Innovations in Supercritical Fluids - ACS Publications - American

Chapter 30

Supercritical Water Oxidation of Pulp and Paper Mill Sludge as an Alternative to Incineration 1

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Carol A. Blaney , Lixiong Li , Earnest F. Gloyna , and Shafi U. Hossain

Downloaded by SUNY STONY BROOK on October 17, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch030

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Long Range Research and Development, Kimberly-Clark Corporation, 1400 Holcomb Bridge Road, 400-4, Roswell, GA 30076 Environmental and Water Resources Engineering and Separation Research Programs, J. J. Pickle Research Campus, University of Texas, Austin, TX 78758 2

This paper summarizes recent research by Kimberly-Clark Corpora tion and The University of Texas at Austin in a new technology referred to as "Supercritical Water Oxidation" (SCWO), an emission -free and discharge-free alternative to incineration, to treat wastes such as pulp and paper mill sludges. SCWO has been shown to be an effective method to treat pulp and paper mill sludge, converting it to usable by-products of clean water (for reuse in paper mill), carbon dioxide gas, and dioxin-free ash (mostly CaCO , which can be used as paper filler or cement aggregate). The majority of all organics are destroyed, including polychlorinated biphenyls and 99+ % of chlorinated organics such as dioxins and dibenzofurans. Economic studies indicate that a commercial SCWO process can compete on a cost basis with fluidized bed incineration. A new reactor design is discussed which avoids the problem of corrosion. 3

The present work relates to the technical feasibility of the process termed "Supercritical Water Oxidation" in oxidizing pulp and paper mill sludge in water under supercritical conditions in order to destroy all organic material, including all trace chlorinated organics such as polychlorinated biphenyls (PCBs), dioxins (PCDDs) and dibenzofurans (PCDFs). In the context of this report, pulp and paper mill sludge refers to that portion of the papermaking process waste stream that contains cellulosic fibrous fines and debris, inorganic materials such as fillers and clay, and small amounts of residual pulping and/or deinking chemicals, in an aqueous medium. Responsible solid waste stream management, of which industrial wastes such as pulp and paper mill sludge constitute a significant part (approximately 70 million bone dry tons per year of pulp and paper mill sludge is generated worldwide), is becoming a regular theme of modern society. Until recently, landfill has been the disposal method of choice for solid wastes such as paper mill sludge due to economic appeal. However, recent legislative trends indicate mat in the future, landfill sites may become harder to obtain and more expensive to maintain, especially for solid wastes containing real or perceived toxic materials. Increased attention has thus begun to be placed on alternative methods of 0097-6156/95/0608-0444$12.00/0 © 1995 A m e r i c a n Chemical Society

In Innovations in Supercritical Fluids; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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SCWO of Pulp and Paper MOI Sludge445

waste disposal such as recycling, waste reduction at the source, various forms of conventional incineration, bioremediation, and most recently S C W O . In the case of paper mill sludge disposal, conventional incineration is commonly thought to be a feasible alternative to landfill, but as presently practiced, incineration may bring with it potential problems, specifically the issue of releasing reportedly toxic compounds such as P C D D s and P C D F s , whose structures are shown in Figure 1, into the environment via the incinerator off-gas emissions and/or fly ash. Since rotary kiln and fluidized bed incinerators are inherently open systems (open to the atmosphere), there is always a potential for fugitive emissions. It has been reported in the literature that conventional incineration may not always destroy these chlorinated organic compounds, and in fact, under certain conditions incineration systems may actually generate more of these compounds [1] . For example, as described by Acharya et al. [1], as long as the minimum required temperature of 871 °C for the complete combustion of organics is maintained, the ratelimiting step in incinerators is often the diffusion of oxygen to carbon monoxide intermediates to form carbon dioxide. Hence the reaction is mixing limited. Often incinerators do not obtain adequate mixing, and thus organics and chlorinated organics present in the feed may not be completely oxidized.


Incineration.

Another potential problem with incinerators is the possible formation of P C D D s , referred to in the literature as "de novo formation of dioxins", and is defined as follows: "dioxin formation from the pyrolysis of a variety of chemically unrelated precursors including naturally occurring precursors together with a chlorine donor" [1]. Likely precursors include, for example, 2,4,5- trichlorophenol, halogenated aliphatics, benzene in the presence of A1C1 , short chain alkanes with HC1, residual carbon with catalyst, and even C 0 plus catalyst in the presence of chlorine compounds (although to a lesser extent), all of which can and often do occur in incinerator fly ash. 3

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Vogg et al. [2] found for incinerators that, in the presence of air, the optimum temperature for P C D D formation from precursors (such as those listed above) was between 250 and 350°C., and for P C D F formation was between 250 and 4 0 0 ° C . These temperatures can often occur in waste incinerators, principally in the energy recovery boilers and the partial quench associated with certain systems. Further experiments by Vogg et al. [2] confirm that both oxygen and water vapor concentrations affect the formation of P C D D s in incinerators. For example, at 300°C., oxygen concentrations over one mole percent exacerbated P C D D and P C D F formation dramatically. And in experiments where moisture was present, predominantly tetra-, penta-, and hexachloro isomers of P C D D s and P C D F s were formed - whereas in dry experiments the more highly chlorinated isomers (e.g. octa-) of P C D D s and P C D F s tended to be formed instead. Other factors affecting dioxin formation in incinerators are: 1) the presence of residual carbon and inorganic chlorides such as NaCl on the fly ash, 2) the presence of HC1, S 0 and C l , and 3) the presence of catalysts (e.g. C u C l , alkali and alkaline earth chlorides, alkali halides and alkaline halide salts). To prevent dioxin formation in the boiler, it was suggested by Acharya et al. [1J to "Use a two-stage incinerator utilizing a turbulent hightemperature oxidative secondary combustor operating with 2 seconds minimum retention time, followed by a rapid full quench to adiabatic saturation (no boiler)." This implies no heat would be recovered, and the system would be energy intensive (requiring fuel). 2

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The largest known commercial S C W O unit is 5 gallons per minute capacity (Huntsman Chemical unit at Austin Research Laboratories, Austin, Texas, constructed by Eco Waste Technologies). Although this size is still very small, the new and emerging technology of S C W O offers definite environmental benefits over incineration, and will possibly mature into being cost-

Supercritical Water Oxidation (SCWO).


INNOVATIONS IN SUPERCRITICAL FLUIDS

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polychlorinated dibenzo-para-dioxins (PCDDs or "dioxins")


Q\

2,3,7,8-tetrachlorodibenzo-para-dioxin (reportedly the most toxic congener) polychlorinated dibenzofurans (PCDFs, or "dibenzofurans")

\/ Clx-1-8

CI

CI.

2,3,7,8-tetrachlorodibenzofuran (reportedly the most toxic congener)

Figure 1. Chemical structure of PCDDs and PCDFs.

Coiled-tube reactor Air-driven oxygen booster

Electric heater Solids (ash) Product Feed tank (sludge) . Low-

[ois™

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Double-pipe heat exchanger Off-gas product (C0 ) 2

circ. loop

Product storage tank

Product water

High pressure diaphragm pump

Figure 2. Pilot plant schematic diagram.


BLANEY ET AL.

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comparable to 'standard* incineration ('standard' incineration here implies minimal pollution control equipment). In a SCWO process environment, the chemical oxidation takes place in a dense water vapor-type medium. Typically the supercritical water mixture's densities are around 0.3 g/cc, and around two-thirds of the hydrogen bonds are destroyed. Under these conditions the supercritical water acts as a powerful organic solvent which penetrates into polymeric networks, surrounding organic moieties. The oxidation reaction is generally complete in less than about a minute if temperatures are in the range of 500 to 600°C. The application of SCWO to treat pulp or paper mill sludge would comprise bringing the aqueous sludge slurry to conditions near or above the critical point of water (T =374°C., P=218 atm = 3204 psi) in the presence of an oxygen donor such as air or hydrogen peroxide. Typical temperatures are around 500°C., and typical pressures are 230 atm (3400 psi). Products are clean water (may contain HC1), clean ash (e.g. deinking sludge would produce predominantly calcium carbonate ash with residual metals if present) and clean vapor containing the carbon dioxide product and any residual oxygen, along with water vapor. Any nitrogen-containing organic material will produce nitrogen gas (Nj) or N 0; little or virtually no NO is formed due to the low temperatures [3]. SCWO is inherently a closed system, so there is less risk of fugitive emissions. If the pressure containment is breached, runaway reactions are not a threat because a complete loss of pressure halts the oxidation reaction. The exothermic heat of reaction may be used to preheat the feed sludge as well as to generate low-pressure steam for process usage. Clean product water can be neutralized and used as process water in the mill. Carbon dioxide may be collected and sold. Ash by-product could be used as a cement extender or sold as calcium carbonate. (Note: unlike the fly ash from conventional incinerators which contains unoxidized carbon which may include PCDDs, PCDFs, PCBs, and so forth, the by-product ash obtained from SCWO has been found in this research to be substantially free of these organics.) At this point in the discussion, the interested reader may refer to any textbook or review paper on supercritical fluids for a discussion of the theory and properties of supercritical water [4,5,6,7]. C


SCWO of Pulp and Paper Mill Sludge

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Experimental Section The University of Texas at Austin currently owns and operates one of the largest known SCWO pilot facilities (40 gallons per hour capacity, continuous flow system). KimberlyClark Corporation has performed numerous experiments at this facility and has shown that paper mill sludges can be converted (oxidized) to clean water, clean calcium carbonate ash, and clean gas (carbon dioxide and residual oxygen). Experiments were performed on deinking (recycled) sludge. Results for virgin sludges are reported elsewhere by Hossain and Blaney [8]. The effect of temperature on paper mill sludge conversion was investigated, holding pressure and flowrates constant. Test conditions included pressures of 245 atm (3600 psi), flowrates of 20 gallons per hour, and two reaction temperatures of 450 and 500°C. The overall organic destruction was quantified in terms of total organic carbon (TOC). The thoroughness of destruction of trace chlorinated organics was quantified by measuring the trace quantities of PCDDs, PCDFs, and PCBs before and after SCWO, using gas chromatography and high-resolution mass spectrometry [9]. Pilot Plant. A schematic diagram of the 40 gallon/hour SCWO pilot plant at the University of Texas is shown in Figure 2.


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Table I. Composition of Feed Sludge and Product Ash 2

Feed Sludge

Expt. A Product Ash

Expt. Β Product Ash

3400 PPM

Innovations in Supercritical Fluids - ACS Publications - American

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