the heat balance

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).