Heat Recovery in Thermo-mechanical Pulping

A substantial amount of steam is produced as a by-product of thermo-mechanical pulping – amounts reaching 0.8 to 0.9 t/t-pulp, or around 2/3 of the power input to the refiner (NEDO, 2008. p. 166). This low pressure steam is usually contaminated but its heat content can be reclaimed for use in other processes. Heat recovery options include:

  • mechanical vapor recompression for integrated mills, where the generated clean steam can be used for drying in the paper machine;
  • direct contact heat exchangers for generating hot water for use in paper machines and as boiler maker-up water;
  • reboilers for producing clean process steam;
  • other devices such as thermo vapor recompression and heat pump systems.

TMP heat recovery is applicable to any mill that uses pressurized refining and currently does not use heat recovery – which usually means older mills, because most modern TMP mills are designed with heat recovery systems ((Kramer et al., 2009. p.99).

Typically, between 1.7 to 2.5 MWh/t of energy is used in refiner main lines. The theoretical maximum values for recoverable energy flows from this is given in the table below:

Typical theoretical maximum heat recovery from total input power in refiner main line (BREF, 2010. p. 461)
Source of Energy Recovery Single Disc and Conical Disc Refiners (as %) Twin and Double Disc Refiners (as %)
High pressure steam recovery 61 79
High pressure steam condensate 11 15
Low pressure heat recovery 20 Not applicable
Low pressure condensate 2 Not applicable
Irrecoverable heat (e.g. due to low value heat in fiber and water, motor cooling losses, radiation losses) 6 6

 

Development Status Products
Commercial

Heat Recovery in Thermo-mechanical PulpingCosts & Benefits

Parent Process: Mechanical Pulping
Energy Savings Potential

One study estimates heat recovery systems for pressurized refiners can generate 1.1 to 1.9 tons of clean steam at dryer pressure per ton of pulp (Kramer et al., 2009. p.99).

Japanese flag For a 150 tpd plant, recovery of 695 GJ of energy in the form of steam and 1175 GJ of energy in the form of hot water is reported (NEDO, 2008. p.166)

CO2 Emission Reduction Potential
Costs

Average installation costs have been estimated to be $21/t-pulp (2000 dollars), with significant increases in operation and maintenance costs.
Payback periods vary widely depending on capital costs, but can be as low as a few months (Kramer et al., 2009. p.99)
The payback time for the investment is usually around 1 year (BREF, 2010. p. 462)

Heat Recovery in Thermo-mechanical PulpingSchematic

Heat Recovery in Thermo-mechanical Pulping Publications

Page Number: 

99

Global Warming Countermeasures: Japanese Technologies for Energy Savings / GHG Emissions Reduction

This revised 2008 version of the publication from New Energy and Industrial Technology Development of Japan includes information on innovative Japanese technologies for energy efficiency and for the reduction of COemissions.  

Page Number: 

166