Biomass Feasibility Project Final Report - Xcel Energy

The industry’s most strenuous challenge, however, comes from Wall Street. Investors consider the
paper industry a poor performer relative to its huge capital assets in timberlands and mills.
Faced with shareholder demands to liquidate assets, paper companies have had to choose the
real estate business or the manufacturing business and so a great round of selling off timberland
and plants is under way. Some companies, like Potlatch – formerly Minnesota’s largest forest
products manufacturer – have sold mills and become real estate investment trusts. Others are
doing the opposite: selling timberland and buying mills. Others are selling both. International
Paper, the country’s largest paper company, sold its Minnesota mill and two Wisconsin mills in
June, 2006, along with 12,000 acres of Minnesota land planted in hybrid poplars. The former
owner of a mill in International Falls, Boise Cascade, left the paper industry altogether and
bought the retail chain Office Max.
Biomass Power in Pulp Mills
Pulp mills fall into two distinct categories: chemical (or kraft) or mechanical (or groundwood).
Chemical mills use a chemical process to separate cellulose for paper from lignin, the organic
glue that holds wood fibers together. The lignin-rich liquid residue of the pulping process, “black
liquor,” is burned in recovery boilers (sometimes called Tomlinson boilers) to make steam and
retrieve pulping chemicals for re-use. Steam from these recovery boilers is expanded through
turbines to generate electricity before it is piped on to serve mill processes.
Recovery boilers in chemical mills currently used have low thermal efficiencies of just over 10%.
Research into black liquor gasification techniques has produced new technologies that raise
thermal efficiencies to 20 to 30%, and gasification also can recover processing chemicals more
efficiently.
Mechanical mills differ from chemical mills in that they don’t separate lignin from pulp. Rather,
they defibrate wood mechanically by steaming and grinding it. Some mills add chemicals in a
thermo-chemical-mechanical-pulping process (TCMP), but that isn’t to be confused with
chemical (or kraft) pulping. Unlike chemical mills, even TCMP mechanical mills leave lignin in the
wood. Therefore they don’t produce black liquor to use as fuel.
Chemical and mechanical mills are alike, however, in their treatment of bark and wood wastes.
They burn them in simple direct-combustion boilers to make steam for plant processes and
turbine generators.
Potential for Increased Power Generation in Pulp and Paper Mills
The problems of the paper industry may discourage some companies from making large
investments in new energy technologies, but on the other hand they may drive some
companies to exploit them. Some paper companies reportedly are studying new energy
technologies, like fluidized-bed combustion, gasification, and biorefining, that could transform
mills from net importers of electricity into net exporters. With biorefining or cellulosic
technologies, they could even make transport fuels from their wastes. There are technology
companies seeking partnerships with paper mills to do exactly that.
The time is right. Simple direct combustion boilers burning black liquor and wood wastes have
proven to be reliable and durable for many decades, but their time is running out. 67 boilers
currently operating in American pulp and paper mills were built before 1960, eight of them
before 1931 (NREL, 2006). Those old boilers now are reaching retirement age. The majority of
recovery boilers will end their 30- to 40-year lifetimes over the next 10 to 20 years, and many
wood waste boilers are older than that. As boilers age and power costs increase, mills have
Identifying Effective Biomass Strategies: Page 75
Quantifying Minnesota’s Resources and Evaluating Future Opportunities