 | | | | EnvirOcycler Sizes and Btu/h Rating
The EnvirOcycler is available in sizes ranging from 15 to 150 Million Btu/h (4 to 40 MWth), consuming between 2 and 20 tons/h of typical 45% moisture content biomass. The EnvirOcycler is rated according to the total enthalpy in its products of combustion with respect to a temperature of 77°F. 77°F (25°C) is a standard boiler reference temperature.
Thus, a 100 Million Btu/h EnvirOcycler discharges a stream of, say, 2,000°F products of combustion, the total enthalpy in the component gases of this stream - the carbon dioxide, water vapor, oxygen and nitrogen - is 100 Million Btu/h when measured from a datum of 77°F.
The assumption is made that the water in the EnvirOcycler's products of combustion (water brought in by the wet fuel as well as water of combustion) "magically" remains in the vapor state, even at
77°F. This assumption is really not restrictive since, in most cases, it is differences in enthalpy during some form of heat transfer process that are of interest.
Net Heating Value of wet solid residues
Building on the principle of how EnvirOcyclers are rated, the Net Heating Value (NHV) of a wet solid waste defines how many Btu's each pound of wet solid waste contributes to the total sensible heat in the products of
combustion leaving the EnvirOcycler. The NHV is calculated by dividing the enthalpy difference between the products of combustion at the EnvirOcycler's discharge temperature, and the same products of combustion at the reference temperature, by the total amount of wet solid waste consumed. As indicated above the normal reference temperature is 77°F. The following calculation illustrates how the NHV is determined. | |
| | | Combustion calculation - principle
This combustion calculation requires the
fuel's Ultimate Analysis - its percent by weight of hydrogen, carbon, oxygen, nitrogen and ash, a discharge temperature (2,000°F in the present case) and a process discharge temperature. It calculates the stoichiometric air required to burn one pound of bone dry fuel. It then calculates, as a function of wet basis moisture content, the excess air, the percent by weight of the various products of combustion and R, the gas constant, in ft-lb/lb°F. |
| | | | | The calculation then assumes that the EnvirOcycler
products of combustion enter some kind of heat exchanger (a boiler, a hot oil generator, etc.) and, during their passage through the heat exchanger, transfer 10 Million Btu/h to the fluid travelling through the other side of the heat exchanger. The EnvirOcycler gases are thereby cooled down to the process discharge temperature (a boiler stack temperature of, say, 350°F).
Having determined the lb/h of products of combustion involved, the calculation then calculates the lb/h of solid fuel, the scfm of combustion air and the acfm/scfm of the products of combustion involved in generating the 10 Million Btu/h.
Finally, the calculation determines the Net Heating Value (NHV) per pound of wet fuel and an equivalent quantity, Hs, the amount of sensible heat each pound of products of combustion transfers to the process, (between the EnvirOcycler discharge
temperature and the process discharge temperature). As pointed out above, a process discharge temperature of 77°F is used to determine the rating of individual EnvirOcyclers.
Combustion calculation - results
The first combustion calculation is a reference calculation for sawmill wood residue. At a typical moisture content of 42% the NHV is 4,062 Btu/wet lb.
2,462 lb/h of wood needs to be burned to release 10 Million Btu/h between an EnvirOcycler discharge temperature of 2,000°F and a datum of 77°F. The corresponding combustion air required is 3,311 scfm. The excess air is 67.1%. The lowest excess air shown is 0.6% at 64% moisture content. The lowest practical excess air at which the EnvirOcycler is normally operated is 15%.
The second combustion calculation, also for wood residue, is for the same 2,000°F
EnvirOcycler discharge temperature but has a process discharge temperature of 350°F, typical of a waste heat boiler. The NHV for 42% moisture content is reduced to 3,550 Btu/wet lb because of the stack heat loss. 2,817 lb/h of wood is required to transfer a full 10 Million Btu/h to the process (boiler feedwater), up from the reference 2,462 lb/h.
Note the right hand column. It shows that 11.443 Million Btu/h, with respect to 77°F must be input to the boiler to transfer 10
Million Btu/h to the water in the boiler. In other words, an additional 14.43% of the heat being transferred to the process must be supplied to allow for the heat lost up the stack. Note also that the ratio of the NHV's, 4,062/3,550 = 1.1442. The difference between this and 1.1443 is due to round off errors. | |
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