Causticizing for Black Liquor Gasifiers

Abstract

The cost-benefit outlook of black liquor gasification (BLG) could be greatly improved if the smelt causticization step could be achieved in situ during the gasification step. Or, at a minimum, the increase in causticizing load associated with BLG could be mitigated. A number of chemistries have been proven successful during black liquor combustion. In this project, three in situ causticizing processes (titanate, manganate, and borate) were evaluated under conditions suitable for high temperature entrained flow BLG, and low temperature steam reforming of black liquor. The evaluation included both thermodynamic modeling and lab experimentation. Titanate and manganate were tested for complete direct causticizing (to thus eliminate the lime cycle), and borates were evaluated for partial causticizing (to mitigate the load increase associated with BLG). Criteria included high carbonate conversion, corresponding hydroxide recovery upon hydrolysis, non process element (NPE) removal, and economics. Of the six cases (three chemistries at two BLG conditions), only two were found to be industrially viable: titanates for complete causticizing during high temperature BLG, and borates for partial causticizing during high temperature BLG. These two cases were evaluated for integration into a gasification-based recovery island. The Larsen [28] BLG cost-benefit study was used as a reference case for economic forecasting (i.e. a 1500 tpd pulp mill using BLG and upgrading the lime cycle). By comparison, using the titanate direct causticizing process yielded a net present value (NPV) of $25M over the NPV of BLG with conventional lime cycle. Using the existing lime cycle plus borate autocausticizing for extra capacity yielded a NPV of $16M.

Direct Causticizing for Black Liquor Gasification in a Circulating Fluidized Bed

“…In some cases, a small fraction of the amount expected was found. We know that under conditions similar to those of Table 3, but at 950°C, causticizing was achieved [9]. We can reasonably assume that at 850°C the causticizing kinetics are slower than at 950°C, and perhaps the short time in the PEFR (1.5 to 5 seconds in Table 3) was not enough for Reaction (1) to take place.…”

“…The NaOH and Na 2 S (along with some unconverted Na 2 CO 3 ) leave the lime cycle as fresh white liquor for the pulping operation. However if black liquor is instead gasified, a portion of the sulfur leaves with the gas stream as H 2 S and COS [9,19]. While this is good for making specialty pulping liquors, it does leave excess sodium which leads to additional Na 2 CO 3 , which adds to the recausticizing load.…”

“…Several studies have shown they are and do [2,3,6,7,[11][12][13][14][15]. Previous work by the authors [9] has evaluated the chemistry at high temperature entrained flow conditions (i.e. 5 seconds at 950C), and at low temperature steam reforming conditions (i.e.…”

“…In this study we seek to overcome the pressure/CO 2 restriction and achieve direct causticizing at 20 bar. pressure [9]. The conversion of carbonate (i.e.…”

“…NPV and IRR as a Function of Fuel Cost 21 D8. Annual Chemicals, Fuel Oil, and Electricity Usages [9] Table D9. Annualized O&M Costs 44 Table D10.…”