Digester Blow/Flash Heat Recovery

In chemical pulping process, steam is produced when hot pulp and cooking liquor is reduced to atmospheric pressure at the end of the cooking cycle. In batch digesters, steam is typically stored as hot water in an accumulator tank. In continuous digesters, extracted black liquor flows to a tank where it is flashed. Recovered heat from these processes can be used in other facility applications, such as chip pre-steaming, facility water heating, or black liquor evaporation.

The black liquor that is flashed in stages from continuous digesters can be used in two ways for chip steaming. Flash vapor from the first stage is normally used to heat the chips in the steaming vessel, while the flash vapor of the second stage can be used instead of live steam in the chip bin.

US flag U.S. regulations state that the vent from the chip bin has to be collected and treated if flash steam is used for chip preheating. 

For black liquor evaporation, flash steam from batch digester blow or black liquor flash from a continuous digester can used for thermal energy in a multi-stage evaporator. This thermal energy will offset the need for steam generated by a boiler for black liquor evaporation (Kramer et al., 2009. p.91-92).

Development Status Products
Commercial

Digester Blow/Flash Heat RecoveryCosts & Benefits

Parent Process: Chemical Pulping
Energy Savings Potential

In one study, for a plant producing more than 650 000 tons of paper, improved heat recovery has been found to have the potential to save 991,700 GJ of of fuel and 743,775 GJ of steam, annually (US DOE, 2003)

CO2 Emission Reduction Potential
Costs

The investment and annual savings for the blow hear recovery system are estimated to be $2 250 000 and $2 235 000, respectively – giving a payback time less than one year (US DOE, 2003)

Digester Blow/Flash Heat RecoverySchematic

Digester Blow/Flash Heat Recovery Publications

Page Number: 

91-92

Digester Blow/Flash Heat Recovery Case Studies

Forest Products - BestPractices Plant-Wide Assessment Case Study: Georgia-Pacific: Crossett Mill Identifies Heat Recovery Projects and Operational Improvements that May Save $9.6 Million Annually

This case study summary outlines the energy efficiency improvement potentials identified in Georgia-Pacific Crossett Paper Operations, which produces more than 650,000 tons of printing paper, board, and tissue products each year.