Solar Thermal Design & Modeling
The Phase I Solchar prototype featured a batch receiver reactor, which was appropriate for household scale applications, but has limited potential to serve more users efficiently. A continuous collection and processing system is required to increase scale. The UCB team developed simplified engineering designs and conducted preliminary feasibility studies on the use of an evacuated tube solar receiver to thermally inactivate and treat human waste. Evacuated tubes are commonly used in solar power generation as the receiver on parabolic trough concentrators. The vacuum insulation of evacuated tubes can offer improved thermal efficiency over other kinds of insulation while still being transparent to solar flux. The novel fiber optic power delivery of the Solchar system permits flexibility in the location of the evacuated tube reactor, which does not have to be mounted on the parabolic trough. The performance of the proposed receiver-reactor was evaluated based on the steady-state solution of a computational fluid dynamics (CFD) simulation, namely the average temperature of the fluid inside the receiver. CUTrace, an open source Monte Carlo ray-tracing program, was used to simulate concentrated solar power delivered by fiber-optic cables to an evacuated tube receiver-reactor. The performance of the evacuated tube receiver-reactor was simulated using ANSYS CFX, a computational fluid dynamics (CFD) simulation software. For purposes of this preliminary study, water was used instead of human feces. Flux distribution profiles were calculated in CUTrace and imported into ANSYS CFX then set as boundary conditions at the receiver and vacuum tubes to simulate the steady-state temperature in the system. This study indicates that temperatures above 700 K, well above the temperature required to thermally inactivate human waste, can be obtained by recirculation of a water-like fluid through a receiver-reactor tube irradiated by fiber optic cables.. A more detailed and thorough investigation should include simulation studies with temperature dependent physical properties, phase change, and experimental verification of the model.
The pyrolysis of feces and fecal sludge is gaining more attention as both a means of disinfecting pathogens for public health safety and producing useful products. Pyrolysis reactions depend on the rate of temperature increase, flow of gas, particle size, etc. Thermogravimetric analysis (TGA) coupled with Mass Spectrometry (MS) has been conducted on a dried and ground feces powder in order to investigate the kinetics of the reaction as well as quantitatively characterize the pyrolysis gases produced. Experiments were carried out at five different heating rates corresponding to a slow pyrolysis process. Quantitative analysis was enabled by using calibration gases to generate a standard curve of the Ion current vs. Gas concentrations. The experimentally determined data were mass loss and current intensity of selected released gases. Pyrolysis gas concentrations at each heating rate and across all experiment temperatures were determined for Carbon Dioxide, Carbon Monoxide, Ethane, Methane, an d Hydrogen. In addition the char, tar/liquid, and gas yields of the pyrolysis of feces were determined. Multiple kinetic modeling approaches were used to determine the activation energy of the pyrolysis reaction, as well as the pre-exponential factor. We expect the results to be used as key inputs in the design of feces and other animal manure pyrolysis plants.
Researchers from the University of Colorado at Boulder have developed a novel prototype process for pyrolyzing human fecal waste into char using concentrated solar power. During the course of the project, the authors developed a COMSOL® multiphysics pyrolysis model as a process design tool for batch pyrolysis of feces to char. The model encompasses many of the key physics in the pyrolysis process: concentrating solar power, variable solar conditions, and reaction kinetics to predict conversion. Several inputs to the model were derived from experiment and literature, and the model is verified by comparison to experiments during prototype testing. The output of this work is an expression for the effective thermal conductivity of untreated human feces, and other wet-solid materials, that accounts for “wet” and “dry” stages and that can be incorporated into any heat transfer model. This research concludes the development a COMSOL heat transfer model that accurately predicts reactor temperatures with variable DNI, based on experiments from on-sun testing of the Sol-Char Reactor charged with water, synthetic feces/raw feces.
In this study, we have ascertained the performance characteristics of the solar concentrator (flux distribution, power delivery from reflective surface to fiber input) and measured the power delivery and durability of existing fiber optic bundles.
Biochar Fuel & Emissions
The developing world faces dual crises of escalating energy demand and lack of urban sanitation infrastructure that pose significant burdens on the environment. This paper 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.
Fecal char fuel briquettes have the potential to be a viable alternative to traditional solid fuels such as store bought charcoal and wood fuel. This research compares the performance of fecal char fuel briquettes and traditional solid fuels in terms of energy efficiency and emissions. Water-boiling tests (WBT)of fecal char briquettes, charcoal and wood were conducted in a controlled environment. The fecal char briquettes are a product of the human waste from the Sol-char toilets. In the tests, carbon monoxide (CO), carbon dioxide (CO2) and particulate matter (PM) data is acquired to calculate energy efficiency as the ratio of CO/CO2 and individually as emissions as indicators of safe burning. Comparison between fuels will give a clear idea as to which fuel is more energy efficient and will help conclude if solid fuel char briquettes from human waste are a practical solution to not only energy needs in third world countries but also a means to address the management of fecal sludge.
Our previous research has shown that fecal char has potential as an energy source. We are investigating the industrial applicability of fecal chars and pit latrine sludge chars as a solid fuel for co-firing with coals. Performance of coals and biomass fuels for use in a boiler is characterized through several standardized tests – we have performed those tests on our chars and are now comparing heating value and ash composition parameters of fecal chars to those of coals and frequently co-fired biomass. A major component of the study is attempting to predict optimal feces-derived fuels to coal ratios for co-combustion, based on fouling propensity and HHV. We are also investigating ash removal protocols that could make high-ash pit latrine sludges more viable fuel options.
Biochar Applications in Agriculture
This study expands on the previous studies of nitrogen sorption to biochar, going a step beyond to observe phosphorus recovery combined with nitrogen recovery. Most phosphorus and some nitrogen recovery from urine has been found to occur via precipitation as struvite (NH4MgPO4), a slow-release fertilizer. Full phosphorus recovery requires addition of a supplemental magnesium source, and various natural sources are currently under investigation as cost-effective and locally available substitutes for magnesium minerals. At the same time, sorption methods provide effective uptake of a majority of nitrogen from urine. This study seeks to combine these two processes in a cost-effective solution for optimum nutrient recovery. Furthermore, two obstacles to the economic viability of urine fertilizer is the amount of storage required (typically 6 months storage suggested) and the loss of nutrients in the form of settled precipitates during initial storage. Column experiments will be carried out with fresh urine to assess the viability of a biochar/compost/magnesium source filter as a more practical method for nutrient recovery and fertilizer production.
The modern agriculture system heavily relies on inorganic fertilizers, with nitrogen, phosphorous, and potassium being the top three elements used. The production of phosphorous containing fertilizers depends on the availability of phosphate minerals which are increasingly difficult to mine and process due to depletion of high grade ores. In addition to the technical difficulty, the cost associated with the purchase and use of commercial fertilizers is a huge burden for all countries and developing countries in particular. For these reasons, many researchers are engaged in optimization of phosphorous fertilizer use and recovery of phosphorous from biomass resources. Human waste (feces and urine) contains significant amount of phosphorous, by some estimates up to 11% of the annual global phosphorous demand (Mihelcic et al. 2011). Over 60% of the phosphorous in human waste is found in urine making it a better target for phosphorous recovery. Urine phosphorous recovery has the added benefit of preventing algae growth in surface waters.
Biochars produced by "low-tech" methods (e.g. top lit up‐draft (TLUD) cookstoves and drum ovens, externally heated retorts) from a range of feedstocks (e.g. biomass fuel pellets, rice husks, woody brush, fecal sludge) were used in batch tests to sorb key nutrient components of real and synthetic human urine. Sorption of ammonia and ammonium using powdered biochar in well mixed batch reactors were used to validate uptake capacities. Bioavailability of sorbed N were then determined by standard methods. This work displays the potential of biochars to be ‘pre-loaded’with nutrients before application to soil, enhancing the biochar’s value and simplifying a farmer’s application procedure for crop enhancement products. Additionally it shows the potential for a more efficient sourcing of nitrogen for agricultural purposes. Nitrogen is abundant in human/animal urine, and is commonly labeled as a contaminant for removal in traditional wastewater processes due to it causing harmful algal blooms in receiving waters. This work shows the possibility of more efficiently using this nitrogen, typically labeled a waste, and offsetting the need for industrial produced nitrogen salts.
The use of fecally derived biochars has caught more attention of late because of its high potential for use in agriculture and as adsorption media. Biochars made from fresh human waste and biosolids are compared with ~20 different biochar feedstocks in terms of chemical/physical characteristics as well as methods for measuring recalcitrance. The robustness of the recalcitrance index (Harvey, 2013) is assessed in this wide-ranging biochar library. Analysis is shown indicating the likely value for fecally-derived biochars in agriculture and as adsorption media.
This study will assess biochar as a possible mechanism for treating wastewater at various stages of treatment. Many anaerobic digestion projects (including Duke) are concerned about treatment of the supernatant (wastewater) and have already shown interest in using biochar columns for this. Additionally, there is growing interest in addressing emerging contaminants in a wastewater polishing process. Based on biochar’s low cost, it could be a desirable alternative to activated carbon for removing trace organic contaminants depending on the relative capacities. This would be valuable to both the developing world, in regions where flush toilets and/or septic systems are common, and to modern-day wastewater treatment in the developed world. Research tasks will include running biochar column experiments and observing treatment capacity of various pathogens and contaminants found in wastewater. Additionally, we will observe biochar’s ability to perform as a media for biological filtration.
Odor & Exhaust
One of the technical issues that need to be considered in treating fecal sludge by pyrolysis is the treatment of the resulting odor. In this research, the odor level was first measured during pyrolysis of fecal sludge over different temperatures and three different heating rates. The temperature ranged from room temperature to 750 degree C. This was done by using a hydrogen sulfide sensor and an odor unit measurement - Olfactometery. The results obtained have been used as an input to odor treatment experiments performed within our project. We also expect the results to help the wider fecal sludge processing industries by providing key inputs of expected odor levels.
The off gas released when feces are pyrolyzed at high temperatures not only contains a potentially toxic mixture of gases, but also has a strong and disagreeable odor. Treatment of this exhaust for odor is essential to make the use of any fecal waste treatment involving drying both feasible and attractive. Hydrogen sulfide is the largest odor nuisance during fecal sludge pyrolysis and can be used to measure the effectiveness of any form of odor treatment. Treatment of hydrogen sulfide using both activated carbon and biofiltration is a common practice in industries, such as wastewater treatment, where hydrogen sulfide is an issue. A combination of biochar, a cheap adsorbent with characteristics similar to activated carbon that can be generated locally, and soil, which is readily available and inherently contains microbes capable of breaking down any number of exhaust components, has the potential to treat pyrolysis off gases for hydrogen sulfide through microbial metabolism. The goal of this research will be to ascertain whether this biofilter is capable of effectively treating the intense odor released during intermittent fecal pyrolysis. In this research, gases from the pyrolysis of feces will be passed through both microbially active and non-microbially active filters multiple times a week for two months. Measurement of odor and hydrogen sulfide concentration will be taken as necessary, both before and after treatment with each filter. Theoretically, the biological activity will dramatically enhance the lifetime of the filter.
Hydrogen Sulfide, the largest odor nuisance during fecal sludge pyrolysis, is also toxic and highly corrosive. Treatment of hydrogen sulfide using activated carbon is a common practice in industries where hydrogen sulfide is an issue. Less is known about the effectiveness of biochar, a relatively cheap adsorbent that can be generated locally, for treating these gases. The goal of this research was to understand how feedstock and temperature affect biochar’s surface properties and hydrogen sulfide adsorption capacity, especially at higher temperatures. In this research, hydrogen sulfide of about 80 ppm concentration was passed through a column packed with various biochars at a controlled flow rate. The effluent gas was then analyzed for hydrogen sulfide content using a chemical sensor. The adsorption capacity of each column was calculated from the resulting breakthrough curve. To correlate the results with specific surface properties BET, SEM, and FTIR analysis were performed. Pre and post adsorption biochar samples were also analyzed for various elements and sulfate content. Results showed that high temperature chars derived from feces provided orders of magnitude higher performance than high temperature wood chars, as well as low temperature wood and fecal derived chars. This showcases a useful application of high temperature fecal chars both in treating odors caused in sanitation systems as well as other industry needs for a hydrogen sulfide adsorption.
This study analyzed disinfection in source separated and hydrolyzed urine. Hydrolyzed urine mainly contains ammonia which comprises of both free ammonia and ammonium ion. The pH and temperature of the urine solution dictate the relative amount of free ammonia and ammonium ion present. Our work has focused on investigating the role free ammonia plays on the inactivation of pathogens in heat treated urine. Viral and bacterial surrogates were added to various urine dilutions and temperatures to quantify inactivation kinetics. The results have shown that it is possible to inactivate urine at the scale of minutes to hours when hydrolyzed urine is heat treated at temperatures as low as 50 degree C. This can alleviate the large urine storage volume needed to achieve inactivation when relying on time and ammonia present in the urine.