Process and system for calcination of high solids kraft paper pulp mill lime mud

Abstract

The invention features methods and systems for calcining wet calcium carbonate lime mud produced in a re-causticizing manufacturing operation, for instance, Kraft pulp mill lime mud (“lime mud”) and converting it to re-burned lime by (a) feeding wet lime mud obtained from a re-causticizing manufacturing process into a flash dryer and then feeding the dried lime mud into a bubbling fluid bed calciner thermally linked by moving media heat transfer (MMHT) utilizing solid particulate media to a circulating fluid bed combustor wherein the MMHT provides heat input for calcination and drying; (b) recycling the media being from said calciner to said combustor wherein said combustor receives mill WWTP sludge, or precipitated lignins, or biomass, or NCGs as fuels to re-heat the solid particulate media; and (c) recovering calcined “soft-burned” lime mud from the fluid bed calciner. Steam and heated boiler feed-water are also generated and exported to the mill's steam distribution and generation system as well as hot process water for use in the mill's boiler house and manufacturing operation. The system for calcining calcium carbonate lime mud produced from a re-causticizing manufacturing operation and converting it to re-burned lime comprises a calciner and a combustor linked by a moving media heat transfer MMHT system or apparatus. The MMHT system or apparatus thermally links separate fluid bed combustion (exothermic) and calcination (endothermic) stages. The system further comprises a flash dryer or spray dryer that utilizes exhausted heat from the calcination stage.

Description

FIELD OF INVENTION[0001]This invention relates generally to an improved process for calcining or “reburning” calcium carbonate “high solids lime mud” as produced in, for instance, a Kraft or other alkali-based paper pulp manufacturing operation and converting it to high-quality re-burned lime while using only low cost biomass and biomass derived waste water treatment plant (WWTP) sludge, non-condensable waste mill gas (NCG), or low cost solid fuels such as coal, petroleum coke, etc. The biomass and biomass related fuels are also “carbon-neutral” with regards to their global environmental impact.BACKGROUND OF INVENTION[0002]The Kraft pulp and paper industry is a major energy consumer, with the majority of that need being met by low cost, carbon-neutral, biomass and biomass related fuels. The conventional lime mud calcination process has, however, not easily been converted to biomass fuels and remains a conspicuous consumer of high cost, greenhouse gas emitting fossil fuels. In the Un...

AI Technical Summary

Benefits of technology

[0033]Additionally, the method may include mixing the wet lime mud feed to the flash dryer with H2O2, to convert sodium sulfide contained in said lime mud into Na2SO4. Aqueous solutions of Na2CO3, or Na2SO4 may also be mixed to change the Na2CO3 / Na2SO4 ratio within the lime mud to a higher melting point. This may be particularly effective to mitigate calciner scaling and fouling and unwanted gaseous emissions. Still further, the method may include injecting the dry lime mud feed to the calciner at the base of the fluidized media bed of the calciner. This may help to maximize dry lime mud particle residence time thereby ensuring thorough calcination.

Problems solved by technology

The conventional lime mud calcination process has, however, not easily been converted to biomass fuels and remains a conspicuous consumer of high cost, greenhouse gas emitting fossil fuels.

It fell into disuse, however, as rotary kiln / LMD technology re-captured the fuel economy lead and FluoSolids installations experienced operability issues and the inability to economically operate at the high unit capacities required by a “world-class” Kraft pulp mill.

Low-cost solid fuels such as raw biomass (tree trimmings, sawdust, etc.), mill WWTP sludge and coal, etc. are not used due to their contaminating ash content.

Wet mill WWTP sludge and raw biomass have the added disadvantage of lower adiabatic flame temperature

Due to technology limitations, attaining future significant fossil fuel consumption / cost reductions in the rotary kiln / LMD calcination process appears difficult.

There is, however, wasted energy within the rotary kiln / LMD calcination process that could be recovered with the proper technical approach.

Such energy efficiency, however, is not possible with the rotary kiln / LMD calcination process since a very large non-variable fuel amount is required to provide the constant endothermic calcination heat-of-reaction enthalpy (at 25° C.) and also heat reaction products (CaO and CO2) to the calcination temperature.

The following kiln pre-heat section, however, has insufficient chain heat transfer ability to absorb available energy from the combustion products and CO2 associated with the aforementioned non-variable fuel component and transfer it into the dried solids entering from the drying zone.

Unfortunately, the current rotary kiln / LMD technology is limited in the ability to economically respond to this fuel saving opportunity and will become less fuel-efficient as filter cake solids content further increases.

The less utilized fluidized bed calcination process never had a solids pre-heat section, and wastefully dissipated this excess heat via a water spray cooler to control lime mud flash dryer exit temperature.

Designs have been proposed to address this dilemma by insertion of a waste heat boiler in place of the spray cooler step, but this approach will likely not be commercialized because of the high surface fouling characteristics of calciner exit gas caused by the presence of low eutectic melting point Na2CO3 / Na2SO4 mixtures.

Many of these emissions have been reduced or eliminated thanks to better manufacturing practices but WWTP sludge (cellulosic, organic, and inorganic matter from waste water treatment) remains a costly disposal problem since it must ultimately be placed in a landfill.

As previously discussed, WWTP sludge cannot be used in existing rotary kiln representing a lost opportunity to conserve fossil fuels.

While NCG combustion in rotary kilns has been widely practiced there are operability problems (kiln deposit “ringing”, SO2 “blow-through”, etc.) that persist at most mills.

These incinerator / boilers, however, are not always available when NCGs are produced so a back-up disposal means is desirable.

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