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Content Type:Q&A

Question:

Do you have advice about converting a boiler from coal-fired to natural gas?  The application is a base-loaded, 80,000 lbs/hr coal-fired boiler producing 120 psig steam. The boiler is an older (1970s era) spreader-stoker type that was originally equipped to burn No. 6 oil as a backup fuel. The proposal is to convert it to interruptible gas, with No. 2 oil as the backup fuel. Pollution control equipment consists of a multiclone and an electrostatic precipitator.

Answer:

To get advice I spoke with Cal Hooper, whose company did many coal-to-gas conversions in the 1970s.  He is now serving on the Steam BestPractices steering committee for the U.S. Department of Energy Industrial Technologies Program.  The following is some of what he had to say:

A boiler equipped with a Detroit spreader-stoker empties ash on the front end. Cal said you remove the header off of the front of the boiler, also the stoker and coal hopper. Remove the feeders and the multiple feeder chutes. Remove the refractory front wall from underneath the front header to the floor. Remove the sifting hopper, stoker, and stoker drive shafts, grates, and chains. The bottom of the boiler is now gone. Install a steel-back, then build a new refractory floor in the boiler. Build a refractory front wall with openings for the gas and oil burners.

Install a duct under the refractory floor so the forced draft fan can provide air to a windbox located in the front of the boiler. The burner likely would consist of multiple gas rings (2 or 3 burners). An oil gun would be inserted from the operating floor through the windbox and the opening in the refractory wall. Modify the duct so it goes directly to the plant stack and bypasses the multiclones and electrostatic precipitator. Flame safeguards are generally installed on the burner package.

Burner manufacturers, such as Coen or Peabody, need to know the dimensions of the boiler in order to design the burner flame pattern. They have flexibility with respect to where they put the holes in the burner tip. You also need to know if the gas pressure is adequate and know the period of gas interruptibility so that size of the No 2 fuel oil storage tank can be determined.

Since superheat is not an issue, no changes are necessary to the heat exchange tubing. The boiler feed pumps, deaerator, etc, operate as before. Once it heats the steam drum, the steam doesn't know if it was produced by coal or natural gas.

Cal suggested finding a company to do the work that has completed many conversions in the past; for example, he knows of one company that has design drawings for boilers of various ratings produced by many manufacturers.

Topic: Steam Systems--General
Topic: Steam Systems--Boilers
Sector: Commercial
Sector: Industrial
Content Type: Q&A
Keywords: electrostatic precipitators, steam boilers, multiclones
ID:  2325