Anaerobic digestion for methane generation and ammonia ?· Anaerobic digestion for methane generation…

ww.sciencedirect.comb i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5Available online at whttp: / /www.elsevier .com/locate/biombioeAnaerobic digestion for methane generation andammonia reforming for hydrogen production:A thermodynamic energy balance of a modelsystem to demonstrate net energy feasibilityDavid M. Babson a, Karen Bellman b, Shaurya Prakash b,*,Donna E. Fennell a,**aDepartment of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901,United StatesbDepartment of Mechanical and Aerospace Engineering, The Ohio State University, 201 W. 19th Ave., Columbus,OH 43210, United Statesa r t i c l e i n f oArticle history:Received 6 October 2012Received in revised form6 May 2013Accepted 24 May 2013Available onlineKeywords:Anaerobic digestionAmmoniaBioenergyBioammoniaHydrogenAnaerobic digestion-bioammoniato hydrogen (ADBH)* Corresponding author. Tel.: 1 614 688 404** Corresponding author. Tel.: 1 848 932 574E-mail addresses: prakash.31@osu.edu (S0961-9534/$ e see front matter 2013 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2013.05.a b s t r a c tDuring anaerobic digestion, organic matter is converted to carbon dioxide and methane,and organic nitrogen is converted to ammonia. Generally, ammonia is recycled as a fer-tilizer or removed via nitrificationedenitrification in treatment systems; alternatively itcould be recovered and catalytically converted to hydrogen, thus supplying additional fuel.To provide a basis for further investigation, a theoretical energy balance for a model sys-tem that incorporates anaerobic digestion, ammonia separation and recovery, and con-version of the ammonia to hydrogen is reported. The model Anaerobic Digestion-Bioammonia to Hydrogen (ADBH) system energy demands including heating, pumping,mixing, and ammonia reforming were subtracted from the total energy output frommethane and hydrogen to create an overall energy balance. The energy balance wasexamined for the ADBH system operating with a fixed feedstock loading rate with C:Nratios (gC/gN) ranging from 136 to 3 which imposed corresponding total ammonia nitrogen(TAN) concentrations of 20e10,000 mg/L. Normalizing total energy potential to themethane potential alone indicated that at a C:N ratio of 17, the energy output was greaterfor the ADBH system than from anaerobic digestion generating only methane. Decreasingthe C:N ratio increased themethane content of the biogas comprising primarily methane to>80% and increased the ammonia stripping energy demand. The system required 23e34%of the total energy generated as parasitic losses with no energy integration, but wheninternally produced heat and pressure differentials were recovered, parasitic losses werereduced to between 8 and 17%. 2013 Elsevier Ltd. All rights reserved.5; fax: 1 614 292 3163.8; fax: 1 732 932 8644.. Prakash), fennell@envsci.rutgers.edu (D.E. Fennell).ier Ltd. All rights reserved.024b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 54941. Introduction1.1. Anaerobic digestion processes and applicationsIn recent years, major attitudinal shifts have occurred inmodern society to allow wastes to be considered resourcesrather than just materials requiring disposal [1e3]. Waste as aresource can be envisioned as a means of reducing energyconsumption by minimizing the need for producing raw ma-terials, reducing pre-disposal processing, and extracting us-able energy from the waste as a feedstock [4e6]. Oneestablished technology for extracting energy from waste isanaerobic digestion (for recent reviews see Refs. [7e11]).Anaerobic digestion of crop biomass, agricultural wastesand residuals, and source-separated mixed organic wasteshave been employed at full scale for decades in Europe (nearly200 digesters, 2010 [12]), China (10,000 digesters, 1986 [13]) andIndia (2,000,000 digesters, 2000 [14]), among other places [15,16].These digesters operate to generate biogas, comprising pri-marily of methane, as a fuel source. In the US, anaerobicdigestion is primarily used for wastewater treatment plant(WWTP) sludges [17], animal manures [18], andmunicipal solidwaste (MSW) in landfills [19]. There aremore than 500 large (>5million gallons per day) municipal wastewater treatment fa-cilities [20] and 176 animalmanure digesters in the US [18]. Oneof the most prevalent large-scale applications of anaerobicdigestion in the US is in landfills where anaerobic conditionsdominate the operational timeline [21]. However, as of June2012, 594 of more than 1700 US landfills utilized biogas for en-ergy production while the remainder flared biogas withoutrecovering biogas energy [22]. The active landfill projects lead togeneration of over 1800 MW of equivalent energy [15].Compared to several other countries around the world,anaerobic digestion has been relatively under-utilized in theUS for a variety of economic and technical reasons. Theseinclude traditionally low energy and/or fuel prices, lack ofgovernmental incentives for implementing new anaerobicpower plants, the need for abundant and suitable land for sitedevelopment facilities and disposal of residuals, the need toprovide high quality reliable heat for the process to achieveacceptable or commercially viable conversion efficiencies,and the reputation of anaerobic processes as odor-generatingand difficult to operate [23e26]. The purpose of this paper is toshow through amodel system that when part of an integratedsystem, anaerobic digestion can be a powerful resource forwastemanagement and energy extraction. Specifically, in thispaper, a thermodynamic energy balance for amodel system ispresented demonstrating that anaerobic processing of wastefor harvesting both methane and ammonia as multiple fuelsources in contrast to methane alone can provide an addi-tional avenue to a net increase in extracted usable energyfrom waste processing.1.2. Inorganic nitrogen mitigation and removalThe environmental advantages of in-vessel anaerobic di-gesters include stabilization of biochemical oxygen demand,generation of biogas, production of digestate as a soilamendment, and reduction of the environmental footprintassociated with land-filling [15,27,28]. However, several envi-ronmental concerns as discussed below dictate post-treatment steps needed for the digestate produced duringanaerobic digestion of organic feedstocks [29,30]. Of particularconcern is ammonia which is toxic to aquatic organisms,causes eutrophication, and exerts oxygen demand in surfacewaters [31]. Further, processes to remove ammoniaenitrogenfrom aqueous effluent can require energy-intensive treatmentwith large reaction vessels and long holding times [32e34].Theammonia that accumulates inanaerobicdigesters existsin two forms, ammonium ion NH4 and free ammonia (NH3),and is in equilibrium in aqueous systems (Equation (1)) [17].NH44NH3 H (1)Total ammonia nitrogen (TAN) is the sum of NH4 and NH3expressed as total N on a mass basis. The ratio of NH3-N toNH4 N in an aqueous system is governed by pH and tem-perature (Equation (2)) [17].NH3 N TAN1 HKa (2)where NH3-N is the free ammonia nitrogen concentration andKa is the temperature dependent dissociation coefficient forEquation (2). TAN accumulates in digesters when proteins,urea, nucleic acids, and other nitrogen-containing com-pounds degrade, and its concentration must be controlled byremoval or by altering feedstock carbon to nitrogen (C:N) ra-tios to prevent inhibition of the microbial process by higherconcentrations of free ammonia [35,36].Anaerobic digestate is frequently used as a soil amend-ment. However, application of anaerobic digestate to land as asoil amendment must be carefully managed to avoid releaseof excess nitrogen to surface waters, infiltration to groundwater, and the atmosphere. Particularly affected by theseproblems are swine, poultry, and dairy operations, where landapplication of digestate is an important disposal route [37e43].Ma et al. (2005), for example, estimated that for TompkinsCounty, NY, USAwith a total dairy herd of 9500 approximately20,000 acres of suitable land would be needed to house di-gesters and solids/liquids handling systems and to provide aland sink for the resulting digestate [24]. With increasingpopulation pressures throughout the world, demand for suchlarge land resources can pose a significant problem for post-processing of N or ammonia-rich waste feedstock. In addi-tion, large domestic WWTP digesters located in metropolitanenvironments are often at considerable distances from suit-able land (>10 km). Consequently, these WWTPs must treatammonia onsite via nitrification/denitrification or haulnitrogen-rich digestate to distant land sinks for disposal [24]causing challenges for energy efficient waste management.Anaerobic digester supernatant currently recycled to theinfluent of some WWTPs may account for as much as 30% ofthe incoming nitrogen loading to the facility [44] and alsoconstitutes a substantial regulatory concern and energy sink[45]. Ammonia contained in leachate is also an importantfactor controlling the long-term monitoring and post-closureconcerns of MSW landfills [46].Conventional biological nutrient removal that combinesnitrification and denitrification requires long solids retentionb i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 495times (on the order of several hours to days) and energy-intensive aeration to accommodate nitrifying bacteria[32e34]. Further, denitrification mediated by heterotrophicbacteria may divert carbonaceous substrates from methanegeneration in digesters or require external electron donoraddition in wastewater applications [47]. Combined processesof partial nitrification of ammonium to nitrite followed bydenitrification of nitrite (e.g., Canon/Sharon Anammox pro-cesses [47e49]) have been developed to reduce energy andoxygen demands and thus eliminate the need for externalelectron donor addition [34]. However, these processes do noteliminate the treatment energy demands completely nor dothey allow for the extraction of ammonia as a potential fuel.Ammonia is a valuable industrial and agricultural chemicalused to produce fertilizer, solvents, cleaning agents, and re-frigerants [50]. Furthermore, ammonia has been proposed as apotential feedstock for hydrogen [51] due to the high density ofhydrogen per unitmass or volume of ammonia, and it can alsobe directly harvested for energy conversion in a direct-ammonia fuel cell [52e55]. Chemical routes for synthesis ofammonia tend to be energy and material intensive [56] andbio-ammonia as a sustainable fuel source is thereforereceiving increased interest for a variety of applications [57].As discussed above, ammonia has a high-density of hydrogenper unit volume on a weight basis of source material(w0.18 g H2/g NH3), and compares favorably to other materialsused for hydrogen storage [58]. However, high temperature(w800e900 C) is required for thermal reforming to generatehydrogen from ammonia i.e. energy input is needed to harvesthydrogen as a fuel.This paper evaluates whether ammonia liberated biologi-cally (bio-ammonia) during anaerobic digestion could be har-vested via stripping [59,60] and utilized as a source of hydrogenas part of a coupled Anaerobic Digester-Bioammonia toHydrogen (ADBH) system (Fig. 1), and be a beneficial operationalapproach in addition to harvesting methane from biogas.Fig. 1 e Model anaerobic digester used for developing thetheoretical model for analysis. This system is referred to asthe anaerobic digester for bioammonia to hydrogen(ADBH). The schematic shows flows of different streamswith details on each stream tabulated in Table 1. Thedotted line around the system represents the controlsurface for thermodynamic analyses.Ammonia-stripping has been used for treating animal wasteslurries [61e63], landfill leachate [64] and fertilizer plant wastes[65]; however, it has not been extensively studied as a meansof ammonia removal from digester effluents [66]. Strippinghas been shown to reduce ammonia in effluents to less than10 mg NH3-N/L [67]. Recovered ammonia gas could thereforebecome the reforming fuel for catalytic reforming as shown inEquation (3).2NH3(g) / N2(g) 3H2(g) (3)Thus, for an anaerobic digester producing biogas contain-ing methane and discharging digestate rich in TAN, the in-clusion of an ammonia recovery and reforming system togenerate hydrogen could allow additional biofuel or providean alternate route to harvesting an important industrialchemical in itself. Consequently, the specific purpose of thispaper is to develop a conceptual model of an ADBH systemgenerating usable energy by harvesting multiple fuel sourcestreams in biogas and validate this concept model through athermodynamic energy balance based on the first law forfeedstocks of varying C:N ratios. Therefore, this paper (1) es-tablishes a theoretical design scheme for an integrated systemto carry out anaerobic digestion and ammonia recovery todemonstrate a quantifiable increase in overall energy gener-ation from waste, (2) characterizes the energy demands andenergy production by focusing on two fuel sources in theforms of methane and hydrogen, (3) considers different casesof energy recovery within the integrated system to improvethe overall operation efficiency of the system from a net en-ergy output perspective, and (4) identifies areas for furtherscientific and engineering research needed to produce a net-positive energy ADBH system. As discussed above, the en-ergy balance estimates theoretical energy inputs and outputsbased on the first law of thermodynamics analyses and doesnot account for process entropy changes.2. Theoretical model framework2.1. Model ADBH system descriptionThe model ADBH system shown in Fig. 1 includes an anaer-obic digester that stabilizes waste, produces biogas containingmethane and carbon dioxide as the main constituents, anddischarges digestate containing TAN. Further, the systemutilizes a solideliquid separator to concentrate the solid con-tent in the digestate and produces liquid leachate containingTAN. In addition, two pH shift reactors were included, to firstincrease the pH of the leachate for converting TAN to NH3,then later to neutralize the pH for recycle back to the digester.After the leachate pH has been increased, gaseous ammonia isrecovered in a stripper. Finally, a combustion-based heatsource uses a fraction of the methane generated by thedigester as an energy source for ammonia reforming to pro-duce hydrogen as an additional fuel. Descriptions of the con-ceptual ADBH system flows indicated by arrows in Fig. 1 aresummarized in Table 1.Stream 1, the solid organic waste flow, and Stream 2, theinfluent additional liquid flow are the input streams to theTable 1 e Theoretical anaerobic digester-bioammonia tohydrogen (ADBH) system flow descriptions andconstraints.Number Type Phase Components1 Influent Solid Organic waste feedstock2 Influent Liquidb i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 497C62nH105nO42nNn 1:5 0:5nH2O nH/3:25 0:75n CH4 2:75 1:25nCO2 nNH4(6)where n, the stoichiometric amount of nitrogen contained inthe feedstock formula (mol N/mol feedstock), was computedfrom the C:N ratio of the influent feedstock by Equation (7).n 6$AWCarbonC : Nratio$AWNitrogen 2$AWCarbon (7)From Equations (6) and (7) it can be seen that there are 6molesC per mol of the organic portion of the feedstock and 2 addi-tional moles of C per mol of feedstock for each mole N added,AWCarbon is the atomic weight of C (12 g C/mol C), AWNitrogen isthe atomic weight of nitrogen (14 g N/mol N) and (C:N)ratio isthe carbon to nitrogen ratio (g C/g N) contained in the desig-nated feedstock.To fully account for carbon and nitrogen flows in themodel, microbial growth was also included [70] in a mannersimilar to other waste treatment mass balance models [71].The net fraction of the feedstock (C(62n)H(105n)O(42n)Nn)electron equivalents incorporated into new microbialbiomass, fs, was calculated according to Equation (8) [17,70,72].fs f 0s $1 1 fd$b$T1 b$T(8)where f 0s is the theoretical fraction of feedstock electrons usedfor synthesis and was taken to be 0.05 [72]; fd is the fraction ofthe microbial biomass that is biodegradable and was taken as0.80; b, the decay rate was 0.03 d1 [17]; and T, the retentiontime was 20 days. The net fraction of electrons used for en-ergy, fe, was fe 1 fs [17,70,72]. Incorporating fs and fe and cellsynthesis modified the overall digester reaction stoichiometry(Equation (9)) [17] toCaHbOcNd 2a d c 0:45efs 0:25efeH2O/0:125efeCH4 a c 0:2efs 0:125efeCO2 0:05efsC5H7O2N d 0:05efsNH4 d 0:05efsHCO3(9)where a (6 2n), b (10 5n), c (4 2n) and d (n) correspond to thefeedstockmaterial (C(62n)H(105n)O(42n)Nn), e 4a b 2c 3d,and themicrobial biomass was assumed to have themolecularformula C5H7O2N, which controlled the amount of N incorpo-rated during synthesis of new cells [17]. The TAN concentrationin the digestate under differing C:N ratios was equal to theNH4 N component of Equation (9). Further, the methanegenerated from each test condition was determined fromEquation (9).Calculations for external heating required to raise thetemperature of the influent material to 55 C and also tomaintain the digester at 55 C are described in Section S1 ofthe Supplementary Materials. The power required for me-chanically mixing the digester, which was assumed to be acontinuously stirred tank reactor (CSTR), is described in Sec-tion S2 of the Supplementary Materials. The solidseliquidseparation unit, handling digestate flow from the digester,was assumed to be a gravity separation unit operating at 95%efficiency [73]. The unit size was a function of the influentdigestate total solids content (%TS), the volumetric flow rate,the average assumed particle size (0.5 mm) [73], and thespecified solids removal efficiency (95%) [73]. Flow through theunit was assumed to be driven by the hydrostatic pressuredrop between the digester and the first pH-shift reactor.2.3. Ammonia gas recovery componentThe ammonia gas recovery system consisted of two pH-shiftreactors, an aqueous ammonia stripper, and Streams 5e14.The NH4 in the digestate leachate (Stream 5) was converted toNH3 in the first pH shift reactor where calcium hydroxide(lime) was added to increase the pH from 7 to 11 (Equations(10) and (11)).CaOH2/Ca2aq 2OHaqDH 16:7 kJ=mol (10)NH4aq OHaq/NH3aq H2ODH 366:6 kJ=mol (11)Heat generated from reactions described by Equations (10) and(11) is sufficient to increase the temperature of the aqueousstream entering the stripper to the solution boiling point fromthe digester discharge temperature of 55 C. NH3(aq) containedin Stream 6 is then stripped to the gas phase in the ammoniastripper using steam, produced from Stream 7, as the strip-ping gas. The alkaline, TAN reduced digestate leachate(Stream 8) flows to the second pH shift reactor and the pH isneutralized using the carbon dioxide in the biogas, Stream 11(Equation (12)).Ca2 2OH CO2/CaCO3 H2ODH97:10 kJ=mol (12)This neutralization process removes carbon dioxide from thebiogas stream and converts it to a dissolved form in theaqueous phase, subsequently producing a concentratedbiogas stream with higher methane content than comingdirectly from the digester (Stream 13). A finite energy input isrequired to operate the ammonia stripper and to homogenizethe reactants in the pH-shift reactors; however, excess pro-cess heat could be recovered from the reactions described byEquations (10) and (12). This is included in themodel based onthe enthalpies of each reaction as discussed below, whichassume that these reactions go forward to completion. Thereactions are assumed to reach completion based on availabledata (the concentrations of CaCO3 at pH 11 andOH at pH 7 areeach less than 107 M respectively), and this assumption isprovided to validate the reaction enthalpy values utilized.However, the model accounts for reaction equilibriums torectify stoichiometry and quantify mass balances. Thus, thecalculated heat values correspond to the predicted equilib-rium of each reaction for which kinetic and equilibrium datahas been previously reported.Elston and Karmarkar (2003) considered aqueous ammoniastripping technologies for selective catalytic reduction appli-cations (SCR) [67]. Comparing air and steam as strippingmedia they identified a linear relationship for power re-quirements for aqueous ammonia stripping (19 wt.% NH3(aq))for a range of aqueousmass flows [67]. Based on their findings,the power requirements for stripping ammonia from thedigestate leachate using either air or steam were estimated tobe 0.265 kJ/kg-aqueous mass treated (kJ/kg-aq.) or 1.574 kJ/kg-aq., respectively [67].b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5498Heat produced during each of the pH shifts was determinedusing the standard enthalpy of formation (DHf) for the reactantsand products [74,75]. Heat from these exothermic reactionswasassumed to be captured and re-used with an internal heattransfer efficiency of 35% [76]. In this work, the energy input foroperating the mechanical components required optimizingthis internal heat utilization, for example, through the use ofadditional pumping to and from a heat exchange unit, wasnot estimated. The sensitivity of this latter assumption forinfluencing the overall energy balance was assessed as part ofthe energy integration cases (see Section 2.7).Mixing of pH-shift reactors was assumed to occur via ve-locity gradient mixing with retention times of 30 s [77] and thepower requirementwas determined as described in Section S3of the Supplementary Materials.2.4. Ammonia reforming componentThe ammonia reforming component consisted of two sepa-rate processes. The first was a combustion-based heat sourceto heat a catalyst within a reformer where the incoming NH3(g)(Stream 10) is catalytically pyrolyzed to yield N2 and H2(Stream 18). A fraction of the concentrated methane biogas(Stream 13) is diverted from the splitter to the combustor(Stream 15), combined with air (Stream 17), and burned, pro-ducing combustion exhaust (Stream 19). The ammoniareforming reaction shown in Equation (13) is endothermic,requiring heat to proceed.2NH3g/N2g 3H2gDH 46 kJ=mol (13)The reaction at 850 C is approximately 90% efficient [78], andthe air stream was assumed to be at 15 C, thus requiringheating of 13.4 kJ/mol-NH3 converted for Equation (13). Theamount of biogas diverted to the combustor is dependent onthemethane content of the biogas, the energy demand for thesystem, NH3(g) flow rate (Stream 10), and the heating value ofmethane, assumed to be 50 MJ/kg (lower heating value ofmethane for combustion) [79].For all reactions listed, a single-step reaction mechanismwithout reaction intermediates or kinetic considerations isassumed. While the analytical model presented in this papercould be optimized further, it nevertheless provides severalvaluable insights into developing anaerobic digesters as apotential waste processing system to generate multiple fuelstreams.2.5. Stream transfer pressure-drop and pumpingrequirementsPower demands resulting from movement of internal gas andaqueous flows were estimated using mass and energy bal-ances for individual streams as described in Section S4 of theSupplementary Materials. The associated power as a functionof the required pressure drop and flow for each internalstream was assessed based on specific process configurationsas case-studies. Values for a specific process configuration orstream were input variables in the model allowing differentconfigurations and operating conditions to be tested analyti-cally. Although not a fluid stream, the energy of transportingthe feedstock (Stream 1) into the digester was calculated bydetermining the work required to raise its mass to the heightof the digester assuming the energy utilization was 35% effi-cient. The dimensions of the model system components weredetermined as described in Section S4 of the SupplementaryMaterials.2.6. Mass and energy balance modelMaterial and energy flows for the conceptual system were afunction of the input flows (Table 2), which were systemati-cally varied to estimate a system-wide energy balance for arange of input values as discussed later. Further, mass flows ofC and N were tracked through the system, and C and N as apercentage of the total C and N entering the system werecomputed for each component flow. The modeled outputswere a function of the system inputs (e.g. input flow rate, size,geometry, retention times, etc.), which were held constantwhile the feedstock C:N ratio was varied (Table 2). Results ofthe energy balance were expressed either as a function of theC:N ratio or as a function of the corresponding TAN asdescribed both in the text and accompanying figures. Tofurther evaluate the system, the methane plus hydrogen fromammonia reforming was classified as net usable energy asthese two materials would act as fuel sources. The energyconsumed by the ADBH system was normalized to themethane production potential for the feedstock at each C:Nratio as determined from Equation (9). It should be noted thatwhile the harvesting of bioammonia presents other avenuesfor useful resources either as direct fuel or an important in-dustrial chemical, those benefits are not accounted for in thenet usable energy estimates discussed in this work. Thenormalization to the methane production potential was cho-sen since methane is the most commonly and widely har-vested fuel source from anaerobic biogas and provides a readyreference for comparison. Therefore, the equivalent usableenergy was calculated as the heat generated during thecomplete burn of the total fuel (methane and hydrogen) dur-ing complete, single-step oxidation.2.7. Model application to different case scenariosThree test operating cases were examined for comparison(Table S1). For all cases, identical inputs (Table 2) were used,but the modeled outputs were calculated based on differentassumptions as discussed next. For Case 1, the heat generatedfrom chemical reactions and the hydrostatic pressure avail-able from each reactor was assumed to be recaptured at 35%efficiency to offset heating and pumping requirements else-where in the process. For Case 2, the same assumptions weremade as for Case 1 with an additional assumption that thesystem utilized the N2/H2 gas stream (Stream 18) as thestripping gas, reducing the stripping power requirementsfrom 1.574 to 0.265 kJ/kg-aq. In Case 3, no internal energy re-covery/integration was included.The thermodynamic energy balance model was then usedto identify the effect of the feedstock C:N ratio on the overallenergy balance for Cases 1, 2, and 3 (Table S1) under steady-state operation. A first law thermodynamic analysis wasconducted to estimate the theoretical maxima for the energyb i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 499output of the envisioned ADBH system to identify conditionswhere net energy output exceeds energy input and potentiallypresents a viable operating case. Other aspects of processviability, such as economics and feasibility of the design toaccount for irreversible (entropy) losses were not included inthe present analysis, but the currentmodel can be extended toenable further work to include a second law analysis and aneconomic analysis to demonstrate the viability of developingsuch systems for fuel generation from anaerobic wasteprocessing.Fig. 3 e ADBH mass percent flux of carbon with feedstockC:N ratio of 3.0. Thicker arrows represent streams withgreater mass flows while the non-bold text displays thebreakdown percentages of the bold category directly aboveit. Numbers in parentheses correspond to stream numberspresented in Fig. 1.3. Results and discussionThe energy balance of the ADBH system was determined forfeedstock C:N ratios (g C/g N) between 3.0 and 136, presentingthe lower and upper bounds for reasonable waste feedstock asa starting point. The relationship between the feedstock C:Nratio and corresponding digester TAN concentration is pre-sented in Fig. 2. Higher C:N ratioswere not considered becauseabove 136, the system nitrogen, even combined with the anaqueous nitrogen loading of 100 mg TAN/L, was too low tosupport microbial cell growth as predicted by Reaction (5).Similarly C:N ratios below 3 caused digester TAN to rise above10,000 mg/L, which would severely limit the viability ofanaerobic microorganisms [35,36]. Next a discussion of thethree specific test cases is presented.3.1. Mass balances on C and NThe effect of system energy integration as tested by comparingCases 1, 2, and 3 (see discussion below) did not change overallsystem mass fluxes of C and N. Thus, Figs. 3 and 4 show themass fluxes for C and N, respectively, for a C:N ratio of 3 as arepresentative example. The greatest mass percent C in theeffluent from the ADBH system was associated with carbonate(32.4%) and methane (31.1%). The remainder was associatedwith digestate (residual feedstock and biomass, 22.3%) andFig. 2 e Comparison of resulting digester TANconcentration and feedstock carbon to nitrogen (C:N) ratio.The limits of the C:N ratios were chosen on ability tosupport microbial growth and viability.carbon dioxide gas (14.2%). Note that the calcium carbonateprecipitate generated in the pH shift reactors accounted for9.9% of the carbon flow. The greatest mass percent N in theeffluent was associated with nitrogen gas (61.1%), and theremainder was associated with digestate (residual feedstockand biomass, 38.9%).3.2. Case 1 assessment (internal heat and pressuredifferentials recovered)3.2.1. Fuel, heat, and energy analysisAs the C:N ratio decreases from 136 to 3 (i.e., the organic ni-trogen content increases with fixed carbon content), theoperating TAN concentration in the digester increases fromFig. 4 e ADBH mass percent flux of nitrogen withfeedstock C:N ratio of 3.0. Thicker arrows correspond tostreams with greater mass flows. Numbers in parenthesescorrespond to stream numbers presented in Fig. 1.Fig. 5 e Net system energy generation attributable tomethane and hydrogen respectively as a function ofsteady-state digester TAN concentration (TAN is a functionof feedstock C:N ratio, as shown in Fig. 2).b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 550020 mg/L to 10,000 mg/L. The model predicts that as the oper-ating TAN concentration in the digester increases, morehydrogen is recovered from the ammonia reformer and moreheat is recovered from the pH-shift reactors. Biofuel energyfromhydrogen increases from 0 to 2.5MJ-biofuel/kg-feedstockand heat output from the pH shift reactors increases from 0.64to 1.02 MJ-heat/kg-feedstock.The heat energy required to operate the digester (to in-crease inlet feed flow temperature from 15 C and to maintainthe digester at 55 C) was not offset by the heat generated inthe pH-shift reactors for TAN concentrations below 7500mg/L(i.e., C:N ratios > 4.3). Thus, the system requires external heatfor operation up to a TAN concentration of 7500 mg/L. Thisexternal heat input decreases as the operating TAN concen-tration in the digester increases, and the need for externalheating becomesminimal at TAN concentrations of 7500mg/Lbecause the heat produced in the pH-shift reactors is a func-tion of the TAN concentration.The mass flow rate of ammonia (kg/h) recovered is pro-portional to TAN concentration increases and the power inputrequired to strip the dissolved ammonia from the aqueousphase. However, since the stripping power required is afunction of the total aqueous mass flow (10,047 kg/h), themarginal increase in the ammonia to be recovered has aminimal impact (Fig. 6 e a) Potential methane production rate as a functionof feedstock stoichiometry controlled by feedstock C:Nratio; b) Energy from methane and hydrogen as apercentage of the potential methane production versusoperating digester TAN concentration (TAN is a function offeedstock C:N ratio).b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 501favorable for feedstock C:N ratios greater than 31 (correspondsto digester TAN less than 1000mg/L). In general, operating theADBH system at TAN concentrations above 1000 mg/L (C:Nless than 31) improves the overall energy available by asmuchas 1.8% at 1000 mg/L and 26.3% at 10,000 mg/L compared toanaerobic digestion with methane recovery alone.3.2.2. Influent feedstock effects on digester TAN and biogasqualityThe effect of the influent feedstock C:N ratio on digester TANand effluent biogas quality was analyzed by assuming andholding constant one set of specific operating conditions(Table 2). Although the operating TAN concentration in thedigester is used as a reference for the energy outputs in thisanalysis, it is the C:N ratio of the dry solids flow that controlsTAN since other inputs are held constant. The designatedwaste stream inputs and operating parameters were based ontypical design values [73] that contain a solid fraction of 10% orless of the total incoming volume. Hence the specific heatingproperties for the waste stream were approximated to be thatof liquid water in agreement with previously establishedmethods [73]. The operating TAN concentration was used as areference because high TAN concentrations (above5000e6000 mg/L) have been reported to decrease methano-genesis as a result of toxicity and inhibition of the microbialcommunity [80]. A C:N ratio between 6.8 and 5.6 wouldcorrespond to digester TAN concentration greater than5000e6000 mg/L based on the input parameters to the modelused in this paper. One caveat should be noted with respect tothe energy balance reported here. The presentmodel does notexplicitly evaluate process trade-offs to account for the rela-tionship between the feedstock C:N ratio and the operatingTAN concentration in the digester which are a function of thesolids flow rate, recycle flow rate, set moisture content in thedigester, retention time, and digester aspect ratio. While sucha detailed system optimization will be no doubt useful it isbeyond the scope of the current effort. Specifically, for an in-dividual C:N ratio (even for low ratios, C:N 1e3), the designcharacteristics of the system could be adjusted to maintain aviable operating TAN concentration (and corresponding freeammonia concentration, the form of TAN thought to imposetoxicity [81,82]) in the digester such that TAN levels would notbecome inhibitory. It is expected that this work will spurfurther debate in the scientific community to design and buildADBH systems accounting for complete system operation andoptimization.The C:N ratio of the feedstock also affects the systembiogas composition by controlling the amount of carbon di-oxide removed from Stream 11 in the second pH-shift reactor.As the C:N ratio increases over the considered range, themolar percent of carbon dioxide removed in the second pHshift reactor rose from 22.0% to 48.2%. The biogas compositionin Stream 11 thus varied from 54.3% to 62.8% methane as theC:N ratio was decreased. For feedstock C:N ratios of 136 and 3,the methane content leaving the second pH-shift (Stream 13)was thus predicted to be 64.3% and 80.7% methane, respec-tively. This result presents another tradeoff to be considered.If purified methane were the desired downstream product,decreasing the C:N ratio of the influent solids material streamcan be seen as a means of improving downstream biogasmethane enrichment. Such an approach would impact theenergy balance for a system seeking to produce a nearly puremethane stream with no carbon dioxide.3.2.3. Power and pressure drop requirementsRequired pressure drops to power fluid flow, compensating forfrictional energy losses along the length of the pipe, wereincluded in the model (see Section S4 and Table S1 of theSupplementary Materials) based on well-established pipeflow models [83]. The magnitude of the frictional losses andcorresponding power required to maintain the calculatedpressure drop was almost exclusively a function of the spec-ified influent feedstock mass flow rate, which was held con-stant throughout the analysis. Thus, it was not unexpectedthat the total power requirements for pumping (sum of thehydrostatic and frictional power requirements), mixing, andFig. 7 e Normalized total equivalent energy generated byADBH system as a function of the operating TANb i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5502ammonia stripping increased by less than 2% (from 86.4 to87.5 kW) as the C:N ratio decreased from 136 to 3. A compar-ison of the relative power demands for pumping, mixing, andammonia stripping showed that the mixing in the digesterand pH-shift reactors required the most power (Table 3). Thesum of the power to sustain mixing in the digester and pH-shift reactors, maintain the pressure drop and facilitateammonia stripping from the aqueous stream was two ordersofmagnitude smaller than the equivalent energy generated asfuel (methane rich biogas and ammonia reformed tohydrogen) and heat. Subtracting the total power required forthese demands from the net energy equivalent generatedconsequently had a relatively minor impact on the ADBHsystem energy generation potential.The overall process fuel and heat energy generation as afunction of the influent solids C:N ratio is shown in Fig. 7. Thenormalized output indicates that above an operating digesterTAN concentration of approximately 2000 mg/L, the totalbiofuel energy (CH4 and H2) output is greater than could beexpected to be produced from anaerobic digestion (CH4) aloneproviding theoretical evidence for an increase in net usableenergy for implementing an integrated ADBH system.concentration in the digester (TAN is a function offeedstock C:N ratio). The solid line represents the expectedexternally usable energy produced by anaerobic digestionalone.3.3. Effects of system energy integration: comparison ofcases 1, 2, and 3The effects of various energy integration scenarios (Table S3)were assessed by comparing the fraction of the energygenerated diverted to power the integrated process as afunction of the operating digester TAN concentration. Thepercent of the energy required increased as the influentfeedstock C:N ratio decreased (corresponding TAN concen-trations in the digester increased) for all cases becausepumping and mixing requirements downstream from thedigester were a function of the mass flow of ammonia. Addi-tionally, the energy content of the feedstock decreased as theC:N ratio decreased, allowing less methane to be generated(Fig. 6a) from approximately 18 kmol/h for a C:N ratio of 136decreasing to nearly 7 kmol/h for a C:N ratio of 3. The totalpercentage of energy diverted remained under 35% for allcases and feedstock C:N ratios. The ADBH system operatedunder the Case 1 and Case 2 scenarios did not generate greateramounts of methane or hydrogen than when operated underthe Case 3 scenario, but as expected, less biofuel (methaneTable 3 eADBH fractional power requirements: demandsfor pumping, mixing and ammonia stripping, as well asrelative requirements of mixing in the digester and pH-shift reactors.Process Fractional power requirementMixing 83%Digester 42%pH-Shift 1 29%pH-Shift 2 29%Stripping 15%Pumping 2%and/or hydrogen) was diverted to provide energy needed bythe ADBH system.Case 3 had the highest fraction (23e35%) of its biofuel beingdiverted to power various systemdemands for all feedstock C:Nratios because available energy integration was not consideredin this case, i.e., heat generated and available hydrostaticpressures to facilitate stream flow were not utilized. With noenergy recovery, the system losses were 23 to 35% of the totalenergy generated (fraction of methane and hydrogen).Recovery of heat and available hydrostatic pressure wereconsidered for integration in Case 1 and Case 2. Case 2 differedfrom Case 1 in that it also utilized a fraction of the availablehydrogen and nitrogen stream (Stream 18) as the stripping gasas opposed to steam alone. For Case 1, liquid vaporization andstripping required 1.574 kJ/kg-aq., whereas for Case 2, usingthe available gas stream eliminated the need for vaporizationto produce steam, and ammonia stripping thus required only0.265 kJ/kg-aq, improving the amount of energy recoverednearly 7 times. When internally produced heat and pressuredifferentials were recovered (Case 1), system losses werereduced to between 8 and 17% in contrast to a high of 35% forno energy recovery. The energy required for ammonia strip-ping in Case 2 assumed that the hydrogen and nitrogenstream (Stream 18) could strip ammonia from the liquidstream as effectively as air. Using steam as the stripping gasallows ammonia to be concentrated by allowing steam to beselectively removed via condensation. It is likely that theminimal (b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 5033.4. Summary, implications and areas for furtherinvestigationThe ADBH system energy balance remained positive (gener-ating net usable energy in contrast to energy consumed) whiledigesting biomass, removing aqueous ammonia, and reform-ing ammonia gas to produce hydrogen for all feedstock C:Nratios considered, but the energy balance favored C:N ratioslower than 31. The model did not account for microbialsensitivity to aqueous TAN concentrations, entropic losses,and, unless otherwise specified, assumed 1-step reactions andArrhenius kinetics. Feedstocks C:N ratios (below 4.0) might bebetter managed by employing a separate hydrolysis-fermentor reactor upstream from the digester. Ammoniarelease during hydrolysis occurs more rapidly than otheranaerobic processes in the digester [68], and operating aseparate hydrolysis reactor could potentially reduce nitrogenloading in the downstream digester. This could preventammonia toxicity and improve digester stability andmethanegeneration, while still allowing bioammonia to hydrogenconversion.As the model and discussion above illustrates, moreresearch needs to be conducted before full-scale ADBH sys-tems could be pursued. The cost for constructing and oper-ating ADBH systems versus the relative benefits of energygeneration and nitrogen removal need to be assessed andevaluated in the context of alternative methods of disposingof TAN. Since aqueous nitrogen species are typically a concernfor digester effluents and may require significant subsequenttreatment, the energy and economic savings of using ADBHsystems versus conventional digestion and downstream ni-trogen mitigation processes can be important. Biologicalmediation of aqueous nitrogen species are typically aerobicprocesses that require organic substrate, and do not offer theprospect of energy recovery. In fact, aeration typically is thegreatest energy sink at wastewater treatment plants and theorganic substrate required could be viewed as substrate that isdiverted from methane generating anaerobic processes.Finally, though not considered explicitly in this work, har-vesting of ammonia as a direct fuel source or chemical sourcecan also be pursued, providing additional avenues to managewaste streams and eliminating the need to divert methane forinternal processes as in the present ADBH system.4. 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