If you want to understand that once-obscure, now infamous pulp byproduct known as black liquor, you can turn to an unlikely source –- the lawyers at the Internal Revenue Service.
In its recent ruling about the water and inorganic content of black liquor (See IRS Ruling Helps Pulp Makers Keep Black Liquor Billions), the IRS’ Office of Chief Counsel presented a pretty clear explanation of how black liquor is created, what it contains, and how it is used in the kraft-pulping process.
The ruling's reference to mixing in diesel fuel is not a usual part of the process but was a trick U.S. pulp makers used to qualify for alternative fuel mixture credits last year. (For the record, I’ve seen paper-company executives handle plenty of liquor, and I’ve never known them to add diesel to it. Tonic water, maybe; diesel, never.)
Anyway, here’s how the IRS describes the process that resulted in at least $8 billion in tax credits for the pulp and paper industry last year and $23.6 billion in alleged “savings” that helped pay for Obamacare (See ObamaCare's Black Liquor Tab: $23.6 Billion.):
To make black liquor, wood chips from debarked tree logs are pulped by using inorganic pulping chemicals (“white liquor”), process water, heat, and pressure to separate lignin and other components of the wood from the cellulose fibers in the wood chips (“kraft milling process”). The wood chips consist of approximately 50 percent water (“chip water”) and 50 percent wood, by weight. The process water is added at one or more stages during the kraft milling process. White liquor and
process water are necessary to create black liquor.
The resulting weak black liquor (including the lignin, spent chemicals, and water) is an aqueous solution. The weak black liquor is concentrated into heavy black liquor by removing much of the water. The removed water (including both chip water and process water) is generally recycled back into the kraft milling process. After being concentrated, the resulting heavy black liquor has a molasses-like consistency and its chemical composition includes organic matter, inorganic solids, and water. At this stage, heavy black liquor is approximately 60 percent dissolved solids and 40 percent water, by volume. The producer then adds a small amount of diesel fuel to the black liquor to produce an alternative fuel mixture, as described in § 6426(e) of the Code.
In the recovery boiler, the diesel fuel, organic matter, and water are burned away from the alternative fuel mixture, generating steam to produce electrical energy that powers the paper mill. The inorganic solids, however, are not burned away but remain in the recovery boiler as smelt (sometimes called “green liquor”). The chemical transformation that changes the inorganic solids into smelt/green liquor consumes energy in the recovery boiler. Despite this energy consumption, however, burning black liquor results in a net production of energy. Originally, the recovery boilers were explicitly designed to recapture the inorganic solids. Currently, the industry recaptures and reuses more than 95 percent of the inorganic solids. Through additional processing, the chemical composition of the green liquor is changed into white liquor, which in turn is recycled into the pulping process.