It may not be operationally efficient to change to larger batch digesters for the smaller mills. Moreover smaller and speciality mills produce variety of pulp that are less suited for continuous digesters. In such cases, several approaches can be considered to reduce energy consumption in batch digesters, such as indirect heating and cold blow.

In indirect heating, cooking liquor is withdrawn from the digester through a center pipe, pumped through an external heat exchanger, and returned into the digester at two separate locations in the vessel, thereby reducing direct steam loads.

In cold blow systems, hot spent pulping liquor is displaced from the digester contents using brownstock washer filtrate at the end of the cooking cycle. Heat is thereby recovered from the spent liquor for heating subsequent cooks, leading to reduced steam requirements for heating the digester contents. Recovered black liquor can be used for preheating and impregnating incoming wood chips or for the heating of other process inputs, such as white liquor or process water (Kramer et al., 2009. p.91).

 In the US, cold blow techniques are regarded technically applicable for all kraft pulping, both for softwood and hardwood. The only negative feature is the cost of complete replacement of the existing digesters. Replacement normally cannot be justified unless a production increase or the condition of the present equipment motivates complete replacement of existing digesters (NCASI, 2001. p.62).