Lots of Wet Calcium Issues and Options – Hot Topic Hour May 14, 2015

There were many participants in our discussion of wet calcium systems, scrubbers and scrubber internal components, balance of plant components, and materials yesterday.

We are defining wet calcium as both lime and limestone and not seawater or ammonium sulfate wet systems.  McIlvaine has a whole separate decision guide for dry scrubbing. Both are linked to the Power Plant Air Quality Decisions.

We prepared five route maps and summaries for the wet calcium decision guide. They are:

Route Maps
 &
  Summaries

§  Overview

§  Systems and Reagents

§  Scrubber and Components

§  Materials

§  System Components

Power Plant Air Quality Decisions (PPAQD) subscribers can link to each of these route maps and summaries.

Overview:  There are large numbers of wet calcium systems which must be upgraded or replaced.  New regulations, technology, and the effects of corrosion and erosion over time are creating a big market.  Some existing systems have gas to gas heat exchangers and dry stacks.  When you upgrade should you eliminate this process and discharge through a wet stack? Bypass is no longer an option, so isn’t it logical to remove the old bypass duct?  Mercury capture in the scrubber adds a whole new dimension. Since we have a mercury decision guide we avoided this subject yesterday.

It is interesting that dry and wet systems are being combined. So dry injection of lime ahead of the scrubber can capture SO₃ and also improve the efficiency of the downstream limestone scrubber. Lime injection ahead of the precipitator will eliminate fluorine. This could change the scrubber materials of construction. So titanium might be an option for the SO₂ scrubber.

TAKE THE HOLISTIC APPROACH AND CONSIDER DSI INJECTION IMPACTS ON DOWNSTREAM MATERIALS OF CONSTRUCTION

Systems and Reagents:  Lime and limestone are the two options.  As Scott Fraley of Carmeuse pointed out, magnesium enhanced lime needs to be considered because it results in a chemically fixed sludge which encapsulates the toxic metals permanently.

CONSIDER WET LIME AS AN ALTERNATIVE OR SUPPLEMENT TO LIMESTONE

Charlie Alack of Semi Bulk pointed out the advantages of purchasing powdered limestone. You pay more for the material but you eliminate the capital cost of ball mills. The reagent choice and mode of operation determines both wastewater and byproduct characteristics.

CONSIDER ALL COSTS INCLUDING REAGENTS, SYSTEM, WASTEWATER TREATMENT, AND BYPRODUCTS

Scrubber and Components. The bulk of the systems in the world are wet limestone with wall board quality gypsum as the end product. Spray towers, tray towers, sump scrubbers (JBR) and now pipe scrubbers are all available. Klaus Baernthaler of Andritz submitted power points showing the advantages of creating holdup or turbulence by placing pipes across the tower and effectively reducing the free area.  The tray scrubber and sump scrubber also rely on turbulence rather than laminar flow.  Older rod deck or pipe scrubbers were displayed including Duck Creek Cilco (a Riley Babcock installation).

Alstom introduced Flow Pac some years ago. It is a sump scrubber like the Chiyoda JBR which causes the flue gas to enter the scrubber below the nominal slurry level and thus creating a turbulent froth.  However, there has been no recent news about this design. The pendulum has therefore swung back toward the middle. The first scrubbers were ping pong ball and marble turbulent scrubbers, then for decades it was open spray towers vs. tray towers. We need B&W to weigh in on whether they use their tray for turbulent froth or just gas distribution (the difference is the open area).  Now Andritz has again championed turbulence. 

SCRUBBER TURBULENCE vs. LAMINAR FLOW IS AGAIN A MAJOR ISSUE

Most scrubbers in place are spray towers using large quantities of slurry. They rely on laminar flow and optimum dispersion of slurry droplets. Stewart McKenzie of Lechler stated that their nozzles provide uniform distribution and atomization in both up flow and downflow configurations.

To ensure laminar flue gas distribution, CFD modeling precedes tower design. Baffles may be needed on the side walls to provide the right air pattern. Gas velocity is also a variable. Some testing shows higher performance at higher gas velocities. (10 fps vs 18 fps). Is this further proof of the value of turbulence? Marsulex and other spray tower suppliers will be asked to address this question.

Mist eliminators are also critical.  Escaping mist adds to emissions and can cause maintenance problems.  Design considerations include:

• Chevron or non-Chevron
• Shape and number of turns
• Spacing between  each chevron
• Number of mist eliminator stages
• Pressure drop vs. efficiency

SPRAY TOWER PERFORMANCE IS DETERMINED BY THE NOZZLES, MIST ELIMINATORS, GAS DISTRIBUTION AND VELOCITY

Materials:  Conditions vary throughout the system. Diagrams segmented the individual areas into mild, moderate, and aggressive. In all system designs the scrubber inlet is an aggressive zone.  The stack condition depends on whether there is bypass or reheat.  The worst condition is with bypass and no reheat. With this arrangement conditions change from wet to hot and back again. Steady 50°F reheat is the least aggressive. Other variables include the use of halogens with the fuel for mercury oxidation, coal composition, and the recirculating chloride concentration in the scrubber slurry.

Crevice corrosion has become a big issue. 2205 duplex stainless scrubber shells resist general corrosion.  However, calcium sulfate buildup over welds results in highly acidic liquids trapped beneath the solids. 2205 does not do as well as C276 or other higher nickel alloys under these conditions.

A wide variety of materials are used in wet calcium FGD systems. Here is one way to classify them.  The first cut is

§  Solid sheets

§  Linings or wallpapering

§  Coatings (thinner than linings)

The second cut is by chemical type

§  Metals

§  Synthetic polymer-based materials

§  Rubber

§  Non-metallic minerals

In the meeting yesterday, we addressed some of the latest insights. The Power Plant Air Quality Decisions Orchard contains many papers, white papers and recorded Hot Topic presentations on this subject. One Hot Topic session was 13 hours in length. There was such intense discussion that one session turned into multiple sessions over the period of several weeks.

The metals include titanium, stainless, and nickel alloys. The price of nickel is down considerably. So decisions should be made with this in mind.  Louisville Gas and Electric has replaced corroded materials with relatively expensive metal alloys but has justified it based on life cycle costs.

The Chinese are using lots of plastic linings. We need to learn more about the condition of these systems. Are there opportunities for metals for replacement or upgrade? Many Chinese suppliers offer lower grade stainless steels. But ATI and other international suppliers are competitive with more expensive alloys? Another upgrade option is FRP. Ashland believes that with their fire retardant resins there is little danger in using FRP.  Apparently there has been some resistance due to lack of ability to distinguish between resins. Are Chinese and other developing countries moving toward tighter regulations and compliance during operation?  If the answer is yes, then there are lots of opportunities to supply more reliable materials.

Robert Aliasso  of  Stebbins points to 40,000 MW of SEMPLATE (nonmetallic mineral lining) which withstands a wide range of temperatures and acids. This includes more than 70,000 ppm of chlorides in the recirculating slurry.

John Davis of Sauriesen discussed FRP linings and Organic Membrane / Inorganic K₂SiO₃ concrete which provides “dual protection” against corrosive, abrasive & temperature environments common to flue gas ducts and stacks.

Kevin Lambrych of Ashland described the extensive activities in FRP.

§  Since 2004 Fiber Reinforced Plastic (FRP) has been used in 70+ FRP stack liners, 24+ FRP jet bubbling reactors, and 75+ limestone slurry systems

§  Ashland thermoset resins are used to fabricate industrial FRP equipment and FRP liners

§  Ashland Performance Materials is the largest global producer of Epoxy Vinyl Ester Resins (EVER) and Unsaturated Polyester Resin (UPR)

§  Ashland works with end users, designers, engineers and fabricators to ensure the correct resin is used (based on case histories and ASTM C-581 corrosion data)

§  Ashland provides resin recommendations and suggested corrosion barrier construction in writing upon request

§  FRP equipment can be manufactured in the field at lower cost vs.

§  FRP composite based on Ashland EVER has the same chemical resistance to acid and better resistance to acid chloride salts than C-276 and is superior to 2205 duplex stainless steel in both cases

§  Ashland resins have been used to repair failed 2205 based scrubbers

Duromar provides a wide variety of coatings for FGD Systems. Daniel Toft focused on a new lining which prevents or reduces lime scaling of ducts in dry scrubbers. Its applicability with wet systems would be for combination DSI and wet scrubbing.

Costs of plastic lining systems in China were provided by McIlvaine along with references to papers in Chinese in the PPAQS Orchard on the performance of various linings at Chinese power plants.

A VARIETY OF MATERIALS MAY OFFER LOWEST LIFE CYCLE COST DEPENDENT ON THE SITE SPECIFIC PROCESS, LOCATION, OPERATION, AND LABOR COSTS

System Components:

The recycle slurry pumps are major power consumers and sources of maintenance.  Here are trends:

§  Only a few pump companies make the very large limestone slurry recycle pumps.

§  KSB, Weir, and Duechting are capable of supplying individual pumps with more than 50,000 gpm.

§  Ceramic is becoming more popular than rubber-lined or hard metal.

§  Steag/Evonik has experience with both and recommended the ceramic in a McIlvaine 2008 Hot Topic Hour. Is this still their opinion?

Jim Burke of Weir supplied data to chronicle the continuous process to make their pumps more cost effective. Axial and radial adjustments have reduced internal velocities. The design plus liberal use of silicon carbide has increased component life.

CERAMICS ARE NOW AN ACCEPTED ALTERNATIVE TO ALLOYS AND RUBBER LINING FOR FGD PUMPS

Valves are used for lime slurries, calcium sulfite recirculating slurries, fresh water, wastewater and with various metering pump systems. The big isolation valves are usually butterfly or gate. 

Brian Perkins and Jason McRell of Crane ChemPharma made the following points:

§  Butterfly valves have been the preferred valve design for wet FGD slurry service in Europe for decades.

Advantages:

o   No need for splash containment – much cleaner installation

o   Much lighter and more compact than knifegate (2 x lighter)

o   Quick, low-torque ¼-turn operation

§  Crane CL-RS has hundreds of successful Wet FGD installations globally and in the United States.

There were two suppliers championing the knife gate design. Chris Swarr of ITT Engineered Valves, discussed experience with the knife gate valve which has been in use for FGD slurry isolation for several decades. They are available in diameters of 2” to 60” in urethane or rubber lined construction.

Darrel DesRochers and Arvo Eliahu of Pentair provided some history on Clarkson valves which have been used since FGD was introduced. ITT slurry valves feature

§  Reliable positive operation

§  Full round port flow

§  Bi-directional

§  Abrasion resistant elastomer or gum rubber sleeves

§  Good track record in sticky and abrasive applications

THERE IS A CHOICE BETWEEN BUTTERFLY AND KNIFE GATE VALVES FOR FGD SLURRY

The decision guide also displayed a number of slides on agitators, oxidation blowers and fans. Due to time constraints these topics were postponed to future webinars.

Wet Calcium FGD Webinar - Hot Topic Hour May 14, 2015

Discussion of wet calcium systems, scrubbers and scrubber internal components, balance of plant components, and materials.

Revision Date:  5/14/2015

Tags:  221112 - Fossil Fuel 化石燃料, Duplex Stainless, Wet FGD, Wet Limestone, Valve, Stainless Steel, Lime, Limestone, Mist Eliminator, Nozzle, Pump, Resin