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

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<ul><li><p></p><p>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</p><p>Available online at w</p><p>http: / /www.elsevier .com/locate/biombioe</p><p>Anaerobic digestion for methane generation andammonia reforming for hydrogen production:A thermodynamic energy balance of a modelsystem to demonstrate net energy feasibility</p><p>David 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,</p><p>United StatesbDepartment of Mechanical and Aerospace Engineering, The Ohio State University, 201 W. 19th Ave., Columbus,</p><p>OH 43210, United States</p><p>a r t i c l e i n f o</p><p>Article history:</p><p>Received 6 October 2012</p><p>Received in revised form</p><p>6 May 2013</p><p>Accepted 24 May 2013</p><p>Available online</p><p>Keywords:</p><p>Anaerobic digestion</p><p>Ammonia</p><p>Bioenergy</p><p>Bioammonia</p><p>Hydrogen</p><p>Anaerobic digestion-bioammonia</p><p>to hydrogen (ADBH)</p><p>* Corresponding author. Tel.: 1 614 688 404** Corresponding author. Tel.: 1 848 932 574</p><p>E-mail addresses: (S0961-9534/$ e see front matter 2013 Elsev</p><p>a b s t r a c t</p><p>During anaerobic digestion, organic matter is converted to carbon dioxide and methane,</p><p>and organic nitrogen is converted to ammonia. Generally, ammonia is recycled as a fer-</p><p>tilizer or removed via nitrificationedenitrification in treatment systems; alternatively it</p><p>could be recovered and catalytically converted to hydrogen, thus supplying additional fuel.</p><p>To provide a basis for further investigation, a theoretical energy balance for a model sys-</p><p>tem that incorporates anaerobic digestion, ammonia separation and recovery, and con-</p><p>version of the ammonia to hydrogen is reported. The model Anaerobic Digestion-</p><p>Bioammonia to Hydrogen (ADBH) system energy demands including heating, pumping,</p><p>mixing, and ammonia reforming were subtracted from the total energy output from</p><p>methane and hydrogen to create an overall energy balance. The energy balance was</p><p>examined for the ADBH system operating with a fixed feedstock loading rate with C:N</p><p>ratios (gC/gN) ranging from 136 to 3 which imposed corresponding total ammonia nitrogen</p><p>(TAN) concentrations of 20e10,000 mg/L. Normalizing total energy potential to the</p><p>methane potential alone indicated that at a C:N ratio of 17, the energy output was greater</p><p>for the ADBH system than from anaerobic digestion generating only methane. Decreasing</p><p>the C:N ratio increased themethane content of the biogas comprising primarily methane to</p><p>&gt;80% and increased the ammonia stripping energy demand. The system required 23e34%</p><p>of the total energy generated as parasitic losses with no energy integration, but when</p><p>internally produced heat and pressure differentials were recovered, parasitic losses were</p><p>reduced to between 8 and 17%.</p><p> 2013 Elsevier Ltd. All rights reserved.</p><p>5; fax: 1 614 292 3163.8; fax: 1 732 932 8644.. Prakash), (D.E. Fennell).ier Ltd. All rights reserved.024</p></li><li><p>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 5494</p><p>1. Introduction</p><p>1.1. Anaerobic digestion processes and applications</p><p>In recent years, major attitudinal shifts have occurred in</p><p>modern society to allow wastes to be considered resources</p><p>rather than just materials requiring disposal [1e3]. Waste as a</p><p>resource can be envisioned as a means of reducing energy</p><p>consumption by minimizing the need for producing raw ma-</p><p>terials, reducing pre-disposal processing, and extracting us-</p><p>able energy from the waste as a feedstock [4e6]. One</p><p>established technology for extracting energy from waste is</p><p>anaerobic digestion (for recent reviews see Refs. [7e11]).</p><p>Anaerobic digestion of crop biomass, agricultural wastes</p><p>and residuals, and source-separated mixed organic wastes</p><p>have been employed at full scale for decades in Europe (nearly</p><p>200 digesters, 2010 [12]), China (10,000 digesters, 1986 [13]) and</p><p>India (2,000,000 digesters, 2000 [14]), among other places [15,16].</p><p>These digesters operate to generate biogas, comprising pri-</p><p>marily of methane, as a fuel source. In the US, anaerobic</p><p>digestion is primarily used for wastewater treatment plant</p><p>(WWTP) sludges [17], animal manures [18], andmunicipal solid</p><p>waste (MSW) in landfills [19]. There aremore than 500 large (&gt;5</p><p>million gallons per day) municipal wastewater treatment fa-</p><p>cilities [20] and 176 animalmanure digesters in the US [18]. One</p><p>of the most prevalent large-scale applications of anaerobic</p><p>digestion in the US is in landfills where anaerobic conditions</p><p>dominate the operational timeline [21]. However, as of June</p><p>2012, 594 of more than 1700 US landfills utilized biogas for en-</p><p>ergy production while the remainder flared biogas without</p><p>recovering biogas energy [22]. The active landfill projects lead to</p><p>generation of over 1800 MW of equivalent energy [15].</p><p>Compared to several other countries around the world,</p><p>anaerobic digestion has been relatively under-utilized in the</p><p>US for a variety of economic and technical reasons. These</p><p>include traditionally low energy and/or fuel prices, lack of</p><p>governmental incentives for implementing new anaerobic</p><p>power plants, the need for abundant and suitable land for site</p><p>development facilities and disposal of residuals, the need to</p><p>provide high quality reliable heat for the process to achieve</p><p>acceptable or commercially viable conversion efficiencies,</p><p>and the reputation of anaerobic processes as odor-generating</p><p>and difficult to operate [23e26]. The purpose of this paper is to</p><p>show through amodel system that when part of an integrated</p><p>system, anaerobic digestion can be a powerful resource for</p><p>wastemanagement and energy extraction. Specifically, in this</p><p>paper, a thermodynamic energy balance for amodel system is</p><p>presented demonstrating that anaerobic processing of waste</p><p>for harvesting both methane and ammonia as multiple fuel</p><p>sources in contrast to methane alone can provide an addi-</p><p>tional avenue to a net increase in extracted usable energy</p><p>from waste processing.</p><p>1.2. Inorganic nitrogen mitigation and removal</p><p>The environmental advantages of in-vessel anaerobic di-</p><p>gesters include stabilization of biochemical oxygen demand,</p><p>generation of biogas, production of digestate as a soil</p><p>amendment, and reduction of the environmental footprint</p><p>associated with land-filling [15,27,28]. However, several envi-</p><p>ronmental concerns as discussed below dictate post-</p><p>treatment steps needed for the digestate produced during</p><p>anaerobic digestion of organic feedstocks [29,30]. Of particular</p><p>concern is ammonia which is toxic to aquatic organisms,</p><p>causes eutrophication, and exerts oxygen demand in surface</p><p>waters [31]. Further, processes to remove ammoniaenitrogen</p><p>from aqueous effluent can require energy-intensive treatment</p><p>with large reaction vessels and long holding times [32e34].</p><p>Theammonia that accumulates inanaerobicdigesters exists</p><p>in two forms, ammonium ion NH4 and free ammonia (NH3),and is in equilibrium in aqueous systems (Equation (1)) [17].</p><p>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 to</p><p>NH4 N in an aqueous system is governed by pH and tem-perature (Equation (2)) [17].</p><p>NH3 N TAN1 HKa</p><p> (2)</p><p>where NH3-N is the free ammonia nitrogen concentration and</p><p>Ka is the temperature dependent dissociation coefficient for</p><p>Equation (2). TAN accumulates in digesters when proteins,</p><p>urea, nucleic acids, and other nitrogen-containing com-</p><p>pounds degrade, and its concentration must be controlled by</p><p>removal or by altering feedstock carbon to nitrogen (C:N) ra-</p><p>tios to prevent inhibition of the microbial process by higher</p><p>concentrations of free ammonia [35,36].</p><p>Anaerobic digestate is frequently used as a soil amend-</p><p>ment. However, application of anaerobic digestate to land as a</p><p>soil amendment must be carefully managed to avoid release</p><p>of excess nitrogen to surface waters, infiltration to ground</p><p>water, and the atmosphere. Particularly affected by these</p><p>problems are swine, poultry, and dairy operations, where land</p><p>application of digestate is an important disposal route [37e43].</p><p>Ma et al. (2005), for example, estimated that for Tompkins</p><p>County, NY, USAwith a total dairy herd of 9500 approximately</p><p>20,000 acres of suitable land would be needed to house di-</p><p>gesters and solids/liquids handling systems and to provide a</p><p>land sink for the resulting digestate [24]. With increasing</p><p>population pressures throughout the world, demand for such</p><p>large land resources can pose a significant problem for post-</p><p>processing of N or ammonia-rich waste feedstock. In addi-</p><p>tion, large domestic WWTP digesters located in metropolitan</p><p>environments are often at considerable distances from suit-</p><p>able land (&gt;10 km). Consequently, these WWTPs must treat</p><p>ammonia onsite via nitrification/denitrification or haul</p><p>nitrogen-rich digestate to distant land sinks for disposal [24]</p><p>causing challenges for energy efficient waste management.</p><p>Anaerobic digester supernatant currently recycled to the</p><p>influent of some WWTPs may account for as much as 30% of</p><p>the incoming nitrogen loading to the facility [44] and also</p><p>constitutes a substantial regulatory concern and energy sink</p><p>[45]. Ammonia contained in leachate is also an important</p><p>factor controlling the long-term monitoring and post-closure</p><p>concerns of MSW landfills [46].</p><p>Conventional biological nutrient removal that combines</p><p>nitrification and denitrification requires long solids retention</p></li><li><p>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 5 495</p><p>times (on the order of several hours to days) and energy-</p><p>intensive aeration to accommodate nitrifying bacteria</p><p>[32e34]. Further, denitrification mediated by heterotrophic</p><p>bacteria may divert carbonaceous substrates from methane</p><p>generation in digesters or require external electron donor</p><p>addition in wastewater applications [47]. Combined processes</p><p>of partial nitrification of ammonium to nitrite followed by</p><p>denitrification of nitrite (e.g., Canon/Sharon Anammox pro-</p><p>cesses [47e49]) have been developed to reduce energy and</p><p>oxygen demands and thus eliminate the need for external</p><p>electron donor addition [34]. However, these processes do not</p><p>eliminate the treatment energy demands completely nor do</p><p>they allow for the extraction of ammonia as a potential fuel.</p><p>Ammonia is a valuable industrial and agricultural chemical</p><p>used to produce fertilizer, solvents, cleaning agents, and re-</p><p>frigerants [50]. Furthermore, ammonia has been proposed as a</p><p>potential feedstock for hydrogen [51] due to the high density of</p><p>hydrogen per unitmass or volume of ammonia, and it can also</p><p>be directly harvested for energy conversion in a direct-</p><p>ammonia fuel cell [52e55]. Chemical routes for synthesis of</p><p>ammonia tend to be energy and material intensive [56] and</p><p>bio-ammonia as a sustainable fuel source is therefore</p><p>receiving increased interest for a variety of applications [57].</p><p>As discussed above, ammonia has a high-density of hydrogen</p><p>per unit volume on a weight basis of source material</p><p>(w0.18 g H2/g NH3), and compares favorably to other materials</p><p>used for hydrogen storage [58]. However, high temperature</p><p>(w800e900 C) is required for thermal reforming to generatehydrogen from ammonia i.e. energy input is needed to harvest</p><p>hydrogen as a fuel.</p><p>This paper evaluates whether ammonia liberated biologi-</p><p>cally (bio-ammonia) during anaerobic digestion could be har-</p><p>vested via stripping [59,60] and utilized as a source of hydrogen</p><p>as part of a coupled Anaerobic Digester-Bioammonia to</p><p>Hydrogen (ADBH) system (Fig. 1), and be a beneficial operational</p><p>approach in addition to harvesting methane from biogas.</p><p>Fig. 1 e Model anaerobic digester used for developing the</p><p>theoretical model for analysis. This system is referred to as</p><p>the anaerobic digester for bioammonia to hydrogen</p><p>(ADBH). The schematic shows flows of different streams</p><p>with details on each stream tabulated in Table 1. The</p><p>dotted line around the system represents the control</p><p>surface for thermodynamic analyses.</p><p>Ammonia-stripping has been used for treating animal waste</p><p>slurries [61e63], landfill leachate [64] and fertilizer plant wastes</p><p>[65]; however, it has not been extensively studied as a means</p><p>of ammonia removal from digester effluents [66]. Stripping</p><p>has been shown to reduce ammonia in effluents to less than</p><p>10 mg NH3-N/L [67]. Recovered ammonia gas could therefore</p><p>become the reforming fuel for catalytic reforming as shown in</p><p>Equation (3).</p><p>2NH3(g) / N2(g) 3H2(g) (3)</p><p>Thus, for an anaerobic digester producing biogas contain-</p><p>ing methane and discharging digestate rich in TAN, the in-</p><p>clusion of an ammonia recovery and reforming system to</p><p>generate hydrogen could allow additional biofuel or provide</p><p>an alternate route to harvesting an important industrial</p><p>chemical in itself. Consequently, the specific purpose of this</p><p>paper is to develop a conceptual model of an ADBH system</p><p>generating usable energy by harvesting multiple fuel source</p><p>streams in biogas and validate this concept model through a</p><p>thermodynamic energy balance based on the first law for</p><p>feedstocks of varying C:N ratios. Therefore, this paper (1) es-</p><p>tablishes a theoretical design scheme for an integrated system</p><p>to carry out anaerobic digestion and ammonia recovery to</p><p>demonstrate a quantifiable increase in overall energy gener-</p><p>ation from waste, (2) characterizes the energy demands and</p><p>energy production by focusing on two fuel sources in the</p><p>forms of methane and hydrogen, (3) considers different cases</p><p>of energy recovery within the integrated system to improve</p><p>the overall operation efficiency of the system from a net en-</p><p>ergy output perspective, and (4) identifies areas for further</p><p>scientific and engineering research needed to produce a net-</p><p>positive energy ADBH system. As discussed above, the en-</p><p>ergy balance estimates theoretical energy inputs and outputs</p><p>based on the first law of thermodynamics analyses and does</p><p>not account for process entropy changes.</p><p>2. Theoretical model framework</p><p>2.1. Model ADBH system description</p><p>The model ADBH system shown in Fig. 1 includes an anaer-</p><p>obic digester that stabilizes waste, produces biogas containing</p><p>methane and carbon dioxide as the main constituents, and</p><p>discharges digestate containing TAN. Further, the system</p><p>utilizes a solideliquid separator to concentrate the solid con-</p><p>tent in the digestate and produces liquid leachate containing</p><p>TAN. In addition, two pH shift reactors were included, to first</p><p>increase the pH of the leachate for converting TAN to NH3,</p><p>then later to neutralize the pH for recycle back to the digester.</p><p>After the leachate pH has been increased, gaseous ammonia is</p><p>recovered in a stripper. Finally, a combustion-based heat</p><p>source uses a fraction of the methane generated by the</p><p>digester as an energy source for ammonia reforming to pro-</p><p>duce hydrogen as an additional fuel. Descriptions of the con-</p><p>ceptual ADBH system flows indicated by arrows in Fig. 1 are</p><p>summarized in Table 1.</p><p>Stream 1, the solid organic waste flow, and Stream 2, the</p><p>influent additional liquid flow are the input streams to the</p></li><li><p>Table 1 e Theoretic...</p></li></ul>