Operations & Maintenance – Business – Hopewell Cogeneration Facility

CO2 tube blasting of HRSGs improves heat rate

Hopewell Cogeneration Facility
IPR-GDF Suez Energy North America

Challenge. The plant was commissioned August 1990 and degradation of heat rate became a major concern to plant staff over time. The idea behind CO2 blasting or any other type of cleaning of HRSG tubes is to remove rust and deposits, thereby increasing heat transfer. This increase in heat transfer will show up in the plant as an increase in performance in the form of better heat rate.

 Solution. HRSG CO2 blasting was carried out in spring 2011. All three HRSGs at the site were cleaned. The material removed by blasting is blown down to the floor of the HRSG and vacuumed up, placed in a container, and recycled. In this way there is nothing left when the job is complete except clean tubes (Figs 7, 8).

Results. This was the first time the tubes were cleaned and the results were dramatic; heat rate improved by 176 Btu/kWh. Based on the price of gas at the time of the cleaning, $5/million Btu, the cost of CO2 blasting (a nominal $150,000 for all three units) was recouped in only two months of summertime operation, suggesting that the HRSGs probably should have been cleaned sooner.

   This prompted personnel to develop a calculator, which indicates an economically beneficial time to clean the HRSGs. Note that this calculator only considers the economic benefit of heat-rate reduction. If plant operations are impacted by emissions limits (for example, mandated load reduction, purchase of emissions credits, etc) that cost impact also should be factored into the decision to clean tubes.

   Assumptions used in developing the plant’s CO2 blast calculator were these:

  • Blasting will restore at least 90% of lost heat rate
  • “Timeframe selected”—one year, for example—is when the work is expected to be done
  • Project an average cost of fuel over the timeframe selected          
  • Project an average value for electric production over the timeframe selected
  • Use a budget type bid to get cost of CO2 blasting and clean-up
  • Degradation is a curve, but for calculating the need for cleaning make the degradation linear to estimate degradation over the years since cleaning or commissioning and project the same rate
  • Cost of cleaning should reduce the heat rate enough to pay for the cleaning in the timeframe selected—such as one year.

   Here is the formula developed by the Hopewell staff:

0.9HRd x FC x 1000 kW/MW x MWy = CC

Where: HRd is heat-rate degradation in Btu/kWh; FC is cost of fuel in $/million Btu; MWy is annual (or other timeframe selected) electric production in MWh; and CC is the cost of CO2 blasting in $.