Page 16 - mobile Workbook-chapter 2

2.9 Energy content

The heat values of refuse are of some importance in resource recovery. The units of

measurements are Btu, Cal, J, Wh. One of the earliest measures of heat energy still widely used
by American engineers is the British thermal unit (Btu), which is defined as that amount of energy

necessary to heat one pound of water one degree Fahrenheit. The internationally accepted unit

of energy is the Joule. Other common units for energy are the calorie and kilowatt hour (kWh) -
the former used in natural sciences, the latter in engineering [6].

In [2], the energy content of MSW can be determined by three ways:
• using a laboratory bomb calorimeter,

• combusting samples in a full-scale boiler and measuring steam output,
• calculation from elemental composition (i.e., ultimate analysis).

The following four factors must be considered when evaluating MSW as a potential fuel [4]:
• Only dry organic matter yields energy

• Ash reduces the proportion of organic fuel per pound of MSW
• Ash retains heat when removed from the furnace, therefore wasting heat

• Water reduces the amount of organic fuel per pound of MSW and requires a significant

amount of energy for removal (evaporation).

An important aspect of calorimetric heat values is the distinction between higher heat value

and lower heat value. The higher heat value (HHV) is also called the gross calorific energy, while
the lower heat value (LHV) is also known as the net calorific energy. The distinction is important

in design of combustion units [2].

Fig 2.5: Some models of bomb calorimeter

The energy stored within the chemical bonds of a material is known as the heat of combustion.

This heat is released when the material is burned. The heat generated by the combustion of a

material in a calorimeter may be determined by measuring the temperature rise that occurs upon
its combustion: U = CV × ∆T/ M

where U is the heat value (cal/g) of the unknown material, ∆T the rise in temperature (°C) from

thermogram, M the mass (g) of the unknown material, and CV the heat capacity (cal/°C) of the
calorimeter (measured using a standardized material).

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