

The various versions of dry scrubbing pose differing problems for operators.  Common problems for DSI systems include
• Sorbent storage and delivery issues
• Sorbent Metering
• Adequate dispersion of sorbent in large ducts using multiple injection ports
• Plugging         
• Carbonization of calcium compounds
• Trona deposition
• There can also be impacts on existing Particulate Control devices, including
• Fabric Filters (solids buildup, increased pressure drop, more frequent bag cleaning and reduced bag life)                 
• ESPs (increased inlet loadings, changes to ash resistivity and particle size distribution, stickiness).

Common problem areas for Spray Dryers Absorbers include
• Lime slaking issues (quicklime quality, slaking water quality and temperature) which can impact the hydrated lime reactivity
• Slurry storage and delivery problems
• Atomization issues (droplet distribution, vibration, atomizer change-outs)
• Plugging and buildup
• Corrosion
• Process control, especially during load swings and Startup and Shut down

Circulating Dry Scrubbers have their own problem areas, including
• Lime hydration
• Lime feeding and metering (weigh belts)
• Water feeding (nozzles and pumps)
• Plugging and buildup
• Corrosion
• Process control issues, especially during load following
• Startup and shut down issues.

A review of recent technical presentations by suppliers, operators and engineering consultants have observed the following problems in dry scrubbers:
• Nozzle alignment (the “broom stick test”)
• Cleaning of nozzle tips
• Erosion of atomization tips
• Buildup on nozzle tips and guide tubes
• Water quality
• Slurry and water pump wear
• Reactor plugging
• Hydrators (water-to-lime ratio control, accurate instrumentation)
• Pebble lime size
• Batch operation of hydrators
• Corrosion
• Atomizer Imbalance
• Atomization disruption (large droplets)
• Atomizer maintenance (loss of scrubbing)
• Load following capabilities and challenges
• Start-up and shut-down (ash inventory for CDS units)
• Multiple module startups and shut downs – emission spikes
• Accurate temperature control (feed forward signals)
• Control accuracy
• Dew point monitoring

Depending on the particular technology employed at any given site, along with other site specifics, various problems can arise.  Virtually all are solvable by employing the correct process design, operating procedures and maintenance attention.

Authored By:
Mike Widico has extensive experience in the energy industry and has been involved with emission control technologies for more than 30 years. Mike has graduate degrees in both Chemical Engineering and Business Administration. He has held various positions with several technology suppliers. Mike currently consults for clients with problem installations.

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February, 02 2016

Sam Barnes says

Great article, Mr. Widico! You close with a list of items that can solve many of the known issues that can arise with ACI and SDA installations. I would add that the opportunity to begin solving those issues is best placed prior to equipment purchase, by first pre-qualifying vendors for the bid list, and second, by creating a bid package and specification that accounts for a range of conditions.

Regarding prequalification of vendors, do your homework on what companies offer technology that fits your application. In addition to the normal areas of prequalification inquiry for any company that sells large, expensive equipment and systems, focus on their solution’s track record of installations that are similar to yours. For ACI and SDA applications, ask for information on their products that reflect your needs and conditions, such as: fuel characteristics, water quality, reagent flowrate, flue gas flowrate, climate of installation, number of similar installations, hours of operation of similar installation, reagent properties, and when possible availability metrics. Answers to these questions will help narrow the field of acceptable equipment providers. With your bid list narrowed, the request for proposal (RFP) can be written to allow some flexibility. Ensure that the range of climatic conditions that the equipment will operate in is adequately described (think of a system that has been successfully deployed in Tennessee showing up in North Dakota without regard to summer/winter conditions). Consider that today’s operating parameters may not stay the same for years to come, so contemplate specifying that the equipment and process work for a range of inputs, such as coal type, process water quality, and turn down ratios. Since coal can change dramatically within the same seam over time, consider placing a “fuel box” or range of coal properties into the specification that could be possible over the foreseeable life of the equipment. With today’s focus on water conservation, consider that your intended water source could someday be replaced with used water from another process. Placing a range of water qualities into the specification will help mitigate future problems. If your system runs at 100% today, resist the temptation to disregard how the system will operate for extended periods at less than maximum capacity. It could be that future operations will need to operate at part loads for extended periods. Make sure that these operating scenarios are described in the specification and establish functional requirements for the equipment running at predetermined part load conditions.

These are just a few examples of how to begin solving many of the issues noted in the article for new installations. Verifying that the bidders are qualified to provide equipment and processes that fit your application and taking time to consider ranges of design inputs is time and money well spent.

-Sam Barnes, P.E., VP – Power Generation and Energy, Commonwealth Associates, Inc.

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