Page 244 - Boiler_Operators_Handbook,_Second_Edition

Consumables 229

requirements of the gas supply. Domestic natural gas is one where you better read the instruction manual and
has a higher heating value of approximately 23,165 Btu be aware that leaks are very hazardous.
per pound, approximately 1,042 Btu per standard cubic Digester gas is actually natural gas, just very
foot. For combustion it requires 11.48 standard cubic feet young natural gas. Like a young bourbon it has a kick,
of air per standard cubic foot of gas, 185 standard cubic lots of things in it that make it less desirable than natural
feet per minute of air per million Btu per hour. gas, which had thousands of years to cure in the ground.
Liquefied petroleum gases (LPG) are primarily bu- Digester gas is a by-product of waste water treatment
tane or propane with propane being the more common. where the water is enclosed in the digester and an-
They are transported as a liquid under pressure. They aerobic bacteria (bugs that don’t like air) literally eat the
combine the clean burning properties of gas with the waste and generate methane and carbon dioxide in the
transportation properties of oil but at a premium in cost. process.
In boiler plants where LPG is used it’s normally as an al- The principal difference between digester gas
ternate fuel for interruptible natural gas. Propane can be and natural gas from wells is the digester gas contains
mixed with air in proper proportions to produce a blend a lot more carbon dioxide and usually has some other
that will fire in natural gas burners without adjustment materials in it that carry over with the gas as its gener-
of the burners. ated. Some of the less desirable materials include water,
Propane has a slightly higher heating value of ap- hydrochloric acid, and solids. Some digester systems are
proximately 21,523 Btu per pound, approximately 2,573 fitted with filters to reduce the solids and separators to
Btu per standard cubic foot and it requires 28.78 standard remove most of the water and acid before it gets to the
cubic feet of air per standard cubic foot of gas, 186.45 boiler plant. The largest variable in digester gas is the
cubic feet of air per million Btu. You’ll note that the air amount of carbon dioxide. It’s basically inert (the carbon
required per million Btuh is about the same for all gases. and oxygen already combined) so it dilutes the methane
All but very large LPG installations will absorb enough content of the gas to reduce its heating value to numbers
heat at the tank to convert the liquid to a vapor. Large in the 250 to 800 Btu per standard cubic foot range, 25%
installations require a vaporizer, a heater fired by vapor to 80% of the energy normally found in natural gas.
off the tank that provides the energy to evaporate a liquid Special considerations for firing digester gas in-
stream for use in the boilers. Propane will condense at clude concern for blockage of valves (especially safety
normal atmospheric temperature (70°F) at 109 psig. shut-offs), regulators, etc. All the piping should be fit-
Butane will condense at 17 psig. On a very cold ted with drains, usually drain pots where the collected
day butane will not vaporize and most installations moisture, etc. can be captured for return to the digester.
require a vaporizer. Butane has a higher heating value The piping also has to be arranged so it can be cleaned
of approximately 21,441 Btu per pound, approximately in the event of an upset in the digester which could
3,392 Btu per standard cubic foot and it requires 37.57 send over considerable quantities of water and solids
standard cubic feet of air per standard cubic foot of gas, (another name for that “s” word) to plug things up. Pip-
184.64 cubic feet of air per million Btu. ing materials may be constructed of stainless and other
You’ve undoubtedly heard a lot about hydrogen as alloys to prevent corrosion by the acids in the system
a fuel lately because it’s the principal fuel for fuel cells, but pre-cleaning usually reduces the acids enough that
those devices used on the space shuttle to generate elec- normal steel can be used. When you do have steel piping
tricity and water. By now you can probably envision one it’s advisable to check its thickness regularly and after
taking on hydrogen and oxygen to produce water and any severe plant upset.
the energy generated comes out as mostly electricity. The large fractions of carbon dioxide can dilute a
Fuel cells do produce some heat but that’s considered a digester gas so much that it will not burn with a stable
by-product in their application. I’ve only had one expe- fire. Special burners are required to pass the larger gas
rience with burning hydrogen in a boiler. It was a waste volumes required to get the fuel value needed for the
gas from a chemical process and we burned it to recover boiler capacity and many of them are fitted with stand-
the energy. You can imagine that at 61,000 Btu per pound ing pilots. Most of the applications I’ve worked on
it was a very hot fuel and burner construction and include real natural gas as a support fuel to maintain
maintenance was very demanding. If I knew then what ignition of the digester gas and to make up any addi-
I know now I would have blended it with something tional energy requirements. Both fuels are fired simul-
before trying to burn it, either natural gas or lots of air taneously and the controls have to be able to cope with
to avoid the terribly high flame temperatures. This fuel that.