NASA Technical Reports Server (NTRS) 19940025433: Electrochemical incineration of wastes

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by  NASA Technical Reports Server (NTRS)

NASA-CR-I95262
NATIONAL AERONAUTICAL SPACE ADMINISTRATION (NASA)
Johnson Space Center
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FINAL REPORT
ELECTROCHEMICAL INCINERATION OF WASTES
NASA RESEARCH GRANT: NAGW-1779
Prepared and Submitted by:
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Dr. J. O'M. Bockris, Principal Investigator t_j m QO
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Dr. R. C. Bhardwaj, Senior Research Associate Z 0
Dr. C. L. K. Tennakoon, Research Associate
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Surface Electrochemistry Laboratory
Texas A&M University
College Station, Texas 77843
Januaw 1993
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ELECTROCHEMICAL INCINERATION OF WASTES
C. L. K. Tennakoon, R. C. Bhardwaj and J. O'M. Bockris
Surface Electrochemistry Laboratory
Department of Chemistry, Texas A&M University
College Station, Texas 77843
INTRODUCTION
There is an increasing concern regarding the disposal of human wastes in
space vehicles. It is of utmost importance to convert such wastes into harmless
products which can be recycled into an Environmental Life Support System [CELSS]
[I], which incorporates the growth of plants (e.g., Wheat) and algae to
supplement the diet of the astronauts. Chemical treatments have proven
relatively unsatisfactory and tend to be increasingly so with increase of the
mission duration. Similarly, the use of heat to destroy wastes and convert them
to CO 2 by the use of air or oxygen has the disadvantage and difficulty of
dissipating heat in a space environment and to the inevitable presence of oxides
of nitrogen and carbon monoxide in the effluent gases [2,3]. In particular,
electrochemical techniques offer several advantages including low temperatures
which may be used and the absence of a__ NO and CO in the evolved gases.
Workers in our laboratory at Texas A&M University, College Station, have
carried out successful research in the electrochemical oxidation of wastes over
the last several years. The commencement of the project was based on the
pioneering work of Bockris et al. [4] where it was shown that cellulose could be
broken down to CO 2 with a current efficiency of nearly 100% and only 2 Faradays
of electricity were involved in the evolution of i mol of CO2, perhaps because
of the preliminary hydrolysis of cellulose to glucose.
2
Thereafter, one possibility would be the electrochemical oxidation of
glucose for which the overall reaction is:
C6HIe06+ 6 H20 --> 6 CO2 + 24 H_ + 24 e-
However, this would predict 4 Faradays to produce 1 mol of CO2. The
remaining discrepancy between this expectation and the observations of the
original paper wasexplained by postulating that preliminary chemical oxidation
reactions are coupled with the degradation of the glucose unit.
CBHIzsO + 0z --> C5 Hi006 + CO2
arabinic acid
C5HI006 + 0z --> C5 Hs07 + CO2
arabino-tri-hydroxy-glutaric acid
Cs HsO
7 + 3 HzO --> 14 H÷ + 5CO z + 14 e-
According to this scheme 2.8 Faradays will be used per mole of CO 2 evolved, a
value that is much more in accord with the experimental results.
The other contributions in the electrochemical oxidation of wastes includes
the conversion of cattle manure and wood chips into pollution-free effluent
containing no CO or oxides of nitrogen by Dhooge [5]. In this study they have
extensively used different redox couples for example, iron (II)/iron (III);
cobalt (II)/cobalt (III); vanadium (IV)/vanadium (V). The oxidation of urea in
urine electrochemically was reported by Tischer et al. [6].
The systematic study of the oxidation of human wastes commenced with a
fundamental investigation into their composition and the oxidizability of the
components by electrochemical means. In this initial studv, Kaba, Hitchens and
Bockris [7,8] showed that an artificial waste mixture could be prepared having
a similar composition to that of human feces, and that the individual components
comprised of cellulose, oleic acid, casein and microbial biomass consisting of
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Torpulina, could all be electrochemically oxidized on a platinum electrode.
Initial experiments were conducted in 5-12 Msulfuric acid, up to a temperature
of 150°C. The total organic carbon decreased by about 95%in 96 hours of
electrolysis. The electrodes used were mainly platinum and lead dioxide. The
latter electrode was shownto be moreeffective than the former.
The effect of the addition of Ce÷3/C÷e4redox couple was also studied. An
oxidation rate increase in the presence of the redox couple about 40%compared
to that in its absencewas reported [8].
At this stage (1990), it was decided to attempt to exclude the use of
H2S04:andto carry out the oxidation of fecal-urine mixture at temperatures less
than 100°C.The supporting electrolyte available in humanurine is largely NaCI.
Whenthis mixture waselectrolyzed, at about 90°C, 95%consumptionof the fecal-
urine content was obtained in about the same time as that used for the
consumptionof waste mixture in concentrated sulfuric acid at 150°C.
This unexpected increase in rate of oxidation may be due to a partial
hydrolysis of the CI2 nowevolved from the urine.
C12+ H20 --° HOCI + HCI
HOCI--÷ H_ + OCI-
Ahypothetical possibility for the partaking of this ion in the oxidation
of organic materials is:
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-q-G- +0el- --_ -c-c- +Cl-
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The CI- would then migrate to the anode and forms chlorine. A catalytic
cycle is thus implied.
Reactions of this type are likely to be the origin of the decoloration and
deodorization which is so strikingly observed when urine is involved in the
oxidation of the solid wastes (Indeed, HOCI is a well known bleaching agent).
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However, it wasnoted at this early stage that PbO2 coatings are attacked in the
presence of urine presumably forming PbC2I.
Thenext phaseof developmentwasto research a suitable electrode material
for the oxidation of the fecal-urine mixture. This work also beenperformed in
our laboratory by Bhardwaj, Sharmaand Bockris [9]. In this study seven
different electrocatalysts were investigated in the electrochemical oxidation of
urine-feces mixtures. The electrodes studied were Pt, PbO2,gold, graphite,
perovskite, tungsten carbide and Ebonex(Ti407) coated with RuO2. Thecurrent-
voltage curves indicated that the waste can be oxidized in the potential range
of 1.2-1.6 V vs NHE. Using the amounts of CO2 generated during long term
experiments, measuring TOCvalues before and after such experiments and by
comparing mechanical stabilities, it was found that the perovskite
(La0.79Sr0.21M_n)Oand RuO2 coated Ebonexare superior to the other electrodes
investigated, in respect to the rate of completion of the oxidation reaction and
mechanical stability. Thesestudies were performed in
'U' type cells.
In 1991, mass balance studies were carried out whilst still working with
'U' tube cells, using platinum electrodes. H2, 02 , CO 2, N2 and CI2 were all
collected and analyzed in respect to the dry weight of starting and final
material. TOC was also measured before and after electrolysis. (Appendix I).
The relative advantages of working at constant potential and constant current
were examined. Theoretically, it would be best to work at constant potentials,
but it was found that improved results are obtained at constant current density
but increasing potential. This is probably due to increased HOCI generation at
higher potentials and possibly due to the generation of OH radicals.
At this stage, Tennakoon, Bhardwaj and Bockris [i0], commenced work on
devices which would increase the rate of consumption per unit cell volume. At
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Fig 1. PARALLEL PLATE FLOW THROUGH CELL
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WITHOUT FLOW 360 I/hr 816 Ilhr
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Fig 2. HYDRODYNAMIC EFFECT ON CURRENT DENSITY
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this point (May 1991), a consultation was held with Dr N. Weinberg, regarded as
the leading electrochemical engineer of the time. Weinberg has extensive
experience in the design of electrochemical reactors.) His view was that an
optimal design could be achieved by the use of a parallel plate flow-through
cell, as shown in Figure i.
The parallel plate cell used was purchased from Electrocell AB of Sweden.
This cell constructed in a similar manner to a plate and frame filter press,
consisted of a cathode, an anode, a separator and plastic frames into which the
electrolyte is introduced. The cell had an active area of i00 cm 2, an inter-
electrode gap of 0.5 cm. A maximum electrolyte flow rate of about 500 liters per
hour could be established through the inter-electrode gap. The two electrodes
were separated by a "celgard" membrane. Turbulence promoting nets provided by
the manufacturers were used in both anodic and cathodic compartments. Anodes used
were graphite, lead dioxide and ruthenium dioxide coated titanium. The cathode
was a stainless steel plate.
Figure 2 shows the current-voltage curves obtained with this flow cell for
various flow rates. A number of attempts were made to increase the flow rate of
the solution through the inter-electrode gap. A large centrifugal pump from the
Chemical Engineering Department of the University was obtained to give a flow
rate of 512 liters per hour. Even under these conditions, it was only possible
to obtain a three fold increase in current density. As such, the research work
using this parallel plate cell was discontinued at this stage.
As the use of the parallel plate flow through cell was not successful, it
was necessary to investigate the use of other electrodes systems available for
electrochemical processes. The use of three dimensional electrodes was
considered as the next alternative. In recent years considerable attention has
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Fig 3. SCHEMATIC POTENTIAL DISTRIBUTIONS.
(a) PACKED BED ELECTRODE
(b) FLUIDI2:][D BED rtECTRODE
o :e_:esen:s =he _c_en:L_! o_ _he me=a! phase wi_h respec: to an e!ec:rcde
cuu3ide =he _ed; and _ is =he pc_en=ia! in =he so!u_ion, in =he f!uidized bed,
_aruic!es are only s_cradical!v in conuac=. Such sysuems are used in eiecur=-
_inning frzm di!uue so!u:iuns.
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been given to cell designs employing electrodes in the form of small particles
rather than planar or cylindrical sheets. The primary advantage of these designs
is to provide a high ratio of electrode surface area to cell volume. The two
main types are the packed bed and the fluidized bed electrodes. These two
electrodes were simultaneously developed at the universities of Southampton and
Newcastle Upon-Tyne in the United Kingdom, by Fleischmann [11-16] and Goodridge
[17-21], respectively.
Out of the two possible electrode systems, the packed bed was chosen for
our study by considering the potential distributions of the two electrode
systems. Figure 3 shows the schematic potential distributions of packed and
fluidized bed electrodes [22]. It is seen that the potential of the metal phase
remains constant in a packed bed, whereas in a fluidized bed the potential change
along the metal phase is significant. This is due to the fact that the particles
in a fluidized bed are having only an intermittent contact with each other.
The first electrode material tested on a packed bed consisted of graphite
particles of diameter about 0.5-1.0 mm supplied by the Electrosynthesis Company
and it was observed that the electrochemical stability of the particles was poor
as the particles slowly disintegrated with oxygen evolution on the surface. The
next electrode material tested was approximately i mm in diameter, Ebonex
particles, coated with RuO 2. O.IM solution of RuCI 3 in 50% aqueous solution of
ethanol was used for coating the particles. The particles were prepared by
crushing a plate of Ebonex and by subsequent separation using standard sieves.
The particles were then washed in distilled water, etched in I0 % solution of HF
and dried at 120°C. The coating solution was then applied on to the particles
taken in a beaker using a brush. Care was taken to see that all the particles
were wet with the coating solution. The solvent was allowed to evaporate at