the heat balanceReading time:
Any installation that consumes or produces energy is governed by a heat balance which is itself raised according to thermodynamic laws; this statement expresses the balance between the enthalpies entering into the system and those exiting the system.
When this type of balance is established, through a succession of iterations and based on parameterable values, it can be used to calculate energy consumption or emission induced by the system together with the materials balance applicable to by-products (solid and volatile).
These enthalpies can be defined as follows:
- incoming enthalpies:
- exothermic reactions released by the oxidation of the products processed (e.g. combustion of the organic matter found in sludge);
- energy recirculated by the process (e.g. in the case of incineration, the preheated fluidisation air enthalpy);
- energy given off as the booster fuel is burnt (amount of booster fuel/hour × net calorific value of the booster fuel);
- outgoing enthalpies:
- endothermic reactions given off by the system; in the case of sludge, this is more specifically the latent heat created by the evaporation of water contained in the sludge brought into the system;
- enthalpy (or sensible heat) from the products of reactions created by the breakdown of matter fed into the system; in the case of sludge, this is mainly superheating the evaporated water and of the products resulting from the complete or incomplete combustion of the sludge, to system discharge temperature;
- enthalpy (or sensible heat) from the products of reactions created by the booster fuel when this is needed by for reaction equilibrium purposes;
- the system’s heat losses; classic heat transfer equations are used to calculate these losses. In a simplified manner and as a rough guide, these heat losses are empirically accepted as equal to 3 % of the sum of outgoing enthalpies.
A balance is achieved through a series of iterations, allowing for:
- the system’s minimum outlet temperature;
- the amount of free O2 in the gas products released by the system (excess air).
This is the specific case of incineration or pyrolysis/gasification systems where a minimum temperature of 850 °C is required and where the free oxygen level is set to a level governed by current regulations (recognised good practice usually implies a 6 % O2 default content in dry gases which is the equivalent of approximately 3 to 3.5 % in wet gases).