Internal fuels 

The energy inputs presented in Table 4 and in the life cycle inventories detailed in the Annex refers only to external fuels supplied to the process. However, many papermills also produce their own internal fuels as a by-product of the pulping processes or from the management of waste streams. In particular, the following are the main internal fuels relevant to the papermaking process:
• Black liquor – a by-product from the kraft process when digesting pulpwood into paper pulp. It is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process
• Bark – from the debarking process 
• Biogas and biosludge – from the wastewater treatment process
• Refuse-derived fuel – from the processed rejects from processing of paper for recycling
The consumption of all fuels and energy (purchased fuels and electricity and internal fuels) for each of the corrugated paper grades is summarised in Table 5. This data is for information purposes only and should not be added to the life cycle inventory for each paper grade as it refers to fuels that are generated and consumed within the “black box” of the papermill. The internal fuels will be used for production of heat (in the form of steam) and electricity at the mill.
 

^{Table 5  - External energy inputs and internal fuels consumed for the production of corrugated base papers}