HRSG Superheater Replacement

Plainfield Renewable Energy (PRE) is a net 37.5 MW woody biomass electrical generation facility located in Plainfield, Connecticut. The facility was experiencing extended forced outages due to tube failures in the high temperature T91/P91 superheater assembly.

PRE utilizes a state-of-the-art staged fluid bed gasification system and an Alstom steam turbine to generate approximately 290,000 MW annually. The plant is equipped with a baghouse for particulate control, urea injection for NOx control, and limestone and hydrated lime injection combined with a circulating dry scrubber for sulfur dioxide control.

The boiler is a modularized HRSG (heat recovery steam generator) type originally provided by an Outotec (formerly Energy Products of Idaho or EPI), with a bubbling fluidized bed gasifier. The facility went into operation approximately December of 2013. The boiler is a natural circulation, superheated, balanced draft design unit firing biomass waste. The boiler was originally designed to produce 366,648 pph of steam at 1550 psig and 955 °F at the superheater outlet.

HBBW Scope of Work

HBBW’s scope included a complete design analysis, fabrication and installation of a shop-fabricated Superheater #1 section in two harps using stainless steel for all tubes and headers. The superheater was re-designed and replaced to improve the reliability with replacement of the T91 tubing and P91 headers with stainless steel.

The boiler design analysis included a thermal and hydraulic analysis to review the existing design. A thermal model was prepared of the steam and flue gas temperature, pressures, and flow profile through the boiler. A hydraulic model was prepared to evaluate the steam side distribution through the harp tubing. The hydraulic flow imbalance of the existing superheater was calculated to be below generally accepted industry standards which resulted in unstable, unpredictable steam flow in the circuits and lead to high tube metal temperatures. Hydraulic flow imbalance to the final leg of the high temperature superheater was improved by underdrilling the inlet header of the final superheater pass.

• Engineering
  • Re-design of the final superheater to eliminate T91/P91 components and replace with stainless steel material
  • Thermal analysis of the boiler (HRSG) to establish pressure, temperature, and flow profiles of the steam and flue gas paths
  • Hydraulic analysis to correct steam side flow unbalance with the addition of orifices
  • Stress analysis to support material change to stainless steel
  • Flexibility analysis to accommodate the thermal expansion characteristics of stainless steel
  • Required ASME Code alteration documentation and calculations
  • Installation/arrangement and shop fabrication drawings
• Complete removal of the existing superheater
• Material procurement
• Fabrication
• Installation of the redesigned superheater and interconnecting high pressure piping
• Refractory, insulation and lagging