Siloxy -- Opportunities -- Bagasse

a NORAM company


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Nonwood Pulp Mills – Bagasse

Background

Bagasse contains silica. When the bagasse is pulped, the silica dissolves into the black liquor by-product stream. 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 also limits the solids concentration that can be achieved during evaporation. As a result, many bagasse mills do not concentrate and burn the liquor, but simply discharge it to a wastewater treatment facility.

Mills that do concentrate their liquor have traditionally used a "Copeland" reactor which has no energy recovery or a "Chemical Recovery Unit" (CRU), commonly called a recovery boiler, which recovers energy as steam and cooking chemicals. In the case of a CRU, the high water content of the black liquor severely reduces the thermal efficiency of the recovery boiler (i.e., less steam is produced). The steam generation efficiency is typically less than 55%.

In a Copeland reactor, the sodium is converted into solid sodium carbonate which needs to be landfilled, or in some cases can be sold. A 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. The high silica content of the lime mud makes reburning difficult. If a lime kiln is employed, a large portion of the lime mud must be landfilled to control the accumulation of silica. If the lime mud is not reprocessed in a kiln, it is landfilled and fresh lime must be purchased.

Mills without a Recovery System

Opportunities

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

The Siloxy desilication process uses 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 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 to produce white liquor.

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

At mills that have a Copeland reactor the Siloxy desilication process can be used to remove the silica from the weak black liquor and the Siloxy oxidation processes would replace the Copeland reactor. At mills with a recovery boiler, the Siloxy desilication process can be used to remove the silica from the weak black liquor and the Siloxy oxidation technology used to produce the CO2 for the desilication process. Removal of silica reduces scaling in the multiple-effect evaporators and decreases the amount of lime mud that must be purged to control silica.

Benefits

Environmental

Reduces the amount of lime mud that must be purged to control silica (mill with a lime kiln).

Economic

	Lowers system downtime and maintenance by improving evaporator and recovery boiler operation

	Lowers the consumption of fossil fuel by increasing the thermal efficiency of the recovery boiler (mills with a CRU).

	Substantially lowers the consumption of fossil fuel by recovering the energy from combustion (mills with a Copeland reactor).

	Substantially reduces the purchase of "fresh" lime.

	Significantly reduces the cost to landfill lime mud (mills with a CRU and lime kiln).