Joining dots to create a circle: Recovering and Reusing Secondary Heat Across Pulp Mill Islands

WORKING ACROSS BORDERS TO OBTAIN ADDED BENEFITS

“When dealing with secondary heat utilization in pulp mills, the focus up until now has been very much on the individual islands, for instance, cooking, evaporation or drying,” says Jussi Piira, ANDRITZ Director, Sustainable Solutions. “However, at ANDRITZ we have been stepping back and taking a more holistic view and working on solutions that cross the borders of the process islands and identifying any secondary heat that can be utilized in another process on another island.”

“This has been quite difficult to explain to customers in the past as they tend to be focused on one area in particular,” adds Paavo Tolonen, ANDRITZ Vice President, Global Product Group Manager, Cooking. "So, it's when we zoom out and take a wider look at the mill, a bigger picture emerges of where secondary heat opportunities can be identified across the process islands."

Basically, the idea is that relatively small-sized equipment is installed around the mill’s “receiving” process islands that can convert secondary heat to generate clean steam for the process without having to use valuable boiler generated steam. This heat is pumped to the process island from the heat exhaust of the “supplying” process island – the one generating secondary heat.

Several benefits are obtained from this heat recovery, including savings in energy, recovery of clean condensates, optimized water, and secondary condensate utilization. Secondary heat can also be recovered from effluent and other sources.

As well as environmental benefits, there are also financial rewards. “The cost benefits come when we return the primary condensate to the boiler house and we don’t need to create boiler water from raw boiler water from raw water,” says Aki Muhli, ANDRITZ Technology Manager, Cooking. “In this way we save energy, raw water and demineralization costs, which can add up to substantial amounts of retained revenue.

“Furthermore, the maximum utilization of raw water is becoming increasingly important as we have seen with record high temperatures and drought being experienced across Europe and other parts of the globe recently.”

MINIMIZED ENVIRONMENTAL IMPACT – MAXIMUM SAFETY

ANDRITZ has developed several solutions to utilize secondary heat across fiberlines at pulp mills by closing loops across the following processes.

DIRECT STEAM REPLACER

The direct steam replacer enables reduced boiler and raw water usage. In this case, clean low-pressure steam is generated from condensate. The heat resource is low pressure steam from the boiler.

Benefits of a Direct Steam Replacer: reduced boiler water consumption, less boiler water to be heated, increased clean condensate recovery and less condensate to wastewater handling.

Highlights:

• Clean condensate from fresh steam is returned to the boiler water.
• Secondary condensates or other suitable clean water streams are used as a steam source for direct steam applications.

Patented Direct Steam Replacer (DSR) process produces steam from mill condensate.

VAPOR REBOILER

Clean condensate is converted to fresh steam using the flash stream as a heat source.

Benefits of a Vapor Reboiler in continuous cooking:

Improved health and safety, decreased evaporation load, and high turpentine recovery.

Benefits of a Vapor Reboiler in batch cooking:

Reduced consumption due to secondary heat recovery, improved yield, reduced shive amount due to steaming, and higher turpentine yield.

Highlights:

• Health and safety is one of the major benefits with the utilization of the Vapor Reboiler as it reduces or eliminates sulfuric compounds, turpentine and tall oil process based risk when collecting gases from the chip bin.
• Flash steam condenses inside the tubes.
• Falling film of condensate keeps heat transfer surfaces clean from fibers.
• Clean steam generated from secondary condensate is removed through a central pipe upwards.

Vapor reboiler converts clean condensate to fresh steam. The heat resource is flash steam.

DIGESTER EVAPORATOR

Steam to the digester is generated from black liquor using MP steam as a heat source.

Benefits of a Digester Evaporator in continuous cooking:
Higher black liquor dry solids content to evaporation, smaller heating surface and steam consumption in the evaporation plant, and increased return rate for primary condensate.

Benefits of a Digester Evaporator in batch cooking:

Energy savings from reduced steam consumption, increases dry solid content in the black liquor and decreased evaporation load.

Highlights:

• Evaporation of water from weak black liquor from digester upper extraction to be used in digester top for heating.
• Medium pressure secondary vapor from DEvap to digester top replaces direct steam injection to the digester top.
• High dry solids liquor from DEvap increases overall dry solids and decrease black liquor to evaporation.

REACHING SUSTAINABILITY GOALS

Maximizing secondary heat usage at pulp mills is now well within reach of all pulp mills and ANDRITZ customers are already seeing rewards with the various technologies being installed around the world. Whether a greenfield development, or an existing mill, the utilization of secondary heat at pulp mills represents an excellent opportunity to further circularize production across the mill, at the same time as make major financial savings.

Piira concludes, “As environmental regulations get tighter, and pulp producers focus on their sustainability goals, it makes perfect sense to utilize every possible area around the mill where reuse of resources is possible.

“Secondary heat utilization is an excellent solution for our customers to assist them as they aim to reach their sustainably goals at the same time as making major financial savings on energy, water, and raw material usage.”