Siloxy -- Opportunities -- Wheat Straw

an Air Products and NORAM joint venture company


	Cotton Linter Rice Straw Wheat Straw Bagasse

Nonwood Pulp Mills – Wheat Straw

Background

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 possible, to reprocess the lime. The mud is typically taken to landfills and replaced with purchased lime.

Mills without a Recovery System

Opportunities

The combination of Siloxy desilicatin and oxidation processes provides a greater return on investment than the use of a recovery boiler. The Siloxy system has a much higher thermal efficiency and makes the use of a lime kiln feasible.

The Siloxy desilication process uses carbon dioxide (CO2) from the oxidation process to remove the silica from the weak black liquor. The desilicated weak liquor is concentrated in the multiple-effect evaporators and is then sent to the Siloxy oxidation process where it is combusted with high-purity oxygen. The energy released in the combustion process is recovered as steam, which can be used in the mill and in the multiple effect evaporators. The sodium in the black liquor is recovered as a sodium carbonate solution (green liquor), which is causticized.

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.

Benefits

Environmental

	Eliminates the black liquor BOD and COD load to the wastewater treatment plant.

	Significantly lowers the discharge of sodium into the effluent receiving water

	Lowers the release of volatile organic compounds into the atmosphere from the aerobic wastewater treatment system.

	Substantially lowers the consumption of water and the amount of effluent by recycling condensate from the multiple-effect evaporator system.

	Lowers greenhouse gases emissions by converting biomass (i.e., black liquor) energy into steam and by reducing the electrical power needed to produce sodium hydroxide.

	Lowers SOx, NOx, and particulate emissions by displacing steam production from fossil fuels.

Economic

	Substantially lowers the purchase of costly sodium hydroxide.

	Lowers the consumption of fossil fuel (e.g., coal).

	Substantially lowers the power requirements of the wastewater treatment system.

	Reduces the capital cost (size) of the wastewater treatment plant (new installations).

Mills with an existing Recovery System

Opportunities

The Siloxy desilication and oxidation processes can be used to improve the economic and environmental performance at wheat straw mills with an existing recovery system. 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.

A portion of the concentrated black liquor from the multiple-effect evaporators is sent to the Siloxy oxidation process where it is combusted with high-purity oxygen to produce the CO2 for the desilication step. The energy released in the combustion process is recovered as steam, which is used in the mill and in the multiple-effect 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.

Benefits

Environmental

	Virtually eliminates the cost to landfill lime mud.*

	Lowers greenhouse gases emissions by increasing the thermal efficiency of the existing recovery boiler.

Economic

	Lowers system down time and maintenance by improving evaporator and recovery boiler operation.

	Lowers the consumption of fossil fuel by increasing the thermal efficiency of the recovery boiler.

	Substantially lowers the purchase of "fresh" lime.*

	Virtually eliminates the cost to landfill lime mud.*

*May require the installation of a lime kiln.