Space Applications of Torrefaction Processing

45th International Conference on Environmental Systems ICES-2015-291 12-16 July 2015, Bellevue, Washington

International Conference on Environmental Systems

Space Applications of Torrefaction Processing

Michael A. Serio1, Joseph E. Cosgrove

2, and Marek A. Wójtowicz

3

Advanced Fuel Research, Inc., 87 Church Street, East Hartford, CT 06108-3720 USA

Jeffrey Lee4, Kanapathipillai Wignarajah

5, and John Fisher

6

NASA Ames Research Center, Moffett Field, CA 94035-1000 USA

A recent study addressed the technical feasibility of a torrefaction (mild pyrolysis)

processing system that could be used to sterilize feces and related cellulosic biomass wastes

(food, paper, wipes, and clothing) in space, while simultaneously recovering moisture,

producing additional water, and small amounts of other useful products (e.g., CO2, CO, and

CH4). This work was done using bench scale torrefaction processing units and examined

different modes of heating (conventional and microwave). The amounts of solid vs. gas plus

liquid products could be controlled by adjusting the torrefaction conditions, especially the

final temperature and holding time. The solid char product from a fecal simulant was a dry,

free flowing powder that did not support bacterial growth and was hydrophobic relative to

the starting material. The proposed torrefaction approach has potential benefits to NASA in

allowing for solid waste sterilization and stabilization, planetary protection, in-situ resource

utilization (ISRU) and/or production of chemical feedstocks and carbon materials. In

particular, the torrefaction char residue has several potential applications in space. These

include production of activated carbon, a nutrient-rich substrate for plant growth,

construction material, radiation shielding, storage of elemental carbon, hydrogen, or oxygen,

and fuel gas (CH4, CO, and H2) production. The current paper provides additional

torrefaction data and analysis. It also addresses the potential space applications of

torrefaction processing, how it compares to other approaches to solid waste management, its

applicability to a range of cellulosic biomass materials, and how the technology could be

integrated with existing advanced life support technologies, such as the Heat Melt

Compactor (HMC) or the Universal Waste Management System (UWMS).

Nomenclature

AR = As-Received

DAF = Dry, Ash Free

DOC = Dissolved Organic Carbon

FC = Fixed Carbon

FTIR = Fourier Transform Infrared

HMC = Heat Melt Compactor

ISRU = In-Situ Resource Utilization

ISS = International Space Station

LPM = Liters per Minute

LB = Lysogeny Broth

MFC = Mass Flow Controller

MFM = Mass Flow Meter

1 President, Advanced Fuel Research, Inc. (AFR), 87 Church Street, East Hartford, CT 06108.

2 Laboratory Manager, AFR, 87 Church Street, East Hartford, CT 06108.

3 Vice President, Clean Energy & Carbon Materials, AFR, 87 Church Street, East Hartford, CT 06108.

4 System Analyst, Bioengineering Branch, MS N239-15, NASA Ames Research Center, Moffett Field, CA 94035.

5 Senior Scientist, SCB Branch, MS 239-8, NASA-Ames Research Center, Moffett Field, CA 94035.

6 Lead Engineer, Exploration Life Support, NASA-Ames Research Center, Moffett Field, CA 94035.


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NIST = National Institute of Standards and Technology

OMPCV = Orion Multi-Purpose Crew Vehicle

SBIR = Small Business Innovation Research

TG = Thermogravimetic Analyzer

TOC = Total Organic Carbon

UTC = United Technologies Corporation

UWMS = Universal Waste Management System

VM = Volatile Matter

WMS = Waste Management System

I. Introduction

A. The Problem and Technical Approach

New technology is needed to collect, stabilize, recover useful materials, and store human fecal waste and other

spacecraft solid wastes for long duration missions. The motivations include crew safety, comfort and resource

requirements, along with planetary protection [1-4]. The current paper addresses a torrefaction (mild pyrolysis)

processing system that can be used to sterilize feces and related cellulosic biomass wastes (food, paper, wipes, and

clothing) and produce a stable char residue that can be easily stored or recycled, while simultaneously recovering all

of the moisture and producing small amounts of other gases. Previous NASA sponsored work [5,6] demonstrated

that torrefaction processing was effective for a fecal simulant using bench scale experiments with both microwave

and conventional heating. In addition, the torrefaction method could be applied to other types of wet or dry

cellulosic biomass (food, paper, wipes, clothing), which could provide for some redundancy in the Waste

Management System (WMS). It would also be important for a torrefaction processing system to be compatible with

other components of the Advanced Life Support system.

B. Potential Advantages versus Current Solid Waste Management Technology

There are many constraints on the WMS for advanced life support systems that can be used for both short and

long duration flights and space outposts. Such systems should be of low volume, low weight, low power and oxygen

consumption, have the ability to handle a variety of wastes, require low maintenance, and be able to reduce volume

and increase stability of the wastes [7-13]. This is a complex optimization problem, which is why it still remains an

area of active R&D. An additional layer of complexity results from the uncertainty of the NASA near term mission

objective, as going back to the moon will place different demands on the WMS than going to Mars or

Asteroids/Phobos. In addition, the final system will require relatively sophisticated control schemes to ensure that it

operates properly, is integrated with the rest of the life support system, and has some system redundancy to mitigate

risk [14].

As discussed by Fisher et al. [15], the need for waste processing varies greatly, depending on the mission

scenario. The near term needs are for the Orion Multi-Purpose Crew Vehicle (OMPCV), which will provide crewed

missions to the International Space Station (ISS) and beyond. The intermediate term needs are for waste

management technologies that could support missions to the moon and, eventually, the establishment of a habitat on

the moon. The longer term missions would be to establish a long term base on the moon and ultimately to go to

Mars. There has been a lot of NASA supported work (both internal and external) on Waste Management Systems,

with a recent emphasis on the near term mission scenarios. These include testing of a plastic melt waste compactor

[16], use of microwaves to stabilize waste and recover water [17], and evaluating water recovery using a microwave

freeze drying unit, a microwave powered ambient dryer, and a recirculating hot air dryer [18]. This represents a shift

in emphasis away from fully regenerative technologies, such as incineration and supercritical water oxidation, which

received a lot of attention in the 1990s [15].

Solid wastes will include inedible plant biomass, paper, wipes, plastic, cardboard, waste water concentrates,

urine concentrates, feces, etc. It would be desirable to recover usable constituents such as H2O, CO2, nitrogen, fuel

gases, solid inorganics, etc. Any unusable byproducts should be chemically and biologically stable and require

minimal amounts of storage volume. Many different processes have been considered for dealing with these wastes:

incineration, aerobic and anaerobic biodigestion, wet oxidation, supercritical oxidation, steam reforming,

electrochemical oxidation, catalytic oxidation, and heat melt compaction [7-25]. All the above approaches have their

advantages, but also disadvantages which have prevented adoption of any single method. For example, incineration

utilizes a valuable resource, oxygen, and produces undesirable byproducts such as oxides of sulfur and nitrogen. In


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addition, incineration will immediately convert all of the waste carbon to CO2, which will require venting excess

CO2.

Pyrolytic waste treatment has several advantages when compared to other possible approaches: 1) it can be used

for all types of solid products [26,27] and can be more easily adapted to changes in feedstock composition than

alternative approaches; 2) the technology is relatively simple and can be made compact and lightweight and thus is

amenable to spacecraft operations; 3) it can be conducted as a batch, low pressure process, with minimal

requirements for feedstock preprocessing; 4) it can produce several usable products from solid waste streams (e.g.,

H2O, CO2, CO, H2O, H2, NH3, CH4, etc.); 5) the technology can be designed to produce minimal amounts of

unusable byproducts; 6) it can produce potentially valuable chemicals, chemical feedstocks, and materials (e.g.,

activated carbon); 7) pyrolysis will reduce the storage volume of the waste materials, while important elements such

as carbon and nitrogen can be efficiently stored in the form of pyrolysis char and later recovered by gasification or

combustion when needed. In addition to being used as a primary waste treatment method, pyrolysis can also be used

as a pretreatment for more conventional techniques, such as combustion or gasification

One of the biggest advantages of pyrolysis (or torrefaction) for near term life support is that it can provide the

baseline functions of a WMS, i.e., reduce volume, stabilize, and recover water, along with several enhancements.

These include thermochemical water and oxygen recovery, production of fuels, and multi-purpose carbon, and in-

situ resource utilization, as shown in Figure 1. Consequently, pyrolysis/torrefaction processing can provide a bridge

technology between near term and long term mission scenarios, as discussed below.

Figure 1. Schematic of Waste Management System (WMS) indicating how a pyrolysis/torrefaction-centric

approach impacts near term, intermediate term and long term mission scenarios, including in-situ resource

utilization.

C. Potential Benefits for NASA

The proposed methodology would make it technically feasible to process human fecal waste and related solid

waste streams in space, which will benefit long term space travel such as an extended Lunar stay or a mission to

Mars. As discussed above and indicated in Figures 1 and 2, the proposed torrefaction (mild pyrolysis) approach is

beneficial to NASA in allowing for volume reduction, solid waste sterilization and stabilization, and water recovery

for near term missions. In the case of longer term missions, more severe (pyrolysis) processing in the same or

similar equipment would allow for enhanced water and CO2 production, production of fuel gases (CH4, CO, and H2)


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and multi-purpose carbon, along with ISRU. The torrefaction processing system is also complementary to the Heat

Melt Compactor (HMC) [28] and could also be designed to be compatible with the Universal Waste Management

System (UWMS) [29], both now under development by NASA.

The primary purpose of future work will be to further evaluate torrefaction processing for NASA in the hope it

that can ultimately be an important component of a WMS. However, there are also important potential ancillary

benefits in terms of increasing the technology base for stabilization and recycling of human fecal waste, as well as

for biochar production from solid waste streams. The recent Gates Foundation “Reinventing the Toilet Challenge”

supporting documents indicated that more than 2.5 billion people worldwide do not have access to safe and

affordable sanitation [30]. The production of biochar from readily available cellulosic biomass and other waste

materials has the potential to be important in efforts to promote soil fertility and carbon sequestration [31,32]. The

support of such an effort is entirely consistent with NASA’s objective of improving life on earth and being an

incubator of dual-use technologies.

Figure 2. Spacecraft uses for torrefaction char residue. Shading indicates products that can be produced with

additional heat treatment in a more severe (pyrolysis) process.

II. Background

Torrefaction can be viewed as pyrolysis performed under mild conditions [ 33]. It is usually defined as a

thermochemical pre-treatment of biomass at 200–300 °C in the absence of oxygen. The typical heating rates used

are lower than 50 °C/min, and the typical torrefaction time scales are one hour or less. Torrefaction has attracted a

lot of interest in recent years, primarily in Europe, and the main reason seems to be a trend towards more sustainable

power generation, in particular co-firing of biomass pellets with coal, but also rising fuel prices. A good overview of

the torrefaction process and technologies can be found in Energy Research Center of the Netherlands reports [34–

36] that are available from their web site. Torrefaction research is also carried out at the National Renewable Energy

Laboratory in Colorado (Nimlos et al. [37]), at the University of Leeds in the UK (Bridgeman et al. [38]), and also

at the Royal Institute of Technology in Sweden (Pach et al. [39]), among other laboratories. The main advantages of

torrefied biomass are: (1) higher energy density, (2) more homogeneous composition, (3) hydrophobic behavior, (4)

improved grindability, and (5) elimination of biological activity.

An understanding of a key benefit of torrefaction (mild pyrolysis) processing for intermediate term missions in

space can be gained by looking at the results from temperature programmed pyrolysis of a representative biomass

material (wheat straw), shown in Figure 3. These are results for the evolution of major products from a standard

pyrolysis experiment at 30 K/min using (up to 900°C) a thermogravimetric analyzer with FTIR analysis of evolved

gases (TG-FTIR) for a National Institute of Standards and Technology (NIST) wheat straw sample. The plots show

the sample weight loss, as well as both the differential and integral curves for several species (H2O, tars, CO2, CH4,

CO, acetic acid, acetaldehyde. The more than 20% by weight of water that can be produced from pyrolysis of


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Figure 3. TG-FTIR analysis results for major products from NIST wheat straw sample heated at 30K/min.


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many common (dried) biomass materials (see upper right panel in Figure 3) is one of the advantages of pyrolysis

processing of solid wastes, especially for intermediate term missions scenarios. However, the penalty for this

additional water evolution is the production of significant amounts of non-condensable gases (e.g., CO2, CO), along

with significant tar evolution, as shown in Figure 3.

However, it can also be observed in Figure 3 that pyrolysis temperatures below 300 °C, which is the usual upper

limit for torrefaction, produce about 50% of the pyrolytic water evolution, and do not lead to significant evolutions

for most other gases or tars. Exceptions include acetic acid and modest amounts of CO2 and CO. The key for

torrefaction processing of feces and other biomass materials is to remove moisture, produce additional pyrolytic

water (~50% of the maximum amount) but to not transition to active pyrolysis, where tar formation becomes

important. In this case, careful control of the sample temperature becomes important, as indicated in Figure 4, which

illustrates the various regimes in the pyrolysis processing of biomass as described by deWild [40]. For many

biomass materials, torrefaction becomes mildly exothermic in the temperature range 250-300°C, which can make

the temperature control a little bit more challenging [33]. Figure 4 also illustrates how torrefaction is different from

pyrolysis and the role that hemicellulose decomposition plays in initiating the torrefaction process [33-38]. Figure 5

contains a time series spectra from TG-FTIR analysis of pure xylan, a major component of hemicellulose, and

provides some additional detail on the initial products that are evolved under torrefaction conditions (<300 °C).

Figures 3 and 5 both indicate that early evolution of both water and acetic acid is characteristic of the torrefaction

regime for biomass materials. This is consistent with the TG-FTIR results on the fecal simulant used in previous

torrefaction studies [5,6], which are shown as bar graphs in Figure 6 and tabulated in Table 1 on an As-Received

(AR) and a Dry, Ash-Free (DAF) basis. Even though the fecal simulant is not a single component biomass material,

it behaves like most whole biomass materials that we have studied.

Torrefaction for

Enhanced wood fuels (250 – 290 °C)

Fast pyrolysis for

Bio-oil (450 – 500 °C)

Carbonization

for charcoal

100 °C 200 °C 300 °C 400 °C 500 °C 600 °C

Lignin

Cellulose

Hemicellulose

Drying

Figure 4. An example of the thermal fractionation of biomass by a step-wise pyrolysis approach. Adapted

from Reference [40].

Figure 5. FT-IR spectra of products of xylan (hemicellulose) from TG-FTIR pyrolysis experiments.


7

Figure 6. TG-FTIR analysis of fecal simulant heated at 30K/min to 900°C. The data in blue are for

cumulative product yields at 300°C, while the data in orange are for cumulative product yields at 900°C.

(b)

(a)


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Table 1. Results for TG-FTIR analysis of fecal simulant at 300°C and 900°C while heated at 30K/min.

Sample: Standard Feces Simulant

Heating Rate: 30 K/min (300 °C) 30 K/min (900 °C)

Run No. AFR5722 AFR5772

TG-FTIR Profile Used: BIOM30 BIOM30

Drying Regime: 30 min at 80 °C 30 min at 80 °C

Sample Size (mg): 54.81 54.81

Product Yield (wt. %)

300 °C (AR) 300 °C (DAF) 900 °C (AR) 900 °C (DAF)

Moisture 17.53 17.53

Ash (AR) 0 0.00

Ash (dry) 0.00 0.00

VM (from TGA data) 10 12.13 65.81 79.80

Char 72.47 87.87 16.66 20.20

Tar (by difference) 3.35 4.06 46.70 56.63

Water (pyrolytic) 4.30 5.22 9.28 11.25

Carbon Monoxide 0.07 0.08 1.42 1.72

Carbon Dioxide 0.90 1.09 3.47 4.21

Methane 0.02 0.02 0.73 0.88

Ethylene 0.00 0.00 0.09 0.11

Hydrogen Cyanide 0.01 0.01 0.15 0.18

Ammonia 0.00 0.00 0.09 0.11

Isocyanic Acid 0.00 0.00 0.06 0.08

Formaldehyde 0.01 0.01 0.51 0.62

Acetaldehyde 0.00 0.00 1.29 1.56

Formic Acid 0.06 0.07 0.31 0.37

Acetic Acid 0.33 0.40 0.27 0.33

Methanol 0.00 0.00 0.00 0.00

Sulfur Dioxide 0.00 0.00 0.00 0.00

Carbonyl Sulfide 0.01 0.02 0.07 0.08

Phenol 0.00 0.00 0.00 0.00

Acetone 0.12 0.15 1.40 1.70

III. Experimental

A. Experimental Apparatus

The initial work was to design and assemble a laboratory system for torrefaction studies on fecal simulant

samples using different heating methods. A schematic of the torrefaction reactor system that was assembled for this

project is shown in Figure 7. The primary components are the reactor unit and the gas analyzers, which include an

On-Line Technologies, Inc. Model 2100 Process FTIR spectrometer and a Stanford Research Systems QMS 100

mass spectrometer. A two-stage condenser (25°C, 5°C) is employed for collecting condensable products (water and

oil) and a wool filter is used to prevent residual particulate species from contaminating the sampling components of

the gas analysis instrumentation. The FTIR gas analyzer has a frequency range of 700 – 6500 cm-1

with a resolution

of 0.5 cm-1

. The instrument is calibrated for quantitative measurements of CO2, CO, CH4, C2H4 and H2O (which is

not completely condensed). The heated (125°C), single pass gas sampling cell (~ 50 cc volume, pathlength = 4

inches) has an exchange rate of 4-5 seconds and is ideal for observing transients in the various gas evolutions during

the torrefaction experiments. The mass spectrometer is calibrated for H2, CO2, CH4 and C2H2.


9

Figure 7. Schematic of the experimental apparatus employed for torrefaction experiments.

A primary goal of the study was to evaluate two different heating approaches for torrefaction of fecal simulant

samples: 1) electrical heating using a conventional tube furnace-type heater and 2) microwave heating. The design,

construction, and operation of both reactors was described in some detail in previous publications [5,6]. The

microwave torrefaction system was adapted from a previously constructed microwave pyrolysis prototype [41,42].

B. Materials

The composition of the base fecal simulant samples used for these experiments is shown in Table 2. This is

similar to a formulation developed by Wignarajah et al. [43], the only difference being that E.coli addition was not

used, for safety reasons. The water content, determined by drying in an 80oC oven for a period of 24 hours, is

generally in the range of 20 – 23%, depending on the type of miso used. Because a “standard” miso source was

temporarily unavailable, the initial three experiments used a low-salt alternative, which resulted in a simulant with

slightly higher water content (~ 23%) than that obtained with the standard miso (~ 20%). Water was added to the “as

prepared” simulant to generate levels up to 50% water content for torrefaction experiments.

Table 2. Chemical composition of fecal simulant.

Component Weight Percent

Cellulose 14.3

Polyethylene Glycol 7.1

Peanut Oil 28.6

Miso 42.9

Potassium Chloride 5.7

Calcium Chloride 1.4

Gas

Analyzers

Nitrogen

Carrier Gas

Power

Meter

MFC MFM

Torrefaction

Reactor

Condenser Filter

Computer

Tc


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IV. Results and Discussion

A. Torrefaction Experiments

Under the recent project, a total of 27 torrefaction experiments were performed, using both electrical furnace

heating and microwave heating. The project focused mainly on fecal simulants, as described in the previous section,

but other materials were also studied, including Milorganite (a sewage sludge-derived fertilizer product), baby

wipes, and cotton fabric (from tee-shirts), using the microwave reactor. Furnace heating experiments involved

approximately 125 – 170 g sample sizes, depending on the sample moisture content, while microwave heating

experiments generally involved 250 – 335 g samples, again depending on the sample moisture content. In both

cases, the sample was purged with a 1 LPM nitrogen carrier gas for a minimum of 30 minutes prior to heating and

the carrier flow maintained throughout the course of the run. As described above, FTIR and mass spectra were

collected and analyzed continuously throughout each run in order to track CO2, CO, CH4, C2H4, C2H2 and H2O.

After cool down, the reaction vessel, condenser, filter, and gas transfer lines were weighed to determine the

remaining torrefied sample mass and condensate produced during each run. Mass balances were in the range 96 –

98 % for all but the first few runs. It should be noted that ethanol was typically also observed in the spectral data, but

was not quantified. Based on its evolution beginning at low temperatures (<100°C), the ethanol is thought to be a

fermentation product present in the miso, which represents the largest fraction of material in the simulant (excluding

water). In addition, the spectra displayed features consistent with the presence of carbonyl compounds, such as

acetone and acetic acid. Both of these compounds were quantified in the TG-FTIR analyses described above (see

Figure 6 and Table 1).

As discussed above, two modes of heating were employed. In some cases, the furnace or microwave heater was

temperature controlled, using feedback from one of the sample thermocouples or the furnace thermocouple, in the

case of furnace heating. In other cases, the heater was operated in a constant power mode. For microwave heating,

experiments were also conducted with and without the aid of a central microwave susceptor (SiC tube). An example

of a torrefied sample of fecal simulant is shown in Figure 8, which depicts a photograph of the untreated simulant

(20% water content) along with the torrefied residue after heating to ~ 300oC. The untreated sample has the

consistency of moist sand. After heating, the simulant is fully charred and reduced to a mass of less than half of its

initial (as-received) value. The amount of mass loss depends on the final temperature and holding time, as well as

the initial moisture content.

Table 3 summarizes the results for the total liquid yields of the torrefaction experiments that were performed on

the fecal simulant, Milorganite fertilizer, cotton fabric, and baby wipes. It includes the key experimental parameters,

including the sample mass, sample moisture content, measured (maximum) sample temperatures, and calculated

(average) sample temperatures. In addition, Table 3 lists whether furnace or microwave heating was employed. The

table also provides the yields for CO2, which was the major non-condensable gas observed in all runs, the yields of

which are an indicator of the severity of the heating profile. As described above, the liquid yields are determined

gravimetrically. A more complete description of these experiments was provided previously [5,6].

B. Excess Water Production Table 3 also provides a calculated ratio of the water recovered compared to the initial moisture content, in the

next to the last column. The water collected was estimated by assuming that all of the liquid collected was 100 %

water, an assumption that was supported by selected analyses, discussed below in section D. In almost every case,

excess water was formed when compared to the moisture that was in the starting material. As discussed in the

Background section, about half of the ~ 20% by weight of pyrolytic water for most biomass materials is formed

under torrefaction conditions, or about 10 wt. % on a dry basis. If this 10 % assumption is made for all of the

materials that were studied, an expected excess water ratio can be estimated and this is given in the last column in

Table 3. In general, the two columns agree pretty well. The exceptions were mainly the non-standard materials that

were tested, such as the cotton tee-shirts or wipes. There were also a few experiments where the torrefaction

temperature was well above 300 oC, e.g., Run No. 3, and in this case the assumption of the liquid being almost all

water would not be correct. If Run Nos. 3, 22, 23, and 24 are excluded, the average of the actual H2O ratios was

1.33, while the average of the expected H2O ratios was 1.26. The microwave experiments were more in line with

expectations, presumably since the temperature control and uniformity were better. These were also done later in in

the study when the mass balances had improved to generally better than 95%.


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Table 3. Summary of liquid yields for all torrefaction experiments. The rows are color-coded based on the

type of material that was torrefied: no shading = fecal simulant, yellow = Milorganite fertilizer, blue = cotton

material and pink = baby wipes.

Run

No.

Sample

Mass

(g)

Moisture

Content

(%)

Heater

Type

Max.

Temp.

(oC)

Ave.

Temp.1

(oC)

Liquid

Yield

(g)

Meas.

CO2

(g)

H2O

Ratio2

(Actual)

H2O

Ratio3

(Expect)

1 126.3 23 Furn. 300 I 34.1 2.55 1.18 1.33

2 125.1 23 Furn. 300 I 35.6 2.95 1.24 1.33

3 125.2 23 Furn. 521 >300 71.3 5.08 2.48 1.33

4 125.6 20 Furn. 310 272 51.6 3.45 2.06 1.40

5 125.2 20 Furn. 310 264 59.9 4.53 2.40 1.40

6 250.2 20 Micro. 342 I 77.6 5.05 1.55 1.40

7 250.2 20 Micro. 202 201 55.9 1.23 1.12 1.40

8 250.8 20 Micro. 274 220 60.4 1.84 1.20 1.40

9 250.5 20 Micro. 341 245 76.0 4.69 1.52 1.40

10 250.2 20 Micro. 304 232 68.4 3.20 1.37 1.40

11 276.9 20 Micro. 320 235 79.5 3.73 1.43 1.40

12 280.8 20 Micro. 304 237 77.2 4.07 1.37 1.40

13 335.9 40 Micro. 311 229 154.4 3.27 1.15 1.15

14 168.1 40 Furn. 334 265 109.6 8.07 1.63 1.15

15 167.9 40 Furn. 211 203 72.2 1.33 1.07 1.15

16 336.1 40 Micro. 235 < 200 141.0 1.49 1.05 1.15

17 168.7 40 Furn. 303 228 106.4 4.76 1.58 1.15

18 327.9 50 Micro. 266 < 200 168.7 0.89 1.03 1.10

19 327.9 50 Micro. 303 232 173.1 2.01 1.06 1.10

20 267.2 40 Micro. 293 240 118.1 0.94 1.10 1.15

21 327.7 50 Micro. 284 232 172.0 2.04 1.05 1.10

22 64.2 4.5 Micro. 278 239 3.8 0.78 1.31 3.10

23 74.6 4.5 Micro. 314 279 21.4 3.40 6.29 3.10

24 163.7 81 Micro. 321 248 129.7 0.65 0.98 1.02

25 300.3 40 Micro. 341 268 141.6 4.17 1.18 1.15

26 301.6 40 Micro. 355 280 141.9 4.06 1.18 1.15

27 302.9 40 Micro. 317 272 142.9 3.55 1.18 1.15

1) Average sample temperature where both sample temperatures are ≥ 200 oC. I = Insufficient data.

2) See Text; 3) See Text

Figure 8. Photograph of a sample of undried fecal simulant (left) and a torrefied (~300 °C) sample (right).


12

C. Torrefaction of other Cellulosic Biomass Materials Of the other materials that were studied, the Milorganite proved to be the easiest material to heat using

microwave processing. However, after heating it to a temperature of 293°C, it appeared visually unchanged. It was

subsequently learned that, during manufacturing, Milorganite is processed at much higher temperatures, ranging

from ~480 to 650°C. On the other hand, the cotton material proved to be more difficult to heat and was not

converted to a powder at the temperatures that were explored. For Run #22 (max. temp. = 278°C), the temperatures

were not sufficient to completely convert the cotton material to char. For Run #23, the torrefaction temperature was

increased to 314°C and maintained for a much longer period, resulting in nearly complete charring of the tee-shirt

material and ~ 50% weight loss, although the fabric structure still remained largely intact. Baby wipes showed

similar results to the cotton using microwave treatment. Both of these materials are mostly cellulose and do not have

the hemi-cellulose content that initiates the early decomposition and nearly complete disintegration of materials that

contain whole biomass. This is contrary to the results for the fecal simulant or plant biomass materials under

torrefaction conditions [33]. Future work will include a more detailed study of the effects of feedstock composition,

particularly with regard to how it affects the physical structure and the water production. The testing will also

progress to closer analogs of fecal matter, such as sewage sludge and actual fecal material, and mixtures that include

more than one type of biomass material.

D. Char and Liquid Analysis Several samples of char and liquids derived from torrefaction of fecal simulants were analyzed by Huffman

Laboratories, Inc. (Golden, CO), to assess their composition and organic content, respectively. Table 4 lists the

results for the solids analyses, showing the elemental composition (C,H,N,O,S) as well as ash, Volatile Matter (VM)

and Fixed Carbon (FC), along with temperature data. It also lists the starting composition for a raw feces simulant

used in a previous project, but made with the same recipe [41, 42]. Note that the data are listed in order of increasing

maximum torrefaction temperature. In general, the compositions for the four samples generated at temperatures

under 285°C are quite similar and only modestly different than the starting material. The fact that the char product

from the torrefaction process retains a lot of its hydrogen content (see blue shaded column) suggests that it might be

better at radiation shielding than activated carbon, although not as good as polyethylene [44]. Some preliminary

measurements were made in this regard and were discussed previously [6]. For the highest temperature case, higher

nitrogen and ash and a decrease in carbon, hydrogen, oxygen, and VM are observed.

Table 5 lists the results for the liquid analyses in terms of Total Organic Carbon (TOC) and Dissolved Organic

Carbon (DOC). The table also shows the TOC values normalized to a moisture content of 50%, the highest moisture

content that was studied in this project. As was the case for the solids analyses, the four samples generated at

torrefaction temperatures less than 285oC are similar in terms of their normalized TOC, ranging from 0.62 – 0.77 %

(see blue shaded column). Some of the organics may be condensed ethanol that is thought to originate from the miso

in the simulant, as discussed above. Nonetheless, these values are very close to the acceptable limits for water

recycling applications in space. It should be noted that the 50% moisture content used for the normalized TOC

measurements is actually on the low end of the expected moisture contents in human feces (65-85 %) [45]. The

modest TOC amounts confirm that most of the liquid product collected is water and that the amount of water is in

excess of the moisture content (see next-to-last column, Table 3). In fact, excluding the highest temperature and

non-simulant runs, the amount of water collected is close to the sum of the moisture content plus 10 wt. % of the dry

solid mass being converted to additional water (see last column, Table 3).

Table 4. Composition of char from several fecal simulant torrefaction experiments.

Run

#

Moisture

Content

(%)

Max.

Temp.

(°C)

Ave.

Temp.1

(°C)

Composition (%)

C H N O S Ash VM FC

RAW 19 ___ ___ 49.70 7.77 0.97 28.15 0.06 13.35 75.60 11.05

7 20 202 201 52.14 7.46 1.04 25.03 0.08 14.25 73.69 12.06

15 40 211 203 56.68 7.65 1.05 19.63 0.06 14.93 70.61 14.46

16 40 235 L 53.33 7.55 1.02 23.02 0.06 15.02 73.15 11.83

21 50 284 232 55.83 7.61 1.09 19.39 0.06 16.02 71.39 12.59

14 40 334 265 47.33 4.66 1.59 11.76 0.05 34.61 37.11 28.28

1. Average sample temperature where both sample temperatures are ≥ 200 °C. L is marked where one of the

thermocouples did not reach 200°C. The analyses are reported on a dry basis, except for moisture (as

received).


13

Table 5. Total organic carbon and dissolved organic carbon of liquid yield from several simulant torrefaction

experiments.

Run # Moisture

Content

(%)

Max.

Temp.

(°C)

Ave.

Temp.1

(°C)

Total Org.

Carbon (mg/L)

Dissolved Org.

Carbon (mg/L)

Normalized2

Total Org.

Carbon (%)

7 20 202 201 17000 16400 0.68

15 40 211 203 9630 9010 0.77

16 40 235 L 7700 7680 0.62

21 50 284 232 7490 7170 0.75

14 40 334 265 29000 28000 2.32

1. Average sample temperature where both sample temperatures are ≥ 200 °C. L is marked where one of the

thermocouples did not reach 200°C.

2. Normalized to 50% moisture content.

The results of these liquid analyses are consistent with visual observations during torrefaction processing. In

general, heavy oils and tars are not observed in the reactor condensers until temperatures approach ~ 300°C. As

indicated by the schematic in Figure 7 and the photo in Figure 9, there is an initial condenser, which consists of an

air-cooled glass tube loosely filled with coarse, stainless steel wool. Figure 9 displays digital images that were

obtained of the first stage condenser during microwave torrefaction of a fecal simulant sample with 40% moisture

content, as the temperature of the sample progressed from room temperature (start) to completion. At 70°C, water is

observed collecting in the condenser, along with a relatively small amount of reddish fluid that has not been

identified. Evolution of the reddish condensate is fairly brief, ending when the sample temperature reaches about

100°C. As the reaction progresses up to about 290oC, the condensate produced is fairly clear and colorless, causing

the color of the liquid in the condenser to gradually fade. As the sample temperature continues to increase beyond

290°C, tar and/or oil becomes noticeable. These changes are most evident at the condenser inlet and outlet.

.

E. Biological Activity Testing Some of the advantages of biomass torrefaction include elimination of biological activity and increased

hydrophobic behavior. In this case, methodologies were established for evaluating these characteristics of the

torrefied fecal simulant samples that were produced in the study [5]. In order to assess the biological activity, agar

plates were dusted with a small quantity of the torrefied powder and monitored for evidence of biological growth,

such as mold. The agar plates were commercial lysogeny broth (LB) agar plates manufactured by EZ BioResearch

LLC. Figure 10 includes photographs of agar plates treated with a sample of as-prepared simulant, and plates treated

with torrefied material heated to 202°C, (Run # 7). Similar comparisons were made of 240°C (Run # 15) and 304°C

(Run #10), as described previously [5]. After 15 days, abundant mold colonies were observed in the untreated

simulant samples, while none were observed in the torrefied samples (see Figure 10).

F. Evaluation of Laboratory Results

The experiments performed, employing both microwave and electrical furnace heating, have demonstrated that

torrefaction provides an effective means of processing a fecal simulant biomass material in terms of elimination of

biological activity, significantly increased hydrophobicity [5,6] of the resulting char material, minimal gas

production, and enhanced water recovery with low organic contamination. These results also indicated that

microwave heating provides faster, more volumetric heating (even heating), with superior control, when compared

to furnace heating [5,6]. However, the use of microwave heating may be constrained if the ultimate objective is to

fully integrate the torrefaction processing into the UWMS. This will require further study.

G. Integration with Advanced Life Support System

The results in Table 3 suggest that torrefaction processing is effective over a wide range of moisture contents and

the implication is that other aqueous waste steams can be included in the process, such as urine, brine, or waste

water. There is literature which suggests that the moisture content, acidity, and the presence of salts have a modest

influence on the torrefaction process [33, 38], so the implications of this mixing of process streams remains to be

seen. The process appears to be fairly robust, since the substitution of air for nitrogen in the torrefaction processing

of the fecal simulant had minimal impact on the results [5, 6]. However, the process sensitivities need to be studied


14

Figure 9. Digital images captured of the 1st stage condenser during torrefaction of fecal simulant (40%

moisture content). A light, reddish liquid is observed at the outlet of the condenser early in the process at ~

70°C, followed by a relatively clear and colorless liquid until ~ 290°C. At 313°C until the end, tar/oil

condensation is visible at the inlet and outlet of the condenser, respectively.

Start

End 313°C

290°C 235°C

70°C


15

Simulant, Day 1 Torrefied Simulant (202 oC), Day 1

Torrefied Simulant (202 oC), Day 15 Simulant, Day 15

Figure 10. Biological activity testing results using agar plates. After 15 days, mold colonies are easily seen in the

untreated simulant sample. No evidence of mold growth is observed in the torrefied sample briefly heated to 202 oC

(Run #7).


16

with better solid waste analogs, including actual human fecal waste and actual ALS aqueous process streams. The

effect of torrefaction processing on odors for these more realistic waste streams remains to be studied.

The presence of acetic acid in the torrefaction liquids is interesting, since it has some useful properties. For

example, it is used in laundry water as a clothes softener, is often used to clean washers, and has some antimicrobial

properties.

Torrefaction could also be used to homogenize solid wastes before they are introduced into other devices, such

as the HMC, or as a post-treatment process for feces in the UWMS. The ability of torrefaction processing to

homogenize materials into a uniform powder is partly dependent on the hemi-cellulose content, as discussed above.

How this would be affected by multi-component mixtures remains a subject of future study.

V. Conclusions

A recent project demonstrated that microwave (volumetric) heating allowed for careful control of torrefaction

conditions for a fecal simulant and related cellulosic biomass materials. The net result was complete recovery of

moisture, some additional water production (typically >10 wt. % of the dry biomass weight), which can lead to a

significant percent increase (10 to 40) in the water recovery (depending on the initial moisture content). There was

also a modest reduction of the dry solid mass and small amounts of gas (CO2, CO, CH4) and hydrocarbon liquid

production. The amounts of solid vs. gas plus hydrocarbon liquid products can be controlled by adjusting the

torrefaction conditions (final temperature, holding time). The solid char product from the fecal simulant was a dry,

free flowing powder that did not support bacterial growth and was hydrophobic relative to the starting material [5,

6]. At temperatures less than ~290°C, the condensate was fairly clear and colorless and had a moderate total organic

carbon content (~0.7%). In addition, the torrefaction method can be applied to other types of wet or dry cellulosic

biomass (food, paper, wipes, and clothing), which could provide for some desirable redundancy in the waste

management system. However, the torrefaction behavior is different for cotton and wipes, which are nearly pure

cellulose.

Torrefaction processing is also complementary to the Heat Melt Compactor (HMC) [28] as a biomass

pretreatment step and is compatible with the Universal Waste Management System (UWMS) [29] as a post-

treatment step, both now under development by NASA. A microwave torrefaction process could also be

accomplished with modest energy requirements, which could be improved even further in an optimized prototype

system. However, it should be noted that the use of microwave processing might be constrained by a complete

integration of the torrefaction unit with the UWMS.

The proposed torrefaction approach will make it technically feasible to process human fecal waste and related

cellulosic biomass waste streams and produce additional water and other useful products in space. This will benefit

NASA in allowing for volume reduction, solid waste sterilization and stabilization, and water recovery for near term

missions. In the case of longer term missions, more severe (pyrolysis) processing in the same or similar equipment

would allow for enhanced water and CO2 production, production of fuel gases (CH4, CO, and H2) and multi-purpose

carbon, along with ISRU. In particular, the torrefaction char residue has several potential applications in space.

These include production of activated carbon, a nutrient-rich substrate for plant growth, construction material,

radiation shielding, storage of elemental carbon, hydrogen and oxygen, and fuel gas (CH4, CO, H2) production.

Acknowledgments

The support of this work by the NASA-Ames Research Center under contracts NNX10CA22C and

NNX13PCA54P is gratefully acknowledged. The authors also wish to thank Ms. Emily Serio, who assisted with the

laboratory work. In addition, the authors wish to acknowledge helpful discussions with Tim Nalette and Thomas

Stapleton of UTC Aerospace Systems.

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Space Applications of Torrefaction Processing

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