Evaluation of Solid Fuel Char Briquettes from Human Waste

Jul 14, 2014 - Human fecal waste can be safely treated and transformed into a ... in a pyrolysis chamber (Thermo Scientific Lindberg Blue M heavy duty...
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Evaluation of Solid Fuel Char Briquettes from Human Waste Barbara J. Ward, Tesfayohanes W. Yacob, and Lupita D. Montoya* Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, Colorado 80309-0428, United States ABSTRACT: The developing world faces dual crises of escalating energy demand and lack of urban sanitation infrastructure that pose significant burdens on the environment. This article presents results of a study evaluating the feasibility of using human feces-derived char as a solid fuel for heating and cooking and a potential way to address both crises. The study determined the energy content and the elemental composition of chars pyrolyzed at 300, 450, and 750 °C. Fecal chars made at 300 °C were found to be similar in energy content to wood chars and bituminous coal, having a heating value of 25.6 ± 0.08 MJ/kg, while fecal chars made at 750 °C had an energy content of 13.8 ± 0.48 MJ/kg. The higher heating values of the studied chars were evaluated using their elemental composition and a published predictive model; results found good agreement between the measured and predicted values. Fecal chars made at low temperatures were briquetted with molasses/lime and starch binders. Briquettes made with 10% starch had an average impact resistance index of 79 and a higher heating value of 25 MJ/kg. These values are comparable to those of commercial charcoal briquettes, making fecal char briquettes a potential substitute that also contributes to the preservation of the environment.

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INTRODUCTION It is estimated that by the end of 2012 about 2.5 billion people worldwide still did not have access to improved sanitation facilities. Although open defecation has decreased since 1990, almost 1 billion people (15% of the world population) continue this practice.1 Common human waste management alternatives include standard pit latrines and shared public toilet blocks. Even when fecal sludge is collected from its deposition site, the waste is often dumped back into the environment untreated.2 Over 1.5 billion people use toilets connected to septic systems or other collection tanks that empty raw sewage into shallow groundwater, surface waters, or open drains.3 Ninety percent of people in developing countries lack sufficient access to reliable energy supplies.4 Like sanitation, energy scarcity disproportionately affects the poorest households, across Africa and India; the poorest economic bracket can spend up to 25% of their total income on fuel.5 In comparison, the lowest household income earners in the US spend less than 9% of their income on household energy.6 Indirect costs of energy access include the overcollection of firewood and the production of wood charcoal, which contribute to ecological deterioration causing deforestation, increased erosion, and higher levels of air pollution. For the more than 2 billion people7 that rely on solid fuels like biomass and coal for cooking, heating, and water boiling, a practical supplementary energy source should come in a solid form that works with existing cooking and heating methods. Human fecal waste can be safely treated and transformed into a solid fuel by thermally decomposing fecal sludge at high© 2014 American Chemical Society

temperature and low oxygen conditions, a process called pyrolysis. Overall, pyrolysis reduces the fecal feedstock into useful and pathogen-free char, high-energy gas and oil. NASA investigated pyrolysis of synthetic human fecal matter for waste volume reduction and reclamation on the space station.8,9 Biochar produced through pyrolysis of various animal and plant waste has been shown to have promising benefits to the environment.10−12 Recent studies have investigated the use of char created from animal manure as a fuel.13−16 Animal manure chars pyrolyzed at low temperatures were found to have heating values (amount of heat released during their combustion) between high and low rank coals, from 13 to 18 MJ/kg of fuel.14 Most studies have focused on use of animal fecal char as an industrial fuel, e.g., grinding char and feeding it back into the pyrolysis reactor to heat the process.16 The use of fecal char (animal or human) as a domestic heating and cooking fuel source has not been reported. Researchers have looked into the processing of coal particulates (coal fines) and wood char particles to make charcoal briquettes. Taulbee et al. identified a range of costeffective binders for pulverized coal; Atlun et al. examined the effect of different binders on combustion kinetics.17,18 Demirbas and Stevenson et al. discussed briquetting of coal for use in the developing world.19,20 Zhi et al. examined improvement of Received: Revised: Accepted: Published: 9852

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emissions when coal briquettes were used in improved stoves.21 Synthetic feces recipes have been used in fundamental research in fecal waste treatment; however, their characteristics and performance after pyrolysis are not well understood. The research presented here was done to support the design of a new toilet built by the University of Colorado Boulder, the Sol-Char toilet, which uses concentrated sunlight to dry and pyrolyze feces. In this study, the energy content of real and synthetic fecal chars produced at different pyrolysis temperatures were evaluated by determining their higher heating values (HHVs). Char energy and elemental composition were determined and compared to those of other common solid fuels; measured HHVs were compared to those predicted by a published model. The strength of fecal char briquettes made with different binders was also assessed.

determined by subtraction. Elemental composition was reported on a dry basis. Oxygen Bomb Calorimetry. The char HHV was determined using a Parr oxygen bomb calorimeter and following ASTM D2015 procedures with several modifications.28 Fecal char samples pyrolyzed at 300, 450, and 750 °C and synthetic char samples pyrolyzed at 300 and 750 °C were evaluated. Molasses and lime binders were also tested to determine their individual energy content. Triplicate calorimeter experiments were performed for each sample, and average values with standard errors are reported here. In the cases of 300, 450, and 750 °C fecal char and 750 °C synthetic char, binders were used to facilitate the formation of a stable pellet for calorimetry. When binders were used to make the pellets, the following equation was used to determine the absolute HHV of the char (without added binder):

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EXPERIMENTAL DETAILS Chars. Char samples included in this research were derived from real feces as well as a synthetic human feces formula developed for NASA.22 Real fecal matter was collected from approximately 25 anonymous volunteers at the University of Colorado Boulder. Institutional Biosafety Committee (IBC) approval was obtained, but Institutional Review Board (IRB) approval was not; therefore, no individual information about the participants, including their diet, was available. The diet was likely a mixture of vegetarian and nonvegetarian. Studies have shown that diets high in vegetable fiber produce feces with higher energy and fat levels than those from low fiber diets.23 Also, people eating high-fiber diets and living in rural areas produce more feces than those living in urban areas eating diets with less fiber.24 The collected material was quickly deposited in a metal container and stored at −20 °C for a maximum period of 2 weeks prior to pyrolysis. Synthetic fecal matter was produced using recipe #2 from Wignarajah et al.22 Metal cans containing the waste were placed in a pyrolysis chamber (Thermo Scientific Lindberg Blue M heavy duty box furnace). Internal temperature of the feces was monitored with a thermocouple and temperature data logger. Pyrolysis was conducted at 300, 450, and 750 °C for all feces with average heating rates of 1.6, 1.6, and 13.9 K/min, respectively. The samples were held at the target temperature for 2 h, and the resulted chars were pulverized and homogenized before briquetting and analyses. Elemental Analysis. Elemental analysis was performed on char samples using a 2400 CHN Elemental Analyzer (PerkinElmer Inc., Waltham, MA). The relative amount of gases produced were determined and then used to calculate the mass percentage of carbon, hydrogen, nitrogen, and ash in the char sample. The sulfur content of the chars was estimated using measured sulfur contents of raw human and synthetic feces from the literature. Reported sulfur percent values by dry weight were 0.06% for synthetic feces25 and 0.5% for real human feces.26 No published values were found for pyrolized human feces; however, a study by Cantrell et al.13 found that the initial percent sulfur of raw swine manure was decreased by about 15% after pyrolysis. Swine has been considered closest in diet and digestive physiology to humans.27 Therefore, we assumed a similar sulfur reduction for human fecal char. Thus, the possible range of sulfur in human fecal char is 0−0.43%. This is less than the highest reported sulfur content in any feces-derived char, which is found in swine manure char (0.85% S).13 The validity of this assumption was also verified during the modeling part of this analysis. Mass percent oxygen was

HHVchar = (HHVpellet − xbinder HHVbinder)/xchar

where HHVchar is the higher heating value of the char, HHVsample is the higher heating value of the char pellet, xbinder is the weight fraction of binder in the pellet, HHVbinder is the higher heating value of binder, and xchar is the weight fraction of char in the pellet. Briquettes. Char briquettes were manufactured using a 1.25 in. (3.2 cm) diameter stainless steel die and a Carver Model C pneumatic laboratory press. Briquettes were about 3 in. (7.6 cm) long, 3.7 cubic inches (60.6 cm3), and contained about 0.53 ounces (15g) of char or a char-binder combination. Several different binders were tested. Starch and a molasses/ lime combination were selected as potential binders based on their likely availability in many parts of the developing world.20 Binding ratios were selected based on their performance in published data.17,29 Table 1 lists the types of binders and the weight ratios tested in this study. Table 1. Binder Types and Ratios Tested binder type starch molasses and lime

binder ratio (by weight) 5% corn starch, 115 °C 3% corn starch, 7% wheat starch, 350 °C 10% and 3.5% 20% and 7%

For both binder types, char was first finely ground with mortar and pestle to obtain a small and uniform particle size. For binding with molasses and lime, the materials were thoroughly dry mixed in a large bowl followed by the addition of molasses. After mixing for approximately 5 min, briquettes were made with a die and press and set aside to cure. For briquetting with starch, the procedure from Henley et al. was adapted for bench-scale use.29 Cornstarch and wheat starch were first dry mixed with char powder, then water was added slowly to the mixture at a ratio of 6:1 water to binder weight. The paste was mixed until homogeneous and then briquetted with the press. Briquettes were subsequently heated in an oxygen limited clamshell oven in order to break down the starch binders and encourage bonds to form in a process called calcination. Briquettes were held at a temperature of 150 °C for 15 h and then raised to and held at 350 °C for 2 h. The 5% cornstarch briquettes were heated in a clamshell oven at 115 °C for 15 h to drive off water and then were lowered back to room temperature and stored. To simulate briquetting pressures achieved by hand presses used in the developing world, a 9853

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Table 2. Char Yield, Elemental Composition, and HHV of Human Fecal and Synthetic Fecal Chars and Binder char type feedstock human feces human feces human feces synthetic feces synthetic feces synthetic feces molasses binder corn starch binder wheat starch binder

pyrolysis temp (°C) 300 450 750 300 450 750

% char yield

%ASH

49 29 30 39 19 14

20.0 37.1 50.0 8.3 20.0 38.3

%C 58.23 50.67 42.03 64.05 57.67 56.17

%H

%N

6.10 1.90 0.44 7.48 1.91 0.38

5.19 4.76 2.44 3.10 5.25 2.72

%S a

0.43 0.43a 0.43a 0.05b 0.05b 0.05b

%O 10.05 5.14 4.69 17.01 15.12 2.38

HHV (MJ/kg) 25.57 17.91 13.83 29.53

± ± ± ±

0.08 0.32 0.48 0.91

18.92 ± 1.30 12.92 ± 0.11 20.5c 20.6c

a %S in human feces char estimated from content of sulfur in human feces26 and adjusting for decrease in sulfur content with pyrolysis.13 b%S in synthetic feces char estimated from content of sulfur in synthetic feces25 and adjusting for decrease in sulfur content with pyrolysis.13 cHHV of starch binders is referenced from literature.35

pressure of 1400 psi was applied to the die.30 HHVs of briquettes made with different binder configurations were calculated using the following equation:

dry mass of nitrogen in the sample, and ASH is the % dry mass of ash in the sample.

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RESULTS Elemental Composition and Energy Content. Table 2 shows the measured HHVs and elemental composition for chars produced at different pyrolysis temperatures. The materials tested included human fecal chars, synthetic chars, and molasses binder. HHVs of starch binders from literature are included from ref 35. The char yield is reported based on dry feedstock mass. Elemental composition is reported on a dry basis. HHVs are presented as mean and standard error (at 95% confidence interval). Sulfur content was not measured but was assumed based on published values.13,25,26 Elemental analysis was not performed on binders or the raw fecal feedstocks. The HHVs of both fecal chars decreased with increasing pyrolysis temperatures tested (300 to 750 °C). Synthetic char had an HHV that was systematically about 4 to 5 MJ/kg higher than human fecal char at each temperature tested. The elemental compositions of chars made from human and synthetic feces varied with pyrolysis temperature. The ash content increased substantially for both chars with higher pyrolysis temperature. For a given pyrolysis temperature, the difference in the % ash content between human feces chars and synthetic feces chars ranged from 12−17%. Synthetic char had a higher carbon content than real human fecal char at every temperature setting. For real chars, the % carbon decreased from 58% to 42% from low to high pyrolysis temperatures. For synthetic chars, the % carbon decreased from 64% to 56%. Oxygen and hydrogen contents decreased with increasing pyrolysis temperature for both types of feces. Nitrogen content decreased with pyrolysis temperature for human fecal char but did not follow the same trend for synthetic char. The elemental analysis also allowed for the determination of elemental ratios H/C and O/C, which can offer information about the combustion efficiency of a fuel. Briquettes with low H/C and O/C ratios produce less CO2, water vapor, and smoke when burned, leading to higher combustion efficiency.36,37 For chars of cellulosic origin, H/C and O/C ratios tend to decrease with increased pyrolysis temperature as carbon in the char transitions from aliphatic hydrocarbons into aromatic elemental carbon structures.36,37 Wood chars typically have O/C ratios ranging from 0.01−0.35 and H/C ratios from 0.03 to 0.7;36,38 coals tend to have O/C ratios of 0.01−0.25 and H/C ratios of 0.4−1.0.39 Figure 1 shows elemental ratios H/C and O/C over a range of pyrolysis temperatures for fecal char,

HHVbriquette = xchar HHVchar + xbinder HHVbinder

where HHVbriquette is the higher heating value of the briquette, xchar is the weight fraction of char in the briquette, HHVchar is the higher heating value of the char, xbinder is the weight fraction of binder in the briquette, and HHVbinder is the higher heating value of the binder. Strength Tests. Briquette durability was determined using standardized briquette strength tests proposed by Taulbee et al. and Richards.17,31 Shatter strength and compressive strength tests were performed after briquettes cured for 4 days. Shatter strength was measured by repeatedly dropping briquettes from a height of 2 feet onto an epoxy resin laboratory countertop. The number of repeated drops until the briquette broke apart was recorded, as well as the number of pieces generated during the test. The impact resistance index (IRI) was calculated using Richards’ methods: IRI =

100 × average number of drops average number of pieces

Four repeat tests were performed for each briquette configuration to calculate IRI. Empirical Model. The elemental composition results for the char samples were entered into a predictive published model that correlates HHV to elemental composition of biofuels.32 Predicted HHVs were then compared to those measured via calorimetry analyses performed on the char samples. Channiwala et al.’s correlation has been highly tested and refined and is often used as a substitute for experimental calorimetry analysis. It predicts HHV using elemental analysis results, reported as mass percentages of carbon, hydrogen, oxygen, sulfur, nitrogen, and ash in the fuel.32−34 The Channiwala model was derived and validated using elemental analysis and HHV data for 275 chars, coals, biomass materials, bio-oils, and fuel gases, and the correlation is shown here:32 HHV(MJ/kg) = 0.3491C + 1.1783H + 0.1005S − 0.1034O − 0.0151N − 0.0211ASH

where HHV is the higher heating value of the fuel, C is the % dry mass of carbon in the sample, H is the % dry mass of hydrogen in the sample, S is the % dry mass of sulfur in the sample, O is the % dry mass of oxygen in the sample, N is the % 9854

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Both molasses/lime configurations and the high temperature starch configuration exceeded the minimum compressive strength of a commercial charcoal briquette of 375 kPa.31 Figure 3 shows the impact resistance index (IRI) for the various

Figure 1. Elemental ratios H/C and O/C as a function of pyrolysis temperatures for human fecal char, synthetic fecal char, typical wood char, and typical ratios for coals.

synthetic char, and typical wood char.38 Typical elemental ratios for coals fall in the shaded area.29 H/C ratios for fecal char, synthetic char, and wood char decreased with pyrolysis temperature; only chars made at temperature ranges between 350 and 500 °C fell within the H/C ratio of coals. O/C ratios for all three chars also showed a general decrease with pyrolysis temperature, and all fell within the acceptable elemental ratio of standard coals. Empirical Model. The elemental composition data for human and synthetic fecal chars measured in this study were entered into the Channiwala model, which predicts HHV for fuels. The results showed that the predicted HHVs were within the average standard error of the measured values. In order to check the effect of sulfur content estimation on the modeled HHV, the maximum possible range of % sulfur (0−0.85%), as estimated in the Experimental Details section, was input into the model. This resulted in predicted HHVs that were within 2.8% of the measured values. Figure 2 shows the predicted HHV from the Channiwala model and the HHV determined in this study. There is agreement at the 95% confidence level between predicted and empirically determined HHV. Briquettes. All briquettes were made with 300 °C human fecal char as its HHV made it the most promising potential solid fuel. The compressive strength of three briquette formulations was found to be adequately high (>375 kPa); briquettes with the highest content of molasses and lime binders tested showed the best resistance to impact. All briquettes (except those made with low temperature starch, which crumbled with handling) remained unfractured at the upper limit of the pneumatic press used for testing (20,000 lbf).

Figure 3. Impact resistance index (IRI) for human fecal char briquettes made from different binder configurations with minimum acceptable IRI of 50 indicated by the black line.

briquettes tested. Briquettes made with 20% molasses and 7% lime binder mixture had a measured IRI that significantly exceeded the industry standard IRI of 50, while 10% molasses/ 3.5% lime briquettes and high temperature 10% starch briquettes had average IRIs higher than the standard but standard deviations that ranged below 50. Briquettes made with low temperature starch binders had an IRI of 10 and shattered on the first drop. The strongest briquettes, (20% molasses and 7% lime binder) had the lowest briquette HHV of 21.3 MJ/kg. The HHV of the original unbound 300 °C fecal char was reduced 17% by the addition of lower-energy binders during the briquetting process. Briquettes made with 10% molasses and 3.5% lime binder had an HHV of 23.4 MJ/kg, an 8.5% reduction in energy from unbound char. On the basis of calorific values of starch in the literature,35 briquettes bound with starch were determined to have higher energy contents than those bound with molasses and lime. Fecal char bound with the high temperature 10% starch method (3% corn starch and 7% wheat starch, 350 °C) briquettes had an estimated HHV of 25.1 MJ/kg, a 2% reduction in energy from unbound char. Briquettes bound with the lower temperature starch method (5% cornstarch, 115 °C) were determined to have the

Figure 2. (a) HHV of human fecal char and synthetic fecal char as a function of pyrolysis temperature determined experimentally and predicted by Channiwala model. (b) Correlation between predicted and experimentally measured HHVs. 9855

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aromatic char as readily. A higher percentage of carbon in fecesderived char is in the form of aliphatic hydrocarbons compared to wood char.46,47 The alkanes present in fecal char are more energy-rich48 but also less thermally stable than aromatic compounds. The upper thermal stability limit for molecules containing long-chain alkyl groups is 349 °C,49 whereas aromatic carbon remains stable, with only slight deformation in graphene sheets at temperatures upward of 2000 °C.50 This degradation and volatilization of high-energy aliphatic hydrocarbons in fecal char is likely the largest contributing factor in the decrease in char energy content with increased pyrolysis temperature. The high H/C ratio of low temperature fecal char is further evidence of the existence of unvolatilized hydrocarbons in the char. The aliphatic carbons present in low temperature chars likely contribute to their comparably high HHV but will likely also contribute to the production of more emissions and a less efficient combustion than chars made at higher temperatures. Further research is being pursued to determine which pyrolysis temperature produces fecal char fuels that maximize energy efficiency and minimize emissions. Briquetting human fecal char with 20% molasses and 7% lime using the pressure of a typical hand press created a durable briquette with an HHV slightly lower than that of commercial charcoal briquettes. Briquettes made with 10% molasses and 3.5% lime binder as well as those with 10% starch binder were less durable but had higher HHVs. The highest performing briquettes tested in this study were made with 10% starch binder, as they met all 4 of the standards for commercially distributed charcoal briquettes set forth by the United Nations Food and Agriculture Organization (FAO).51 All fecal char briquettes met FAO standards for moisture and ash content, but the 10% starch briquettes exceeded the FAO energy standard of 22 MJ/kg and were the only ones to meet the 10 wt % limit for added binder. Briquettes made with 10% starch did not perform reliably above the industry standard IRI, however, which could mean that they are not durable enough for transport or rough handling between manufacture and use. The binders used in this study are meant to be representative of what is available in the developing world, but they are not the only ones that can be used and may not be the best for this application. A variety of locally available binders could be tested to optimize briquette quality and cost at a specific location and climate. The Channiwala model was found to accurately predict experimentally determined HHVs for fecal chars produced in this set of experiments within the average standard error of the experimental measurements. Its use might save time and cost in future fecal char research. Developing a recipe for synthetic human feces that behaves similarly to real feces when pyrolyzed would also be useful to researchers of human fecal waste management. Results from this study suggest that NASA #2 synthetic feces may not be ideal as a model for the energy content of human fecal char. At 300 °C, the synthetic char was 4 MJ/kg higher in energy content than fecal char, and at 750 °C, synthetic char was 5 MJ/kg higher in energy than fecal char. Synthetic and real feces start with similar calorific values before charring; estimated HHV of raw NASA synthetic is 18.8 MJ/kg,25 while human feces has been measured at 19.4 MJ/kg.52 This initial similarity in energy content suggests that the difference in HHV is not due to innate differences in energy contained in the raw feedstocks. More likely, the difference in energy content

highest HHV of 25.3 MJ/kg but were deemed too physically weak for consideration of the low temperature starch method as practical for the binding of fecal char. The three binder configurations robust enough to stand up to manual handling have been included in Figure 4, which illustrates how the HHV

Figure 4. HHVs of common solid fuels and human fecal char briquettes manufactured in this study. Bars indicate a range of reported values from the literature. The black vertical line indicates the minimum FAO briquette HHV standard.

of briquettes evaluated in this study compare to those of other solid fuels presently used in the developing world: commercial charcoal, anthracite coal, bituminous coal, sugar cane bagasse, wood, and lignite coal.40,41 Human fecal char briquettes are comparable in energy content to commercially available charcoal briquettes and bituminous coal.

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DISCUSSION The significance of this study lies in determining the high energy content of fecal chars and the successful briquetting protocol, both critical in evaluating pyrolysis of fecal chars as an attractive treatment option for application in the developing world. Char produced from human feces at 300 °C had almost twice the energy content as the fecal char pyrolyzed at 750 °C. Synthetic fecal char pyrolyzed at 300 °C had 1.5 times higher energy content than the char pyrolyzed at 750 °C. Decreasing HHV with increased pyrolysis temperature is a trend that is rarely observed with char production from conventional feedstocks. Only noncellulosic, high ash content feedstocks like animal manure and algae chars have HHVs that decrease with increasing pyrolysis temperature.13,42 Pyrolysis of wood and other cellulosic biomass usually produces higher energy chars with increasing pyrolysis temperatures.34,42−45 It is likely that the decreasing HHV with increasing temperature trend is due to the degradation and volatilization of energy-rich mobile aliphatic hydrocarbons as pyrolysis temperatures increase from 300 to 750 °C. In previous char pyrolysis studies, the %C has been shown to increase with pyrolysis temperature34,43−45 as more volatile compounds are driven off and the remaining lignocellulosic carbon arranges into stable aromatic graphene structures. Because feces are primarily composed of carbohydrates, proteins, and lipids instead of cellulose or lignin,46 they do not form stable, highly 9856

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(2) Strauss, M.; Montangero, A. Applied Research on the Management of Sludges from On-Site Sanitation Systems in Developing Countries: Rationale, Issues and Project Overview; Swiss Federal Institute for Environmental Science & Technology: Duebendorf, Switzerland, 2002. (3) Baum, R.; Luh, J.; Bartram, J. Sanitation: A Global Estimate of Sewerage Connections without Treatment and the Resulting Impact on MDG Progress. Environ. Sci. Technol. 2013, 47 (4), 1994−2000. (4) Barnes, D. F.; Floor, W. M. Rural Energy in Developing Countries: A Challenge for Economic Development 1. Annu. Rev. Energy Environ. 1996, 21 (1), 497−530. (5) D-Lab Fuel from the Fields: Charcoal Background. http://d-lab. mit.edu/sites/default/files/Charcoal_BG.pdf. (6) Average Annual Expenditures of All Consumer Units by Income Level: 2009; Statistical Abstract of the United States; U.S. Census Bureau: Washington, DC, 2012. (7) Betts, K. How Charcoal Fires Heat the World. Environ. Sci. Technol. 2003, 37 (9), 160A−161A. (8) Serio, M. A.; Kroo, E.; Bassilakis, R.; Wójtowicz, M. A.; Suuberg, E. M. A Prototype Pyrolyzer for Solid Waste Resource Recovery in Space, 31st International Conference on Environmental Systems, Orlando, FL, 2001. (9) Serio, M. A.; Kroo, K.; Wójtowicz, M. A.; Suuberg, E. M.; Filburn, T. An Improved Pyrolyzer for Solid Waste Resource Recovery in Space, 32nd International Conference on Environmental Systems, San Antonio, TX, 2002. (10) Cao, X.; Ma, L.; Gao, B.; Harris, W. Dairy-Manure Derived Biochar Effectively Sorbs Lead and Atrazine. Environ. Sci. Technol. 2009, 43 (9), 3285−3291. (11) Fraser, B. High-Tech Charcoal Fights Climate Change. Environ. Sci. Technol. 2009, 44 (2), 548−549. (12) Renner, R. Rethinking Biochar. Environ. Sci. Technol. 2007, 41 (17), 5932−5933. (13) Cantrell, K. B.; Hunt, P. G.; Uchimiya, M.; Novak, J. M.; Ro, K. S. Impact of Pyrolysis Temperature and Manure Source on Physicochemical Characteristics of Biochar. Bioresour. Technol. 2012, 107, 419−428. (14) Ro, K. S.; Cantrell, K. B.; Hunt, P. G. High-Temperature Pyrolysis of Blended Animal Manures for Producing Renewable Energy and Value-Added Biochar. Ind. Eng. Chem. Res. 2010, 49 (20), 10125−10131. (15) Troy, S.; Nolan, T.; Leahy, J.; Healy, M.; Kwapinski, W.; Lawlor, P. Pyrolysis of Separated Solids of Pig Manure; TEAGASC: Cork, Ireland, 2011; pp 24−29. (16) Sánchez, M.; Martínez, O.; Gómez, X.; Morán, A. Pyrolysis of Mixtures of Sewage Sludge and Manure: A Comparison of the Results Obtained in the Laboratory (Semi-Pilot) and in a Pilot Plant. Waste Manage. 2007, 27 (10), 1328−1334. (17) Taulbee, T. D.; Patil, D.; Honaker, R. Q.; Parekh, B. Briquetting of Coal Fines and Sawdust Part I: Binder and Briquetting-Parameters Evaluations. Int. J. Coal Prep. Util. 2009, 29 (1), 1−22. (18) Altun, N.; Hicyilmaz, C.; Kök, M. Effect of Different Binders on the Combustion Properties of Lignite Part I. Effect on Thermal Properties. J. Therm. Anal. Calorim. 2001, 65 (3), 787−795. (19) Demirbas, A. Sustainable Charcoal Production and Charcoal Briquetting. Energy Sources, Part A 2009, 31 (19), 1694−1699. (20) Stevenson, G. G.; Perlack, R. D. The Prospects for Coal Briquetting in the Third World. Energy Policy 1989, 17 (3), 215−227. (21) Zhi, G.; Peng, C.; Chen, Y.; Liu, D.; Sheng, G.; Fu, J. Deployment of Coal Briquettes and Improved Stoves: Possibly an Option for both Environment and Climate. Environ. Sci. Technol. 2009, 43 (15), 5586−5591. (22) Wignarajah, K.; Litwiller, E.; Fisher, J. W.; Hogan, J. Simulated Human Feces for Testing Human Waste Processing Technologies in Space Systems. Training 2006, 2009, 01−26. (23) Kelsay, J. L. A Review of Research on Effects of Fiber Intake on Man. Am. J. Clin. Nutr. 1978, 31 (1), 142−159.

between synthetic char and fecal char is due to the difference in % ash of the chars. Feces contains about 18% ash before charring,46 while the NASA synthetic recipe calls for only 5 dry % inorganics, contributing to a low inorganic mineral composition in the resulting char. A new formulation of synthetic feces with an increased inorganic content could be developed to better model the energy content in fecal char. On the basis of the energy content, the predicted combustion efficiency, and briquette quality of charred human feces in this study, we found the material to have potential as a supplementary, renewable energy source in the developing world. Successful use of fecal char as a renewable energy source, however, will require energy efficient ways to produce it. Greater than 95% of the theoretical energy needed to convert feces into char goes in to drying the feces. Thus, inexpensive drying methods that take advantage of sunlight and high ambient temperatures can be used to meet the energy requirements. Pyrolysis of human waste, as any biomass pyrolysis, produces exhaust that can contribute to air pollution. Thus, effective exhaust treatment should be implemented for human waste pyrolysis systems. Additionally, though the technical feasibility of fecal char briquettes used was demonstrated in this work, its social dimensions are yet to be determined. Users would need to accept this as a safe and competitive alternative to traditional charcoal. As increasing demand for solid fuels continues to drive deforestation and air pollution, and as inadequate urban sanitation continues to pose a major threat to water quality and public health, solutions for treating human waste and providing renewable and affordable fuel sources will be necessary. The treatment of human feces by pyrolysis at 300 °C has been shown to yield char that can be made into briquettes with comparable energy content to solid fuels currently in use in the developing world and can potentially become an attractive fuel alternative.

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AUTHOR INFORMATION

Corresponding Author

*(L.D.M.) E-mail: [email protected]. Tel: +1 303 492 7137. Fax: +1 303 492 7317. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication is based on research funded in part by the CU Engineering Excellence Fund and in part by the Bill & Melinda Gates Foundation. The findings and conclusions contained within are those of the authors and do not necessarily reflect positions or policies of the Bill & Melinda Gates Foundation. The elemental analyses were conducted at the North Carolina State University Environmental and Agricultural Testing Service laboratory. The authors would like to thank the University of Colorado’s Sol-Char Toilet team for support and access to human fecal material used in this study. We also express gratitude to Josh Kearns for lending his biochar data and knowledge and also to the Chemistry and Biochemistry Department at CU Boulder for providing calorimetry expertise and equipment.

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