Page 229 - Mechanical Engineers' Handbook (Volume 4)

218 Furnaces

Distribution of fuel demand by use is estimated at

Power generation 20 quads
Space heating 11 quads
Transportation 16 quads
Industrial, other than power 25 quads
Other 4 quads
Net demand for industrial furnace heating has been about 6%, or 4.56 quads, primarily from
gas and oil fuels.
The rate at which we are consuming our fossil fuel assets may be calculated as (annual
demand)/(estimated reserves). This rate is presently highest for natural gas, because, besides
being available at wellhead for immediate use, it can be transported readily by pipeline and
burned with the simplest type of combustion system and without air pollution problems. It
has also been delivered at bargain prices, under federal rate controls.
As reserves of natural gas and fuel oil decrease, with a corresponding increase in market
prices, there will be an increasing demand for alternative fuels such as synthetic fuel gas
and fuel oil, waste materials, lignite, and coal.
Synthetic fuel gas and fuel oil are now available from operating pilot plants, but at costs
not yet competitive.
As an industrial fuel, coal is primarily used for electric power generation. In the form
of metallurgical coke, it is the source of heat and the reductant in the blast furnace process
for iron ore reduction, and as fuel for cupola furnaces used to melt foundry iron. Powdered
coal is also being used as fuel and reductant in some new processes for solid-state reduction
of iron ore pellets to make synthetic scrap for steel production.
Since the estimated life of coal reserves, particularly in North America, is so much
greater than for other fossil fuels, processes for conversion of coal to fuel gas and fuel oil
have been developed almost to the commercial cost level, and will be available whenever
they become economical. Processes for coal gasification, now being tried in pilot plants,
include

1. Producer gas. Bituminous coal has been commercially converted to fuel gas of
low heating value, around 110 Btu/scf LHV, by reacting with insufficient air for combustion
and steam as a source of hydrogen. Old producers delivered a gas containing sulfur, tar
volatiles, and suspended ash, and have been replaced by cheap natural gas. By reacting coal
with a mixture of oxygen and steam, and removing excess carbon dioxide, sulfur gases, and
tar, a clean fuel gas of about 300 Btu/scf LHV can be supplied. Burned with air preheated
to 1000 F and with a flue gas temperature of 2000 F, the available heat is about 0.69 HHV,
about the same as for natural gas.
2. Synthetic natural gas. As a supplement to dwindling natural gas supplies, a syn-
thetic fuel gas of similar burning characteristics can be manufactured by adding a fraction
of hydrogen to the product of the steam–oxygen gas producer and reacting with carbon
monoxide at high temperature and pressure to produce methane. Several processes are op-
erating successfully on a pilot plant scale, but with a product costing much more than market
prices for natural gas. The process may yet be practical for extending available natural gas
supplies by a fraction, to maintain present market demands. For gas mixtures or synthetic
gas supplies to be interchangeable with present gas fuels, without readjustment of fuel/air
ratio controls, they must fit the Wobbe index: