(170b) New Process for Recovery of Renewable Energy and Chemicals from Black Liquor – A Review

In the pulp and paper industry, large quantities of forest biomass are being used.  The by-products or residues which result include black liquor, bark, and forest logging residues. These can be used for energy purpose to produce electricity, heat, steam, and biofuels. The kraft pulping process accounts for almost 60% of all pulp production. Black liquor from kraft process represents a potential energy source of 250-500 MW per mill. This paper describes an exciting emerging technology that uses low temperature steam re-forming instead of traditional combustion to recover energy and chemicals from black liquor boosts high thermal efficiency, low emissions (including particulate matters and odorous TRS gases), and low power consumption.  This versatile process, a very good example of pollution prevention at the source through process changes, is now available for commercial scale processing of black liquors generate at the pulp and paper industry.  The black liquor steam reforming process utilizes indirect heating of a steam fluidized bed of sodium carbonate solids to recover cooking chemical and energy.  Black liquor is sprayed directly into the bed where the liquor droplets uniformly coat the bed solids, resulting in high rates of heating, pyrolysis and steam reforming.  The process steam reforms black liquor in the absence of oxygen, producing a hydrogen-rich, medium-Btu gas with a heating value of 300-350 Btu/SCF.  Bed temperatures are maintained at 1060 to 1150 ⁰F (565-620 ⁰C), thereby avoiding liquid smelt formation and the associated smelt-water explosion hazards.  In this dry (no smelt) recovery process, the sodium sulfate in the liquor is reduced by reaction with the steam-reforming products, principally carbon monoxide and hydrogen.  The reduced sulfur form is unstable in the reformer environment and decomposes rapidly to gaseous hydrogen sulfite (H₂S) and solid sodium carbonate (Na₂CO₃).  The product gas is scrubbed with a solution derived from the sodium carbonate bed solids to produce green liquor for re-use in the pulping cycle.

Heat required to reach operating temperature and for the endothermic steam-reforming reactions is supplied by heat exchangers immersed in the fluidized bed.  The heat exchangers consist of bundles of pulsed heater resonance tubes.  A portion of the product gas is burned in the pulsed heaters to supply the necessary heat, thus making the steam reformer self-sufficient on its own fuel.  Pulsations in the resonance tubes produce a gas-side heat transfer coefficient which is up to five times greater than a conventional fire-tube heater.  This efficient heat transfer reduces the size and cost of the heat exchangers and reformer vessel.  The hot combustion gases leaving the pulsed heaters are sent to a waste heat boiler to generate steam and to preheat the pulsed heater combustion air.  The product gas from the reformer is passed through a cyclone to remove particulate matter, then cooled in a waste heat steam generator, scrubbed and quenched.

By separating the sodium and sulfur elements, this process provides an imaginative and chemically attractive alternative to conventional kraft recovery.  The process separates the individual unit operations of the conventional recovery boiler and allows independent control of each operation under its own optimized conditions, producing a well-controlled, more efficient modular process plant.  It is applicable to both incremental and full-scale production facilities.  The process also provides attractive energy alternatives, since the product gas is compatible with combined-cycle cogeneration operations.