Cogeneration
The production of steam for both electric power generation and some other use at the same time.

Efficiency
The percentage of the energy production in a process (such as incineration, or burning) that can be put to use.

Enthalpy
A measure of an amount of heat per pound of material; in this example, the number of BTUs required to produce a pound of steam.

Incineration
Burning to ashes, by combustion, or fire.

Landfill
The city dump.

Mass burning
The burning of garbage in order to produce steam as a source of energy, available for a variety of other uses.

Municipal refuse
The garbage that is picked up by a city.

Municipal refuse is no longer just something to be gotten rid of.  We are running out of places for it to go.  At the same time, our steam and electrical energy production needs are growing, and satisfying them is becoming more of a problem as fossil fuels (coal, oil, and natural gas) become more expensive to use.  People in the United States generate a lot of trash, over three pounds per person per day.  That's 150 million to 170 million tons a year, and the amount is growing day by day.   Combining refuse incineration with power generation or with steam and power cogeneration is a successful solution to both problems.

The problems in this reading put you in the seat of the city engineer.  As the city engineer, you are faced with the problem of waste disposal in the community.   After all the options have been discussed, the most appealing solution seems to be mass burning.  Therefore, your job is to show the mayor and other officials how much the city can gain in energy production and money by using mass burning to get rid of all that garbage.

You need to go to the City Commission at their next meeting armed with the figures that will convince them that mass burning is the most profitable, socially responsible solution.  You have municipal refuse with a heat content of 4500 BTU per pound.

Given:
Municipal refuse available = 500 tons per day
Boiler efficiency = 70%
Enthalpy = 1109.6 BTU/lb
Hours of operation = 24 hours per day

PROBLEM 1
How many pounds of steam per hour can the system produce?  Use the following formulas to find out how much fuel is available per hour.

Tons per day/24 hours per day = tons per hour
500 tons per day/24 hours per day = 20.833 tons per hour
Convert tons per hour to pounds (lb) per hour:
20.833 tons per hour x 2000 lb per ton = 41,666 lb per hour of fuel

Using 4500 BTU per pound of fuel, convert pound of fuel available to BTUs per hour, using the following formula:

lb of fuel per hour x BTUs per pound = BTUs per hour

41,666 lb of fuel per hour x 4,500 BTUs per pound = 187,497,000 BTUs per hour

With 187,497,000 BTUs available per hour and a boiler efficiency of 70%, find the number of pounds of steam generated per hour, given enthalpy of 11090.6 BTUs per pound of steam.

Pounds of steam per hour = (total available BTUs x boiler efficiency) / enthalpy

187,497,000 BTUs per hour x .70 / 1109.6 BTU per lb = 118,284 lb per hour

PROBLEM 2
If 1000 pounds of steam are valued at $1.25, determine the value of the steam generated on (a) an hourly basis and (b) a yearly basis if the plant operates 50 weeks a year, 24 hours a day.

Solution (a)

1000 lb of steam

118,284 lb per hour x $1.25 / 1000 lb = $147.86 per hour

Solution (b)

$147.86 per hour

50 weeks per year

7 days per week

24 hours per day

With the value of steam at $147.86 per hour and the plant operating 24 hours per day, seven days per week, and 50 weeks per year, find the yearly value of the steam:

$147.86 per hour x 24 hours per day x 7 days per week x 50 weeks per year
= $1,241,982 value each year