anaerobic digestion for methane generation and ammonia .anaerobic digestion for methane generation

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Anaerobic digestion for methane generation andammonia reforming for hydrogen production:A thermodynamic energy balance of a modelsystem to demonstrate net energy feasibility

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,

United StatesbDepartment of Mechanical and Aerospace Engineering, The Ohio State University, 201 W. 19th Ave., Columbus,

OH 43210, United States

a r t i c l e i n f o

Article history:

Received 6 October 2012

Received in revised form

6 May 2013

Accepted 24 May 2013

Available online

Keywords:

Anaerobic digestion

Ammonia

Bioenergy

Bioammonia

Hydrogen

Anaerobic digestion-bioammonia

to hydrogen (ADBH)

* Corresponding author. Tel.: 1 614 688 404** Corresponding author. Tel.: 1 848 932 574

E-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 t

During 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 it

could 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 from

methane and hydrogen to create an overall energy balance. The energy balance was

examined for the ADBH system operating with a fixed feedstock loading rate with C:N

ratios (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 the

methane potential alone indicated that at a C:N ratio of 17, the energy output was greater

for the ADBH system than from anaerobic digestion generating only methane. Decreasing

the 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 when

internally produced heat and pressure differentials were recovered, parasitic losses were

reduced 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.024

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1. Introduction

1.1. Anaerobic digestion processes and applications

In recent years, major attitudinal shifts have occurred in

modern society to allow wastes to be considered resources

rather than just materials requiring disposal [1e3]. Waste as a

resource can be envisioned as a means of reducing energy

consumption 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]. One

established technology for extracting energy from waste is

anaerobic digestion (for recent reviews see Refs. [7e11]).

Anaerobic digestion of crop biomass, agricultural wastes

and residuals, and source-separated mixed organic wastes

have been employed at full scale for decades in Europe (nearly

200 digesters, 2010 [12]), China (10,000 digesters, 1986 [13]) and

India (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, anaerobic

digestion is primarily used for wastewater treatment plant

(WWTP) sludges [17], animal manures [18], andmunicipal solid

waste (MSW) in landfills [19]. There aremore than 500 large (>5

million gallons per day) municipal wastewater treatment fa-

cilities [20] and 176 animalmanure digesters in the US [18]. One

of the most prevalent large-scale applications of anaerobic

digestion in the US is in landfills where anaerobic conditions

dominate the operational timeline [21]. However, as of June

2012, 594 of more than 1700 US landfills utilized biogas for en-

ergy production while the remainder flared biogas without

recovering biogas energy [22]. The active landfill projects lead to

generation of over 1800 MW of equivalent energy [15].

Compared to several other countries around the world,

anaerobic digestion has been relatively under-utilized in the

US for a variety of economic and technical reasons. These

include traditionally low energy and/or fuel prices, lack of

governmental incentives for implementing new anaerobic

power plants, the need for abundant and suitable land for site

development facilities and disposal of residuals, the need to

provide high quality reliable heat for the process to achieve

acceptable or commercially viable conversion efficiencies,

and the reputation of anaerobic processes as odor-generating

and difficult to operate [23e26]. The purpose of this paper is to

show through amodel system that when part of an integrated

system, anaerobic digestion can be a powerful resource for

wastemanagement and energy extraction. Specifically, in this

paper, a thermodynamic energy balance for amodel system is

presented demonstrating that anaerobic processing of waste

for harvesting both methane and ammonia as multiple fuel

sources in contrast to methane alone can provide an addi-

tional avenue to a net increase in extracted usable energy

from waste processing.

1.2. Inorganic nitrogen mitigation and removal

The environmental advantages of in-vessel anaerobic di-

gesters include stabilization of biochemical oxygen demand,

generation of biogas, production of digestate as a soil

amendment, and reduction of the environmental footprint

associated with land-filling [15,27,28]. However, several envi-

ronmental concerns as discussed below dictate post-

treatment steps needed for the digestate produced during

anaerobic digestion of organic feedstocks [29,30]. Of particular

concern is ammonia which is toxic to aquatic organisms,

causes eutrophication, and exerts oxygen demand in surface

waters [31]. Further, processes to remove ammoniaenitrogen

from aqueous effluent can require energy-intensive treatment

with large reaction vessels and long holding times [32e34].

Theammonia that accumulates inanaerobicdigesters exists

in 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 to

NH4 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 and

Ka is the temperature dependent dissociation coefficient for

Equation (2). TAN accumulates in digesters when proteins,

urea, nucleic acids, and other nitrogen-containing com-

pounds degrade, and its concentration must be controlled by

removal or by altering feedstock carbon to nitrogen (C:N) ra-

tios to prevent inhibition of the microbial process by higher

concentrations of free ammonia [35,36].

Anaerobic digestate is frequently used as a soil amend-

ment. However, application of anaerobic digestate to land as a

soil amendment must be carefully managed to avoid release

of excess nitrogen to surface waters, infiltration to ground

water, and the atmosphere. Particularly affected by these

problems are swine, poultry, and dairy operations, where land

application of digestate is an important disposal route [37e43].

Ma et al. (2005), for example, estimated that for Tompkins

County, NY, USAwith a total dairy herd of 9500 approximately

20,000 acres of suitable land would be needed to house di-

gesters and solids/liquids handling systems and to provide a

land sink for the resulting digestate [24]. With increasing

population pressures throughout the world, demand for such

large 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 metropolitan

environments are often at considerable distances from suit-

able land (>10 km). Consequently, these WWTPs must treat

ammonia onsite via nitrification/denitrification or haul

nitrogen-rich digestate to distant land sinks for disposal [24]

causing challenges for energy efficient waste management.

Anaerobic digester supernatant currently recycled to the

influent of some WWTPs may account for as much as 30% of

the incoming nitrogen loading to the facility [44] and also