Recent Laser Metal Deposition (LMD or Laser Cladding) innovations in the WTE boiler industry, by American Cladding Technologies.  ACT is a “Service Provider” and “Laser Equipment Integrator”, as such we are legally bound by our customer NDA's and our own IP Intellectual Property (because of these restrictions, specific confidential details have been omitted to maintain confidentiality). 

Overview of Laser Metal Deposition or LASER CLADDING FOR WASTE TO ENERGY BOILER TUBES

In the power generation industry, one of the most extreme and severe environments is in the “Waste to Energy” (WTE) boilers.  There are many types of WTE energy production.  The most common WTE boilers use a fuel: such as wood or construction debris and even MSW (Municipal Solid Waste).  The pressurized WTE boiler components operate at high temperatures (870°- 1,100°C), high pressures (850 – 1200 psi) and fuel that is both highly corrosive and erosive.  The main corrosive component is the high chlorine concentration of the flue gas from the incinerating the fuel source. 

Chlorine, tons per month
One USA based WTE facility American Cladding Technologies has worked with processes 1,000 tons of chlorine through its boilers each month.  The erosion is accelerated by fly ash impingement, and the cleaning cycles of the soot blowers. Compounding this corrosion / erosion, the fly ash debris builds up onto the boiler components and begins to block flue gas pathways.  This fly ash builds up dramatically reduces the heat transfer across the boiler components causing thermal “hot spots” in the boiler.  As more of the flue gas pathway are choked off, the remaining open pathways flue gas is increased to a high velocity fly ash impingement to nearby boiler component surfaces.  Pressure components such as super heater tubes, super heater platens and water wall panels are all subjected to this hostile environment and must therefore be replaced at regular intervals and at a significant cost to the energy producer.

THE NEED FOR REDUCTION OF WEAR
To extend component life and reduce replacement costs, the WTE industry has started cladding boiler pressure components with alloys that help reduce the wear of corrosion and erosion.  The most common cladding alloy used in the power generation industry is Inconel 625.  Inconel 625 is “nickel-based alloy”.  Inconel 625 also is high in chromium and molybdenum to give a high level of resistance to pitting and crevice corrosion caused by chloride contamination. Inconel 625 is used in power plants that include coal fired, biomass, nuclear and of course Waste to Energy.  In WTE typical cladding thickness is 1.75 mm to 2.54 mm thick on pressurized boiler components.  Inconel 625 has some resistance to erosion.  

WHY USE CLADDING?
The first benefit of cladding is financial, saving money. It costs less to use a common alloy (example: SA213-T22) as the primary boiler tube substrate.  The low cost SA213-T22 is clad with a layer of Inconel 625.  This clad tube will cost much less than buying the entire tube or component fabricated from a solid Inconel 625. 

Another advantage is the cladding alloy properties can be controlled, to give unique properties.

AMERICAN CLADDING TECHNOLOGIES USA BEGINS LASER CLADDING DEVELOPMENT
In February 2011, American Cladding Technologies (ACT) began working with a North American WTE power producer to create and apply a laser coating that would outperform Inconel 625 on pressurized boiler components.  Even though Inconel 625 was effective, Inconel 625 did not achieve the target service lifetime.  The superheater (SH) lifespan with Inconel 625 was 16 – 24 months.  At the end of the service life, usually the entire primary and secondary SH was replaced at significant cost. Working with American Cladding Technologies the WTE power producer’s “boiler reliability” engineers, ACT “laser process” engineers, a manufacturer of metal powdered alloys and university materials testing facilities.  Finally ACT developed powdered alloy composition and begin operational trials.  Operational trials started with individual segments (3-meter lengths) installed in various boiler pathways on primary and secondary super heater pendants.  Periodic boiler inspections were performed on the test samples to track performance.

Project goals were as follows:

  1. Extend boiler pressure component lifetime by a minimum of 2X.
  2. Eliminate or reduce the use of shielding on the lead SH tubes.
  3. Optimize laser cladding process to be cost competitive with existing cladding methods.


Photo of a leading SH tube from a test sample inspection.  Boiler inspection performed after approximately 14 months of operation.  No shielding was installed on test samples

Photo was taken after water wash cleaning of SH tubes.  NOTE:  Lay lines of the spiral wound laser clad are still clear and visible, meaning little wear. 

 



 

By the end of 2011, the performance data on the test samples showed sufficient encouraging results to warrant moving ahead with continued laser metal deposition of SH tubes.  American Cladding Technologies accepted their first purchase order to laser clad a set of 2.5” Ø   SA213-T22 superheater tubes for delivery in January 2012.   

As of May 2017, more than 5,000 SH tubes and over 100 SH platens have been laser hard-faced and installed in more than 28 WTE boilers across the U.S. 

Another boiler component benefiting from this process is the “Soot Blower Lance”.  American Cladding Technologies has also begun testing in both the Biomass and Pulp & Paper industries.  This performance data has also been encouraging. 


49 MONTH BOILER LIFE IS ACHIEVED MORE THAN 2X Inconel 625

In May 2016, various laser clad SH tubes were removed for evaluation from a USA WTE facility. 

At the time of removal, the SH tubes had been in operation for 49 months.  As of this writing, May 2017, there are SH tubes that have been in operation for more than 5 years without failure.

 

Shown above is one of the evaluation tubes removed for metallurgical examination after 49 months of operation.


 

Metallurgical cross section from one of the “49 month” SH tubes removed for evaluation.  Original coating thickness was approximately 1.00 mm – 1.12mm thick

FINDINGS AND RESULTS
To maintain customer confidentiality, American Cladding Technologies will not release or discuss specific values regarding the realized cost savings, power plant operating values or any other data that might be considered sensitive to our customers.  However, a general overview of the performance results is provided below.

Improved thermal efficiencies compared to the standard Inconel cladding
a.           Elimination or reduction of shielding on leading SH tubes has reduced ash build up in the low flow areas between the shield and tube face and therefore improved heat transfer.

b.           Reduced heat input of the laser cladding process results in a very low dilution of the base material.  This allows for a reduced coating thickness which aids in thermal efficiency.

Reduced maintenance costs.  

a.           Reduced or eliminated costs associated with shielding procurement and installation.

 b.          Reduced the need for (and costs of) unplanned outages by extending boiler component life. 

Other advantages of laser hard-facing over preexisting Inconel claddings.  

a.           Reduced fly ash erosion due to the increased wear resistance of the coating.  The typical laser applied cladding is 66 – 70 Rc.  

b.           By eliminating or reducing SH tube shielding, hot spot formation has been reduced, thereby reducing localized failure points.

c.           Soot blower cleaning cycles have been reduced.  Some facilities have reported a 79% reduction in cleaning cycle frequency.

 d.          For continuous inline soot blower lances, component life has been extended by up to 6 times their preexisting lifespan.

Production cost per linear meter of the laser hard-facing can be equal to or less than the linear meter cost of traditional Inconel 625 claddings.  Cost reductions of 40% have been realized.  These cost swings are largely dependent on the following variables:

Total production quantity.  
Required coating thickness, which is significantly less than typical Inconel wire claddings and therefore leads to reduced filler material costs and cladding cycle times.
Laser deposition rates, which continue to increase as the development effort continues.
Geographic region within the US.  (Cladding costs vary per region)

A typical cross section photo (50x) of a process qualification sample for ASME Section IX. Base material is SA-213-T22. Photo shows good cladding density, no cracking, no Lack of Fusion (LOF), very low dilution and minimal HAZ.


 

SUMMARY

▪            Because of the rapid solidification of the weld pool, microstructure of the laser cladding is superior to traditional cladding.  Hardness over 66 HRc are possible.

▪            Low or minimal heat input from the laser results in very low base material dilution.  It is possible to achieve full material properties in as little as 0.125mm coating thickness.  These thin claddings may not always be economical for the end user.  For a typical SH cladding, a cladding thickness of greater than 0.750 mm performance is superior to traditionally clad Inconel 625.   

▪            Corresponding with the low heat input, the “Heat Affected Zone” (HAZ) associated with the welding is also minimized.  

▪            Thickness of overlay greater than 3.0mm can be clad successfully.  Not always recommended in every application.  Factors include:

Where the component will be used 
Clad geometry of component 
Mass of the component 
Coating quality requirements.

Leading and sometimes trailing SH tubes will generally require a thicker coating than the 0.750 mm normally applied to most SH tubes.

American Cladding Technologies has recently qualified a weld repair process for this cladding.  Coatings can now be repaired with conventional welding equipment in the field.  \

For more information, please contact to stephen.booth@bexxonglobal.com

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Waste to Energy

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