Appendix - Ecoprofile methodology (cont'd)

A11. Calculation assumptions

In the course of the analysis, a number of assumptions were made and the aim has been to apply these consistently across all of the production processes examined. The assumptions used and the reasoning behind them are given below.

Many processes produce steam as a co-product along with the materials outputs. When this steam is recovered for use elsewhere on the site, the process generating it is given an energy credit equal to the specific enthalpy of the steam recovered. If the pressure of the recovered steam is known, the energy associated with it is as shown in Table A6. When the pressure of the steam is not known, then an energy of 2.75 MJ/kg has been assigned. This energy credit is shown as a separate entry in the energy and fuel tables. No air, water or solid waste emissions are assigned to this recovered energy; all of these burdens are assigned to the chemical reaction.

When such co-product steam is taken into another process, usually the steam/power plant, the receiving plant is charged with an energy equal to the credit given to the generating plant.

The effect of this procedure is that recovered steam energy is effectively being treated as if generated with an efficiency of 100%; any inefficiencies in production and extraction of steam from the reactor are attributed to the chemical process. Any reported losses associated with transporting the recovered steam were attributed to the plant receiving the steam.

Table 6.
Specific enthalpy of steam at different temperatures and pressures. ¹⁰

It is important to recognize that energy recovery from chemical plants is only practised because several plants producing different products usually exist on the same site and each will have a different demand for steam. Thus surplus steam produced by one plant can often be used to satisfy the demand for steam by another process. Therefore when examining only one single production sequence on such a site, the recovered energy may appear as either a positive or a negative quantity depending upon whether the sequence is a net generator or a net consumer of recovered energy. Averaged over the whole site, the net recovered energy is expected to be zero.

A11.2. Steam condensate

Condensed steam is frequently recovered from processing operations for use either as process hot water or as a hot water feed for boilers. The energy content of this steam condensate has been calculated by assuming that it is hot water at 100°C. The specific heat capacity of water is 4190 J/kg/K and so, if the water from which the steam was produced was originally at a temperature of 15°C, then the heat required to raise the temperature of this water to 100°C is (100-15) x 4190 J/kg = 0.356 MJ/kg. In some instances, a condensate temperature of less than 100°C has been reported and in such circumstances the above calculation has been modified as necessary.

When condensate appears as an output from a unit process, an energy of 0.356 MJ/kg (or the relevant value for lower temperature condensate) has been attributed to it and the process is given an energy credit in the same way as for co-product steam.

Similarly, when steam condensate appears as an input to the process, this same energy is charged to the process. As with a steam co-product, no air, water or solid waste burdens have been attributed to the recovered energy.

A11.3. Co-generation of steam and electricity

The on-site co-generation of steam and electricity was a feature of many of the plants that supplied data. For such plants, information was available for the total input of fuels and the total outputs of steam and electricity. In some instances, the inputs also included condensate and steam generated in other chemical processes. (The method for handling condensate and co-product steam have been described earlier).

The calculations partitioned the total energy input between steam and electricity using energy as a basis. Output steam was assigned an energy content based on the steam pressure (see Table 6). Where the steam pressure was not known, a notional 2.75 MJ/kg was used. The steam energy was converted to a primary fuel equivalent based on a conversion efficiency of 80%, measured as output energy as a fraction of input energy. This primary energy equivalent was subtracted from the total energy input and the remainder was assigned to the electricity. Where more than one fuel input was identified, the proportions of the different fuels assigned to steam were based on the relative proportions of the different input fuels.

A11.4. Waste incineration

In all processes, the total inputs and emissions from reactions have been assigned to the usable or saleable products using the procedures outlined earlier. Some of the inputs will, of course, appear as solid or liquid wastes which are sent for incineration.

When incineration is used simply as a method of destroying the waste then the incineration plant is treated as an additional operation associated with the production process. Sometimes, however, energy recovery is practised at the incinerator. When energy is recovered in this way, it represents a conversion of feedstock energy to fuel energy and this conversion has been taken into account in the calculations by treating the energy recovered in exactly the same way as that recovered from steam co-products and condensate recovery as discussed earlier.

There are some instances where waste is sent off-site for incineration but no information is available about the incinerator; i.e. performance characteristics, or whether energy is recovered or not. In these circumstances, the waste is classified as waste to incinerator, but no allowance is made in the calculations for the incineration process.

¹⁰ Energy World Yearbook 1994. The Institute of Energy, London.