Silica is present in wheat straw. When wheat straw is pulped, the silica is dissolved into the black liquor. If the black liquor is concentrated in a multiple-effect evaporation system, some of the silica precipitates and deposits onto the evaporator heating surfaces, rapidly decreasing their capacity. The presence of silica in black liquor can also cause high black liquor viscosity, which limits the solids concentration that can be achieved during evaporation. As a result, many mills do not concentrate and burn the liquor. They simply discharge it to a wastewater treatment facility.
Mills that do concentrate their liquor used a "Chemical Recovery Unit" (CRU), commonly called a "recovery boiler," to combust the black liquor, produce steam and recover cooking chemicals. The low solids/high water content of the black liquor severely reduces the thermal efficiency of the CRU. The steam generation efficiency is typically less than 55%. In many cases, fossil fuels must be co-fired with the black liquor to sustain combustion.
The recovery boiler produces green liquor, a solution of sodium carbonate in water which is causticized with fresh lime (CaO) to produce white liquor, a solution of sodium hydroxide in water. In the causticizing process, the fresh lime is converted to lime mud (CaCO3). The presence of silica in the green liquor causes "settling" problems within the causticizing plant.
The lime which has been converted to lime mud (CaCO3) would normally be reprocessed into fresh lime by "reburning" it in a lime kiln. At a wheat straw mill, the high silica content in the lime mud makes it very difficult, if not impossible, to reprocess the lime. The mud is typically taken to landfills and replaced with purchased lime.
Mills with an existing recovery system can use a combination of the Siloxy desilication process and CO2 capture technology to improve the economic and environmental performance at wheat straw mills. The desilication process uses carbon dioxide (CO2) to remove the silica from the weak black liquor (i.e., before the multiple-effect evaporator). Removal of silica reduces scaling in the multiple-effect evaporators and allows for an increase in the solids content of the black liquor from the evaporators.
The lower silica content and the lower viscosity of the black liquor make it possible to increase the solids content of the liquor from the evaporators. The higher black liquor solids content translates into an increased recovery boiler thermal efficiency. The low silica content of the green liquor reduces settling problems in the causticizing process and makes possible the reburning and reuse of the lime mud.
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