demonstration of plasma assisted waste conversion to gas

49th International Conference on Environmental Systems ICES-2019-128 7-11 July 2019, Boston, Massachusetts

Demonstration of Plasma Assisted Waste Conversion to Gas

Anne Meier, Ph.D.1 and Malay Shah2

National Aeronautics and Space Administration, Kennedy Space Center, FL, 32899, USA

Kenneth Engeling3

University of Michigan, Ann Arbor, MI, 48109-2104, USA

and

Katerina Quinn4

University of South Florida, Tampa, FL, 33620, USA

The National Aeronautic and Space Administration Science Technology Mission

Directorate Center Innovation Project at Kennedy Space Center funded a one year

investigation for the development of a DC plasma torch to test the conversion of solid and

liquid mission waste to gas. The volume reduction of mission waste is needed to advance waste

processing for vent gases on board space vehicles and space habitats for long duration

missions. The selected plasma torch operated with an input voltage of 120 VAC and a plasma

pilot arc power of approximately 400 W using air as a baseline carrier gas. To date, the team

has demonstrated early concepts of plasma assisted waste conversion of the following

materials: cotton hygiene material, astronaut clothing, plastics (i.e. polyethylene and nylon),

astronaut food packaging, paper, fecal waste simulant, and degrading plant matter (i.e.

inedible biomass waste). The reactions took place in a quartz cylindrical test cell, where waste

was loaded into a quartz crucible and monitored with optical video. The initial reactions

included a multi-stage process that was primarily plasma combustion. The reaction product

gas was qualitatively and quantitatively analyzed with a gas chromatograph and Fourier

transform infrared spectroscopy instrument. The initial results of the system show the volume

reduction from solid to gas in the form of useful products such as carbon monoxide, carbon

dioxide, methane and light hydrocarbons. This paper will discuss the project development and

results regarding waste conversion, power performance and volume reduction for a plasma

space waste processing system.

Nomenclature

AC = Alternating current

Al = Aluminum

Ar = Argon

CH4 = Methane

C2H4 = Ethylene

C2H6 = Ethane

CIF = Center Innovation Fund

CO = Carbon monoxide

CO2 = Carbon dioxide

1 Analytical Laboratories Branch Lead, Laboratory, Development & Testing Division, Mail Stop NE-L3. 2 Mechanical Engineer, Engineering Analysis, Mail Stop NE-XY. 3 Ph.D. Candidate, Department of Nuclear Engineering and Radiological Sciences. 4 B.S. Student, Department of Chemical and Biomedical Engineering.

International Conference on Environmental Systems

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COTS = Commercial Off the Shelf

Dry GC = Dry golden cherry tomato biomass

Dry GD = Dry golden dwarf tomato biomass

EVA = Extra vehicular activity

FP = Food Packaging

FTIR = Fourier Transform Infrared Spectroscopy

GC = Gas chromatograph

H2 = Hydrogen

He = Helium

HDPE = High density polyethylene

ISS = International Space Station

JSC = Johnson Space Center

K = Kelvin

Kg = Kilogram

KSC = Kennedy Space Center

kVA = Kilovolt-ampere

LEO = Low earth orbit

MAG = maximum absorbency garment

MSW = Municipal solid waste

N2 = Nitrogen

NASA = National Aeronautics and Space Administration

OES = Optical emission spectroscopy

SBIR = Small Business Innovative Research

SLMP = Standard liters per minute

STMD = Science Technology Mission Directorate

Syngas = Synthetic gas

TCD = Thermal conductivity detector

TGA = Thermal gravimetric analysis

US = United States

VAC = Volts AC

W = Watts

I. Introduction

The National Aeronautics and Space Administration (NASA) Science Technology Mission Directorate (STMD)

Center Innovation Fund (CIF) Project at Kennedy Space Center (KSC) funded a one year investigation for the

development of a reactor utilizing a DC plasma torch to test the conversion of solid and liquid mission waste to gas.

Advanced waste processing technologies are necessary to reduce the overall volume of solid mission waste on a

crewed spacecraft or habitat by turning it into a safe vent gas. Plasma gasification is theoretically the most ideal

technology to convert waste to a synthetic gas (syngas) or inert gas (such as permanent gases), due to the ionization

of waste and a clean product gas stream. Rather than burn, plasma gasification uses high heat from the plasma to

degrade materials to their basic molecular elemental form. The primary gas products of organics after plasma

gasification are usually H2, CO, CH4, and CO2. Inorganics are melted and reduce to an inert slag. Most logistical waste

on the International Space Station (ISS) produced by astronauts is low in inorganic material. A low power plasma

application is key for novelty of the technology development for plasma waste conversion applications, since power

is a major design constraint for long duration travel on board spacecraft and habitation systems. Here we describe

some of the background of space mission waste, plasma and other low power applications, as well as the

experimentation that took place with a DC plasma torch at KSC to convert various space feedstocks into gaseous

products.

Colonization of other planets, terrestrial bodies, or hotels in space have been frequently discussed from commercial

space ventures. On Earth, municipal solid waste management is one of the most imperative services a city can provide

to its residents, which would likely parallel to large-scale colonization in space. The effects of mismanaging waste

processing culminates in higher costs downstream compared to the initial cost of managing waste properly at

inception. Since waste is often tended to at a local government level, developing countries often spend the largest

portion of budget on solid waste management1. According to the United States (US) Environmental Protection Agency


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(EPA), approximately 2.03 kilograms (kg) of municipal solid waste (MSW) are generated per person per day, which

equated to 262.4 million tons in 2015, primarily consisting of paper and plastics2. This MSW value is increasing to

approximately 1.3 billion tons per year and estimated to reach 2.2 billion tons per year by 20251. In the US,

approximately 53% of the MSW is sent to landfills, while 25% is recycled, 13% is combusted for energy recovery,

and 9% is composted. These US statistics do not include the new restrictions of China’s elimination of recycled mixed

plastic programs, which now leaves much of the recycling, formally destined for China, back into US landfills as of

2019. In 2014 China generated approximately 178.6 million tons of MSW3. In general, affluent countries produce

more MSW per capita, and in 2012 some of the highest generation per capita included Switzerland, Denmark, and

Cyprus4. MSW generation has increased from the start of recording the data since 1960 and continues to steadily

increase. MSW includes items such as corrugated boxes, food, yard waste, sofas, computers, tires and refrigerators,

but not everything that is landfilled – such as construction and demolition debris, wastewater sludge or non-hazardous

industrial waste – is reported and captured in the MSW values.

Waste reduction and conversion techniques are not only imperative for Earth based landfill reduction and waste

management, but will also be imperative for sending humans beyond Low Earth Orbit (LEO) for Space Missions,

especially for future colonization. For a crew of four on a 1-year mission, a total of ~2,600 kg of crew-related waste

mass including life support system consumables and crew consumables are estimated to be generated5. Waste will

include items such as clothing, hygiene items, food, food packaging, food storage containers, extra vehicular activity

(EVA) and medical supplies, feces, brine (urine, sweat), and life support system supplies. It has been previously

described how NASA is investing in technologies6 to down select for future outpost waste conversion systems, as well

as scenarios for different waste management schemes at various space destinations7,8. Plasma for waste conversion

has only been recently introduced and considered for this space application9. We still seek to convert waste from solid

to gas for safe venting or as fuel generation in this plasma application.10

Traditional thermal waste processes typically include gasification, incineration, and pyrolysis. Incineration

temperatures can reach up to 1,500 ºC, where a series of ignition, complete oxidation, neutralization, condensing and

ash generation occurs. The primary product of incineration on the industrial level is usually heat generated from the

cooling process that creates steam to generate electricity and is one of the lowest efficiencies in the thermal process at

~35%4. Pyrolysis uses limited or no oxygen and has one of the highest efficiencies of thermal utilization between 80-

90%, but a low gas yield at less than 40% (i.e. using biomass) due to high tar vapor production11. Temperatures can

reach approximately 900 ºC during pyrolysis, with primary products as hydrocarbons, but have the disadvantage of

producing harmful byproduct chemicals and are costly at the industrial scale to clean up. Gasification can range from

600 to 1,100 ºC to produce syngas for electrical and heat energy. The partial oxidation process of gasification must

maintain high temperature to avoid dangerous formation of dioxins, furans, polychlorinated biphenyls (PCBs), and

other catastrophic and harmful compounds that form below the reaction temperature. Plasma has gained more attention

recently in combination with gasification and combustion as it can destruct solids into syngas products, while forming

the inert slag material.

Plasma is sometimes referred to as the fourth state of matter with two general classifications: non-thermal and

thermal plasma. Plasma consists of electrons, excited atoms, ions, radicals, photons, and neutral particles. Non-thermal

(non-equilibrium) plasma contains electrons with higher energy than the surrounding particles whereas the

temperature of thermal plasma is near unity12. High temperature, thermal plasmas are typically utilized in solid waste

conversion in a single-stage or two-stage system. The plasma components of the single-stage system includes the

plasma torch and reactor chamber, while a two-stage system employs a secondary plasma treatment after the waste

gasification. The second stage is to finalize conversion of the crude gas products. North America, Europe, and Asia

all have plants utilizing thermal treatment facilities that process waste on a municipal level, with facilities that were

commissioned as early as 199713.

Plasma treatment has been applied to space applications. For example, Marshall Spaceflight Center utilized the

Small Business Innovative Research (SBIR) contracts to develop a microwave plasma pyrolysis application to recover

H2 from the ISS Carbon Dioxide Reduction Assembly that vents CH4, and to also assist in removal of accumulated

carbon and carbonaceous build-up using CO2 gas with the plasma system14. The system demonstrated a single-pass

CH4 conversion efficiency of over 90%, but only ~17% chemical energy efficiency for CH4 to acetylene and H2. The

microwave plasma application was last reported to be ongoing for continued testing for both CH4 processing and CO2

plasma cleaning to remove carbonaceous material due to favorable results. Laboratory and municipal scale plasma

systems for waste conversion applications are in use but still require technology development for power and thermal

optimization. In China, thermal plasma waste conversion technology has been in use since the 1980’s, beginning with

smelting applications and a three face alternating current (AC) plasma torch. Designs shifted in the early 2000’s to

DC torches for plume length and stability. One DC torch utilized conversion of plastics where the primary products


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were H2, CO, acetylene, ethane (C2H6), ethylene (C2H4), and solid yield. The solids declined when introducing steam

as an effective medium to produce more gas. The carrier gas in the DC system was N2 and electric power input was

between 26.4-62.5 kVA11. RF torches have been investigated with polymer waste via plasma pyrolysis for gas

productions of H2 and CO. The resulting carbon char was 85% carbon that can be used for reinforcing carbon black13.

CH4 to methanol has been successfully reported with plasma using varying reactor designs15. A thermal plasma reactor

achieved single step, non-catalytic selective synthesis of methanol likely due to the dielectric barrier discharge in the

reactor with selectivity of methanol ranging from 40-64%. RF torches have also been demonstrated for biomass

pyrolysis for improvements in thermal energy savings and produced gas mixtures of H2, CO, CH4, CO2, and light

hydrocarbons, which are similar to the products produced in DC thermal plasma11. With such promise on the

laboratory scale for Earth applications, we take plasma conversion and apply it here for space waste conversion

applications.

II. Experimental Design

A. Process Design

The primary goal of this

work was to generate a

laboratory scale proof-of-

concept for DC plasma waste

conversion, starting with

single stream mission waste

materials and increasing to

mixed waste streams. The

experimental flow diagram of

the plasma conversion system

is displayed in Figure 1. The

plasma torch feed gas was air

which entered the plasma

torch where the plume was

generated; waste conversion reactions occurred in a test cell located immediately below the plasma plume. The gas

flow was regulated with an MKS Type 247 4-channel readout connected to a MKS mass flow controller and had a

nominal flow setting of 5 standard liters per minute (SLPM). The effluent gases that resulted from the plasma reactions

continued through a heat exchanger, where water or other condensables (if any) were collected in a vessel. The

remaining room temperature gas passed through a Thermo Fisher Scientific Nicolet iS50 Fourier Transform Infrared

Spectroscopy (FTIR) instrument, equipped with a PIKE Technologies Short-Path Gas Cell. After the FTIR, the gas

was collected and analyzed in a three channel INFICON Micro Gas Chromatograph (GC) Fusion gas analyzer with a

thermal conductivity detector (TCD). The GC data was collected in a tedlar bag that was attached to the output of the

system. The tedlar bag was opened during the plasma reaction and then closed and run in the GC immediately

following the tests. It is important to note that there is significant air dilution for the current reported gas data. All

remaining gases were vented to the laboratory fume hood. The GC column characteristics are listed in Table 1.

Temperatures (T1-T5) were measured with surface probes and in-line K-type thermocouples utilizing a multi-channel

Extech thermometer logger.

Optical emission spectroscopy (OES) measurements were made using a Photon Control Spectrometer SPM-002

coupled with a 200 µm core fiber optic cable that was 2 meters in length. The data was collected using the SpecSoft

V4.0 for Photon Control spectrometers software. The fiber optic was located against the quartz cylindrical housing

and an exposure time of 1 second was set to collect data. Plume characteristics such as gas and electron temperature

were diagnosed using the OES techniques. Visual observations were recorded with a GoPro Hero 6 camera. The video

was used to observe differences in the combustion and gasification process depending upon the selected material.

Table 1. GC Column Characteristics

Module A: Carrier Gas: Ar 99.997%

Column Type: Backflush 1.0 ul, Rt-Molsieve 5 A, 0.25 mm (10m) [Rt-Q-BOND (3m)] TCD2;

Inject time: 30 ms, backflush: 10 s. Heaters: 80 ºC, Pressure: 30 psi, Ramp Temp: 0 sec to 90 ºC, hold for 120 s.

Module B: Carrier Gas: He 99.9999%

Figure 1. Schematic of plasma reactor system.


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Column Type: Backflush 1.0 ul, Rt-Molsieve 5 A, 0.25 mm Injector: Inject time: 0 ms, Backflush: 10 s.

Inject time: 30 ms, backflush: 10 s. Heaters: 80 ºC, Pressure: 30 psi, Ramp Temp: 0 sec to 90 ºC, hold for 120 s.

Module C: Carrier Gas: He 99.9999%

Column Type: Variable volume, Rt-Q-Bond, 0.25mm (12m), TCD2

Inject time: 30 ms, heaters: 80 ºC, pressure: 22 psi, Ramp Temp: At Run time: 0s: 65 s, hold time 50 s; At Run time: 50 s: Ramp rate: 1 ºC/s,

135 s, hold time 0 s.

B. Plasma Torch and Test Cell

DC plasma torches are commonly used in academic and scientific plasma research but each torch is operated under

various conditions. The differing operating parameters lead to changes in plasma characteristics. Therefore it was

important to characterize the plasma and the equipment settings. The selected plasma torch was the Plasma Pilot PD-

E-051 from Applied Plasma Technologies, Corporation. The plasma torch operated with an input voltage of 120 VAC

and a plasma pilot arc with power ranging from 50 to 400 W using compressed air as a baseline carrier gas. The torch

had standard fittings to plug into a high voltage terminal line and carrier gas input. The commercial-off-the-shelf

(COTS) parts enabled minimal engineering design and machining for test cell set up. The carrier gas flow may be

varied on this torch, but the standard run parameter was air at 5 SLPM and operated in a continuous high voltage

glow-to-spark discharge. Although 5 SLPM is a high flow rate for considered scale up, at this time the entire system

is in its early stages, and not optimized fully for power and

throughput considerations in depth. The torch data

acquisition was collected with a COTS power supply

(DAT210) featuring constant power mode and compatible

PlasmaView software from Plasma Technics,

Incorporated®. The system was kept in a constant power

output feature which was determined to be ~ 200 W with the

5 SLPM air flow. All waste conversion reactions occurred in

a clear quartz cylindrical test cell with a stainless steel

housing as displayed in Figure 2. The test cell had an inner

diameter of 32 mm and was 35 mm tall. The quartz test cell

was selected for its high temperature properties, and allowed

for observing reactions with the video. The quartz material is

not ideal for insulating the system or for higher temperature

gasification operations. For each experiment, waste was

loaded into the 10 mL quartz crucible with a layer of quartz

wool below the crucible to enable air flow through the

system, and monitored with the video camera. All waste

conversion experiments were run in triplicate, with minimal

variability excluding the high fidelity waste simulant.

C. Waste Simulants and Characterization

The materials studied with this test cell, along with its

starting sample mass, are listed in Table 2 and include items

found as logistical waste on a space mission which have been

previously described in detail regarding composition and

ingredients16. Most waste materials here include the general

majority of mission logistical waste. Food packaging (FP)

was actual space mission food packaging obtained from the

food production laboratory at Johnson Space Center (JSC).

FP contains several layers of polymers and a metallic

(aluminum) layer. The maximum absorbency garments

(MAGs) simulate crew diapers worn on extravehicular

activities and consist of absorptive salts and cotton. In this

series of experiments, a COTS adult diaper was used which

contains sodium polyacrylate as the adsorptive salt. The

MAG experiments were tested in a single waste stream,

Figure 2. Plasma torch and test cell. Left:

Laboratory hardware and experimental set up;

Right: Computer aided drawing of the torch and

test cell.

Table 2. Waste simulants used in experiments and

corresponding mass loading into the quartz

crucible. HDPE = high density polyethylene. FP

= Food packaging, Inedible biomass: Dry GC and

GD.

Waste simulant Starting Mass, g

Cotton (hygiene wipes) 0.50

HDPE 0.50

Nylon 0.50

FP 0.50

Foam 0.25

MAG 0.50

Inedible biomass 0.50

Urine 1.0

Food 2.0

Fecal 2.0

HFWS 1.5


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rather than separating the salt and cotton components. The foam material is a proprietary closed cell foam used to pack

and store logistical materials and science experiments safely during launch from Earth. The foam material was

obtained from the JSC Logistics group. As extended habitation of space continues to be explored, sustainable food

sources are needed in order to supplement shipments from Earth. In doing so, plants have begun growth on the ISS

and it is therefore important to examine the feasibility of gasification of inedible biomass. In this work, dry biomass

was obtained from the KSC plant production team. The biomass was reported as maximum absorbency garments

(Golden Heirloom Cherry Tomato plant) and GD (Ground Dew Dwarf Tomato plant from the University of

Minnesota). Each plasma experiment for waste conversion had a starting mass of 0.5 g of waste material, until the

more dense material (i.e. urine, food and fecal simulants, and the high fidelity waste) was tested.

In this work, gasification was intended to take place in the presence of the plasma high temperature plume and

carrier gas ions from air. The waste simulant was diverse and mainly include materials from cellulose and polymer

raw materials, with additions of metallic and salts. The general gasification reaction in air is represented by Equation

1, which proceeds with multiple reactions and pathways.

𝐶𝑥𝐻𝑦𝑂𝑧 + 𝑂2 (21% 𝑜𝑓 𝑎𝑖𝑟) + 𝐻2𝑂 (𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒) → 𝐻2 + 𝐶𝑂 + 𝐶𝑂2 + 𝐶𝐻4 + 𝐶𝑐ℎ𝑎𝑟 + 𝑡𝑎𝑟 (1)

Waste simulants were characterized using thermal gravimetric analysis (TGA) via a TGA Q5000 and analyzed

within TA Instruments Universal Analysis 2000 software. The reported data had a ramp setting of 5 ºC per min to 700

ºC.

III. Results and Discussion

A. Waste Characterization

The moisture content for selected waste simulants is displayed in Figure 3. The fecal, food, and urine waste

simulants were greater than 50% liquid.

Otherwise, the majority of waste is dry,

with less than 10% moisture.

The TGA results of selected waste

simulants are shown in Figure 4. The

products undergo much faster

decomposition than the TGA data, but

provide general behavior for

decomposition at varying temperature

markers. For example, materials of

cellulose are expected to convert faster in

the air environment at lower

temperatures, which include the MAG

(white portion of diaper), washcloth, t-

shirt, and MAG (grey portion of diaper).

The materials degrading at higher

temperature contain polymer derivatives,

including the food packaging (Al food),

nylon, polyethylene and packaging foam.

The food packaging (Al food), is not expected to fully decompose as it had aluminum layering that will remain as slag

and solid form in the quartz crucible. The washcloth data terminated at ~480 ºC due to external laboratory air flow

that caused the particles to leave the holding tray during data collection.

B. Visible Observation and Plasma Characteristics

Flow rates of the feed carrier gas were investigated from 2.5 to 50 SLPM to determine the highest achievable

temperature in the quartz cell within the general operating guidelines of the COTS torch in air. The flow rate of 5

SLPM yielded the highest observable system temperature (thermocouple T3) after three minutes of operation. A

sample of the five thermocouple (T1-T5) readings collected during the 5 SLPM empty quartz cell trial is displayed in

Figure 5. At 190 seconds (s), the torch was powered off and temperature declined. Maximum steady-state temperatures

Figure 3. Moisture content of waste simulants.


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at the reactor outlet, T3, ranged from 60 ºC

(40 and 50 SLPM) to 285 ºC (5 SLPM).

Gas temperatures increased with a

decreasing flow rate due to the longer

residence time of the plasma within the

discharge region. This was a desired

characteristic for gasification reactions.

The lower flow rates permitted higher

thermal transfer within the discharge

region before thermally neutral particles

entered and absorbed thermal energy. The

5 SLPM flow rate, shown in Figure 6 of the

empty quartz cell test, had the greatest

plume depth of ~12.5 mm into the quartz

cell, and was therefore selected for initial

testing in this work. The plume depth was

important due to the high electron

temperatures within the plasma. Flow rates

below 5 SLPM decreased the plume depth,

and no plume was observed exiting the

torch at flow rates below 2.5 SLPM.

Typical color of atmospheric plasma is

purple due to the abundance of nitrogen,

but the plasma generated by the torch was

observed as purple and blue. A test was

performed without the quartz cell and test

cell apparatus. Within the discharge region

without the test cell (not observable in

Figure 6), the plasma was purple and did

not gain the bluish color. This observation

concluded that the blue plasma evolved

once the test cell was installed and

interaction with the surrounding medium

of the test cell occurred. Product analysis

was therefore important to observe during

empty test cell experiments to use as a

baseline for comparison. The power

consumption of the DC plasma torch for all

of the experimental trials reported here was

consistent at 190 W with some slight variation as the power supply adjusted for the constant power setting.

The electron temperature was calculated from the slope of a Boltzmann line plot using the OES techniques and

was found to be ~ 0.3 eV, or nearly 3227 ºC (3,500 K). The gas temperature analysis and characterization is left for

future work with a collisionally radiative model but is typically on the order of 727 ºC (1,000 K)17. The techniques

involved in obtaining plume characteristics are outside the scope of this paper and will be discussed in later work.

Video screenshots of the waste experiments were obtained for each test. The general behavior included vapor

generation, gasification of the vapor, flame-like combustion, and soot or black char formation on the inner test cell

and quartz crucible walls. The ‘end of reaction’ was validated for each experiment via the emission lines gathered

from OES, in which the intensities returned to background levels. The cotton experiments displayed in Figure 7 capture

the combustion and gasification initialization in the region of the plasma plume. First observations during the cotton

experiment included the formation of a ‘vapor’ cloud throughout the test cell and intense light emission from the

cotton itself. The vapors were not collected as water after the reactor outlet in the heat exchanger, and may have

actually been oil formations. After a few seconds, the vapor dissipated and moisture was seen to collect on the inner

surface of the test cell between the combusting waste within the crucible and the surroundings. After observation of

the moisture, the combustion of the cotton continued with an orange-yellow color for an additional 10 s. Finally, the

plasma plume returned to its baseline colors, indicating the end of the reaction process. The MAGs (Figure 8)

Figure 4. TGA data of a 5°C/min. ramp rate up to 700°C

Figure 5. Temperatures as a function of time during the empty

housing, 180 second trial at 5 SLPM.


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performed similarly to the cotton trials in which vapor

was followed by condensation and flame-like

combustion, covering the test cell in soot. The

adsorptive salt may have led to the extended period of

reaction time that was observed visually and with

OES for the MAG material.

During HDPE experiments, the waste material

immediately showed movement due to the extreme

heat exposure. After 2 s of exposure, the volume of

the waste crucible brightened to extreme intensities

and vapor or oils filled the quartz test cell as observed

with cotton. A noticeable difference with HDPE was

that the vapor from the material ignited upon

interaction with the plasma plume and caused further

breakdown of HDPE and significant vapor

accumulation on the test cell surface. After nearly 20

s of plasma interaction, the gas in the test cell

appeared circular in motion, with a rotating flame from the generated waste gas. The rotating flame continued for

about 1 min, and caused soot build-up on the majority of the test cell inner surface. An additional 10 s of reaction

activity was observed by OES. The continued burn after physical waste was no longer visible suggests combustion of

the vapor. The nylon material produced similar results to the HDPE waste with slightly less soot and was excluded

for brevity.

Vapor formation was observed with dry biomass (Figure 8), followed by a simultaneous formation of condensate

and soot formation on the surface of the quartz crucible. The FP material included a vapor cloud formation, followed

by moisture, as the polymer components of the packaging were gasified. The aluminum FP layer did not reach high

enough temperatures to vaporize, and only direct contact with the plume led to slight aluminum slag pooling

formations. The plasma plume was left in operation for 10 min to observe if further thermal heating would occur in

the test cell, but had no success. The plasma plume did interact with the aluminum as an electrostatic charge was

generated, and promoted movement of the clippings in the first few seconds of plasma exposure.

The torch operation time for the fecal simulant was increased to 5 min to ensure a high mass conversion rate. The

time was determined by visual observations and OES data.

Moisture build-up on the inner test cell wall was observed with the food simulant (Figure 9) and the reaction

continued for 3.5 to 4 min. The longer reaction times may be attributed to the density of the material, the increased

starting mass, and the high energy compounds of the simulant. Discovering that the food items have the potential to

react for significantly longer times is important for future optimization. For example, the power of the torch may be

increased to decrease reaction time. T3 was seen to be comparable with the other materials even though the operating

duration was significantly higher. This suggested steady-state had been reached, with energy going into the food

simulant mass as opposed to extra thermal energy.

The HFWS required between 2.5 and 3 min for complete solid conversion. The visual observations were similar

to the food and fecal simulant with the exception of more significant char build-up at earlier time steps than observed

with the fecal or food simulants. Future analysis of the leftover char will provide insight into the byproducts of the

mixed waste and how best to manipulate the plasma in order to sure higher gasification percentage as well as less soot

build-up.

The liquid urine simulant (not shown) pooled at the bottom of the crucible and upon torch plume initiation, and a

significant amount of moisture build-up was observed on the inside of the quartz crucible. The moisture condensed

into small droplets as the operation continued, with crystalline structures remaining in the quartz crucible. The crystal

powder was collected for future elemental analysis and microscopic evaluation. The overall images acquired from the

plasma interaction with the urine simulant had shown stochastic discoloration as plasma interacted with the crystalline

deposits but otherwise remained similar to the empty quartz crucible plasma plume.

Foam material resulted in the fastest vaporization (within the first 4 s of torch utilization) compared to any of the

previously described materials, yet had the lowest average mass reduction. The soot residue generated from the foam

waste remained within the waste crucible with some slag-like material pooling at the bottom.

Figure 6. Plasma plume images into empty quartz housing

as a function of flow rate.


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Figure 7. Video screenshots of cotton, HDPE, and nylon experiments to show the start of the experiment (set up) in

the torch off position, the torch initialization (t = 0 seconds), the reaction process, and the final state of the test cell

after all apparent combustion or other thermal reaction is complete.

Figure 8. Video screenshots of the tomato, MAG, and FP experiments.

MAG

Golden Heirloom

Cherry Tomato

Food Packaging


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Figure 9. Video screenshots of the fecal simulant, food simulant, and HFWS experiments.

C. Product Analysis

Using the DC plasma test cell for waste conversion, two main product streams were obtained: a combustible gas

and a solid char or soot material. Almost no tars were produced in the process. The product gases consisted of a

mixture of air (left over in excess from the carrier gas) H2, CO, CO2, CH4, C2H4, C2H6 and light hydrocarbons. Char,

a minor product, was observed and collected for the solid conversion calculation. The presence of char however does

demonstrate incomplete conversion of the solid mass during gasification and can likely be further reduced with

improved thermal insulation on the system. Equivalence ratio (ER, ratio of airflow required for stoichiometric

combustion of the mass) will influence the gasification products as well, but contradictions in literature have yet to

convince our team that higher ER to yield higher gas production overall is the best approach18. Higher ERs have also

been shown to decrease the lower heating value and decrease the quality of desired syngas products. In our work, the

lower air flow rate was selected due to the higher temperature values obtained to yield more gasification products.

This can be studied in further detail in future work. The gaseous product distribution during the various waste

experiments at 5 SLPM of air with the plasma torch is summarized in Table 3.

Cotton, HDPE, and Nylon had over 98% solid conversion (gas yield), while food, urine and fecal were over 90%

solid conversion. Since FP contained a layer of aluminum which did not convert to a gaseous product, a lower solid

conversion of about 71% was achieved. The foam, which resulted in the fastest vaporization, had the lowest solid

conversion at 59.57%. In general, these results are encouraging for volume reduction using a plasma source with air


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as the working plasma gas. HDPE, FP, fecal and HFWS had the highest observed reaction output temperatures at T3,

which may be due to the high molecular weight (high energy density), and a more efficient ER ratio.

Table 3. Product distribution and solid conversion for waste conversion reactions in plasma test cell. Plasma

working gas = Air at 5 SLPM.

Time delineated FTIR results for foam are displayed in Figure 10. The foam material is a thermoset with

irreversible thermal degradation characteristics that may have been evolving into products not detected by the

calibrated GC, and also unable to reach temperatures high enough for gasification. We do not know the full

composition of the foam material, as it was provided to us by another interested NASA customer for demonstration,

so we are unsure of the potential composition breakdown at this time. Light hydrocarbons were not detected in the

foam material via GC, but FTIR confirmed the presence of hydrocarbons that were higher than carbon double bond

compounds such as cyclic materials. FTIR confirmed the presence of CO, CO2, and CH4 in the gas product stream for

waste materials that had similar observations in GC.

Figure 10. FTIR plot of foam during plasma test experiment with air at 5 SLPM. Confirms the presence of low CO2

production and other hydrocarbon formations.


12

The HDPE FTIR results confirmed the presence of additional hydrocarbons, while the biomass FTIR confirmed

the presence of permanent gases and ethylene. Time delineated FTIR results for the MAG material is shown in Figure

11, confirming hydrocarbon and CH4 formation.

Figure 11. FTIR plot of MAG during plasma test experiment with air at 5 SLPM.

Particulates did accumulate in the quartz cell and before the heat exchanger lines. Ash materials are typically low

reactive mineral components that decrease the carbon conversion efficiency. Filters were put in line with our system

to prevent particulate from clogging the FTIR or entering the downstream GC.

IV. Future Work

Future work will investigate a variety of fresh biomass and dry biomass, which are important for the in-situ

resource utilization of long-duration space habitation. Further investigation of the by-products of plasma gasification

and combustion will allow for capture, filtration, and re-use of the gaseous compounds. Microscopy and elemental

analysis of solid product formations will take place to confirm ash and soot composition, although assumed at this

time to be highly carbonaceous in composition. The ER can also be manipulated by adjusting the air input into the

system with power adjustments to optimize the reaction parameters. Another future test will include varying the carrier

gases to steam, CO2, and mixed gases (i.e. the syngas product stream). Varying the carrier gas will change the plume

behavior but also help to enable a semi-closed or closed loop system that can integrate with other gases on a spacecraft.

After these additional studies are performed, an energy balance analysis will be performed to evaluate the energy

efficiency of logistical waste treatment using the DC plasma system.

V. Conclusion

The laboratory experiments described here illustrate the possibility of plasma gasification and combustion as an

effective means for waste volume reduction on a spacecraft. The conversion of the solid mass to gaseous products is

enhanced by a slower flow feed gas flow rate so that higher reaction temperatures and longer residence time can be

achieved. Further improvement on the energy efficiency is needed since combustion was a primary reaction result,

and higher temperatures can likely be achieved with additional insulation. The solid mass conversion was over 80%

for most logistical waste products, and greater than 90% solid conversion was achieved for cotton, HDPE, nylon, food,

urine and fecal waste simulant. Product gases from these experiments are useful building blocks for follow-on fuel

and commodity use (H2, CO, CO2, CH4, C2H4, C2H6 and light hydrocarbons). If the product gases are not chosen for

additional use, they are at least observed at this stage for ease of venting off of a spacecraft as permanent gases.

Acknowledgments

The team acknowledges funding from the NASA KSC 2018 STMD CIF as well as funding from the Advanced

Exploration System Logistics Reduction Program at JSC. Many thanks to Dr. Igor Matveev for experimental set up


13

and technical feedback on results, the Plant group at KSC known as Veggie, Elizabeth Barrios for obtaining TGA

results and Conor O’Bryan for moisture analysis.

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