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Avai lab le a t www.sc iencedi rec t .com

ht tp : / /www.e lsev ier . com/ loca te /b iombioe

Modeling the performance of the anaerobic phased solidsdigester system for biogas energy production

Joshua L. Rapport a,*, Ruihong Zhang a, Bryan M. Jenkins a, Bruce R. Hartsough a,Thomas P. Tomich b

aDepartment of Biological and Agricultural Engineering, University of California at Davis, 1 Shields Avenue, Davis,

CA 95616, USAbAgricultural Sustainability Institute, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA

a r t i c l e i n f o

Article history:

Received 27 October 2009

Received in revised form

14 December 2010

Accepted 15 December 2010

Available online 20 January 2011

Keywords:

Anaerobic digestion

Biogas

CNG

Waste treatment

Financial modeling

Renewable energy

* Corresponding author. Tel.: þ1 (530) 752 01E-mail addresses: jlrapport@ucdavis.edu

brhartsough@ucdavis.edu (B.R. Hartsough),0961-9534/$ e see front matter ª 2010 Publidoi:10.1016/j.biombioe.2010.12.021

a b s t r a c t

Aprocessmodelwasdevelopedtopredict themassandenergybalancefora full-scale (115 t d�1)

high-solids anaerobic digester using research data from lab and pilot scale (1e3000 kg d�1wet

waste) systems.Costs and revenueswere estimated in consultationwith industry partners and

the 20-year project cash flow, net presentworth (NPW), simple payback, internal rate of return,

andrevenuerequirementswerecalculated.TheNPWwasusedtocomparescenarios inorder to

determine thefinancial viabilityof using a generator for heat andelectricityor apressure swing

adsorption unit for converting biogas to compressed natural gas (CNG).

The full-scale digester consisted of five 786 m3 reactors (one biogasification reactor and

four hydrolysis reactors) treating a 50:50 mix (volatile solids basis) of food and green waste,

of which 17% became biogas, 32% residual solids, and 51% wastewater. The NPW of the

projects were similar whether producing electricity or CNG, as long as the parasitic energy

demand was satisfied with the biogas produced. When producing electricity only, the

power output was 1.2 MW, 7% of which was consumed parasitically. When producing CNG,

the system produced 2 hm3 y�1 natural gas after converting 22% of the biogas to heat and

electricity which supplied the parasitic energy demand. The digester system was finan-

cially viable whether producing electricity or CNG for discount rates of up to 13% y�1

without considering debt (all capital was considered equity), heat sales, feed-in tariffs or

tax credits.

ª 2010 Published by Elsevier Ltd.

1. Introduction scale MSW digesters in the U.S. has been attributed to low

Anaerobic digestion (AD) has been used throughout the U.S.,

primarily to treat agricultural and municipal wastewater [1].

Inmany European countries and in parts of Australia, Canada,

and Japan, full-scale digesters have begun treating municipal

solid waste (MSW) over last 10e20 years [1]. The lack of full-

02; fax: þ1 (530) 752 2640(J.L. Rapport), rhzhang@

tptomich@ucdavis.edu (Tshed by Elsevier Ltd.

tipping fees, high costs, and limited governmental support.

However, those conditions appear to be changing. In Cal-

ifornia, state regulations have mandated increased diversion

of waste from landfills since 1999, and the recent passage of

the Global Warming Solutions Act [2] may make AD an

attractive option for controlling greenhouse gas emissions

.ucdavis.edu (R. Zhang), bmjenkins@ucdavis.edu (B.M. Jenkins),.P. Tomich).

Nomenclature

A reactor surface area (m2)

Acf after-tax cash flow ($ y�1)

EC capital expenses ($ y�1)

CG generator capital cost ($)

CP heat capacity (kJ kg �1 K�1)

EO ordinary expenses ($ y�1)

EG generator operating expenses ($ y�1)

fMCPC moisture content of press cake (% wet basis)

h heat transfer coefficient (Wm�2 K�1)

M depreciation ($ y�1)

Mbg molar mass of biogas (gmol�1)

mBGd mass of dry biogas produced (t)

mf mass of feedstock (t)

MH2O molar mass of water (gmol�1)

mMC mass of water in feedstock (t)

mPC mass of press cake (t)

mTS mass of total solids in feedstock (t)

mVS mass of volatile solids in the feedstock (t)

mWV mass of water vapor exiting via biogas (t)

mWW mass of wastewater (t)

PSat(Tamb) saturation pressure of water vapor at ambient

temperature (kPa)

PSat(Tr) saturation pressure of water vapor at the reactor

temperature (kPa)

Pstd standard pressure (101.325 kPa)

Qf heat required for heating a fixedmass of feedstock

(kWh)

Ql heat loss rate (W)

R revenues ($ y�1)

Rgas gas constant (8.315 Pam3mol�1 K�1)

Rval insulation thermal resistance (m2 KW�1)

Tamb ambient air temperature (K)

TF federal income tax rate (%)

tG annual duration of generator operation (h y�1)

TI income taxes ($ y�1)

Tr internal reactor temperature (K)

TS state income tax rate (%)

Tstd standard temperature (273 K)

VBGamb volume of wet biogas after condensation (m3)

WG generator size (kW)

Y volumetric dry biogas yield (m3 t�1 VS); all gas

volumes were normalized to standard

temperature (Tstd) and pressure (Pstd)

hPC solid/liquid separation solids recovery efficiency

(% of dry total solids recovered)

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 21264

from organic waste treatment while producing renewable

energy.

Researchers at the University of California at Davis, CA (UC

Davis) have built a 3 t d�1 two-stage AD system known as the

Anaerobic Phased Solids (APS) Digester capable of degrading

organic matter with low moisture content [3e5]. Past studies

of the APS Digester found the system to be marginal

economically according to cash flow and revenue requirement

models using preliminary cost estimates, but the results were

highly sensitive to the assumed tipping fees and capital and

operating costs which may have been overestimated [6,7]. A

more recent cost estimate was used for the current model

which was designed to calculate the net present worth (NPW)

and internal rate of return (IRR) as well as the cost of energy

(COE) and cost of waste treatment (COT). For this study, the

aim of the research was to model and evaluate the economic

performance and mass and energy balance of a full-scale APS

Digester, assuming the biogas was used either for electricity

generation with heat recovery or as a compressed natural gas

substitute (CNG). A pressure swing adsorption (PSA) unit

containing a carbon molecular sieve that can remove most of

the carbon dioxide (CO2) and many other trace contaminants

was included in the model. This technology, originally devel-

oped for the natural gas industry, has been used for upgrading

landfill gas to renewable natural gas [8]. A separate H2S

scrubber was also included.

2. Assumptions and calculations used todevelop the model

The purpose of this study was to determine the mass balance

and energy input required for a consistent digester size and

feedstock quality and then compare the energy balance and

financial performance of different biogas usage configurations.

The design of the APS digester has been described previously

[3,9e11]. The system consists of four equally sized hydrolysis

reactors fed solidwaste in phased batches. A recirculation loop

continuously feeds the single biogasification reactor with

leachate from the hydrolysis reactors and recycles inoculum

and undigested material from the biogasification reactor. The

pilot-scale system used as the basis for the theoretical scale up

was developed to treat 3e8 t d�1 of the organic fraction of

municipal solidwaste (OFMSW). For this study, a full-scale APS

Digester was modeled treating 115 t d�1 of a 1:1 mixture (vola-

tile solids basis) of food and green waste based on the average

characteristics and biodegradability of the separate feeds

(Table 1). Samples of food and green waste from municipal

sources in the Bay Area of California have been characterized

and tested separately and together in bench-scale batch and

continuous APS anaerobic digestion trials [12e14]. A model,

implemented inMicrosoft Excel�, was used to size the digester,

calculate themassandenergybalance, andestimate thecapital

and operating costs, revenues and financial performance of the

system.

2.1. APS digester system design

The APS digester system as modeled here included a feed

hopper capable of auguring pre-sorted food and green wastes

into a chopper pump for delivery into one of four hydrolysis

reactors. Two additional chopper pumps were included for

mixing the hydrolysis reactors, and one additional pump was

included formixing the biogasification reactor and transferring

leachate between reactors. A hydrogen sulfide scrubbing and

passive water removal system would be included in all system

designs. A boiler was included in each system in order to

provide heat as needed when recovered heat from the

Table 1 e Characteristics and anaerobic degradability of municipal food waste (FW) and green waste (GW) collected fromthe Bay Area in California.

Reference source [11] [10] [12]

Experimental design 30 day, 1 L batch Theoretical calculation Lab-scale APS Theoretical calculationa Lab-scale APS

Substrate FW GW Mix (1:1 VS basis) FW GW Mix (1:1 VS basis) Mix (1:1 VS basis)

TS (% wet basis) 31 25 27 31 27 29 34

VS (% of TS) 85 88 86 87 78 82 88

VS (% wet basis) 26 22 24 26 21 24 30

Dry biogas yieldb (m3 t�1 VS) 530 360 450 570 420 530 500

Dry biogas CH4 content (%) 73 55 65 64 58 61 69

a The values in this column were used to populate the current model.

b Biogas yields were reported in the literature aswet biogas yields. The dry biogas yield was calculated assuming that biogas was saturatedwith

water vapor at the gas collection temperature.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 2 1265

generator was not available. For processing the residuals,

a screw press was included for solid/liquid separation with

a sequencing batch reactor for removing ammonia and oxygen

demand from the liquid fraction prior to discharge to the local

wastewater treatment system.

Four scenarios were evaluated for conversion of biogas to

energy: pure electricity, pure CNG, CNG plus heat, and CNG

plus heat and electricity. For electricity production, an engine

generator with heat recovery sized 10% larger than needed for

the amount of electricity to be produced would be added. For

CNG production, a two-stage PSA unit with one compressor

per stage would be added along with a compressor for pres-

surizing the CNG to 24.8 MPa for storage and automobile

fueling or gas-line injection.

The total working volumewas calculated based on the daily

volatile solids (VS) throughput rate expanded over the entire

year assuminga sustainedvolatile solidsorganic loading rate of

8 kgm�3 d�1, which had been determined for the APS digester

system in lab-scale trials [12,14]. The total reactor volume was

assumed to be 15% larger than the working volume to allow

headspace for biogas. The ratio of total biogasification reactor

volume to total combined hydrolysis reactor volume was fixed

at 1:4. The diameter of each reactorwas set equal to the reactor

height in order to minimize the surface to volume ratio.

2.2. Mass balance

The mass balance was calculated for solids and gasses by

assuming that all of the VS destroyed became biogas and none

of the fixed solids were consumed. The water balance was

determined assuming that water vapor exited the digester via

saturated biogas at the reactor temperature and that there

was no biochemical production or consumption of water. For

themass balance calculation, themass of dry biogas produced

was calculated as follows using the normalized volumetric dry

biogas yield (VS basis) and the molar mass of biogas, which

was taken to be the weighted average of the molar masses of

CH4 and CO2 in the biogas (neglecting the contribution of trace

gasses such as H2S, NH3, and VOCs). All variables and symbols

are defined in the Nomenclature.

mBGd ¼ mVSYMbgPstd

�RgasTstd

��1(1)

This mass was set equal to the mass of VS destroyed which

was then used to calculate the mass of residual solids.

Residual water was calculated by subtracting the water vapor

removed by the exiting biogas (Eq. (2)) from thewater added to

the digester via wet feedstock. The saturation pressure for

water at the specified temperature was determined using

steam tables [15].

mWV ¼ mVSYMH2OPstdPSatðTrÞ�Pstd � PSatðTrÞ

��1�RgasTstd

��1(2)

The residual solids and water were then distributed

between the press cake and the wastewater streams from the

solideliquid separation unit based on the solids recovery

efficiency of the screw press (80%) and themoisture content of

the press cake (60%) and the mass balance.

mPC ¼ ðmTS �mBGdÞhPC

�1� fMCPC

��1(3)

mWW ¼ ðmMC �mWVÞ þ ðmTS �mBGd �mPCÞ (4)

The volume of wastewater produced was calculated using

the specific volume of water (1.0 m3 t�1). The volume of wet

biogas leaving the digester after water had condensed out was

calculated using Eq. (5) in order to accurately estimate the

power requirement for the pressure swing adsorption unit. Eq.

(5) adjusts the dry biogas yield for saturation with water vapor

at the reactor temperature and then adjusts for condensation

as the biogas cools to ambient temperature, assuming it

remains saturated with water vapor.

VBGamb¼mVSYPstd

�ðTamb�TstdÞ�Pstd�PSatðTambÞ

�þ1��Tstd

�Pstd

�PSatðTrÞ��Pstd�PSatðTambÞ

���1(5)

The volume of CNG produced was calculated by adjusting

the volume calculated in Eq. (5) for CO2 removal (98%) and

methane loss (6.8%) from the PSA unit (Xebec Adsorption Inc.,

Quebec, Canada).

2.3. Energy balance

The amount of heat and electricity produced by the system

were calculated based on the amount of methane produced

(from themass balance calculations), the higher heating value

of methane (15.451 MWhkg�1) [16], and the assumed elec-

tricity conversion and heat-capture efficiencies of the gener-

ator (30% and 60%, respectively) and boiler (60% heat capture).

The generator size was calculated based on the number of

days per year of generator operation over which the electrical

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 21266

capacitywould be produced (nominally 90% or 329 d y�1). Feed

heating was modeled by assuming the food and green waste

would be heated from the mean annual ambient temperature

to the reactor temperature, assuming the heat capacity of the

feedstock was temperature independent over the small range

of temperatures to which it was exposed.

Qf ¼ mfCpðTr � TambÞ�0:2778 kWh MJ�1

�(6)

Theheat capacity of the feedstockwas assumed to equal that

of water (4.0 kJ kg�1 K�1). The 2007 mean annual temperature

for California was 289 K, according to the National Climatic

Data Center, which was used as the ambient temperature for

this model. The average reactor temperature was 328 K.

Heat losses were calculated using an idealized lumped

model which assumed that reactor contents were homoge-

neous, isotropic, and well mixed, the effects of radiation and

biochemical heat generationwere negligible, and therewas no

contact resistance between surfaces. Heat loss through the

pipes was not included. Heat loss through the heat exchanger

was included in the heat recovery factor of the boiler. The rate

of heat loss was modeled by setting the heat conduction from

the inner wall through the insulation equal to the heat

convection from the outer reactor wall into the ambient air,

resulting in the following equation:

Ql ¼ R�1valh

�R�1val þ h

��1AðTr � TambÞ (7)

The reactor surface areawas calculated based on the system

design. The insulation’s thermal resistancewas assumed to be

that of one-inch thick polyurethane foam as reported by the

manufacturer (1.74 m2 KW�1). The heat transfer coefficient

was nominally estimated to be 50 Wm�2 K�1 as an aggregate

annual average, although this would change depending on

reactor configuration, materials, and site conditions. It was

assumed that the capacity factor applied only to loading, and

that the reactor temperature would be maintained even if the

capacity factor were less than100%. Therefore, the annual

maintenance heat requirement was calculated by extending

the heat loss rate over 365 days, regardless of the actual

number of operational days. The feed heat, however, was

a function of the capacity factor which determined the total

amount of feedstock loaded in a year.

The electrical consumption of the feedstock handling,

circulation, andmixing systems was 2.030 MWhd�1, based on

the pumps’ power ratings and expected durations of operation

as designed by Onsite Power Systems, Inc. This was assumed

to be a function of the capacity factor. The electrical require-

ment for the PSA unit was based on the electrical requirement

of the three compressors required: two for purifying the

biogas (0.116 kWhm�3 each) and one for storing it at the CNG

fueling station pressure of 24.8 MPa (0.212 kWhm�3). The

storage compressor power rating was adjusted for the reduc-

tion in gas volume during purification and combined with the

purification compressors to generate a lumped electrical

power rating per unit volume of biogas processed for the PSA

unit of 0.348 kWhm�3.

2.4. Financial performance

The results of themass and energy balance were fed into the cost

and revenue calculators. The costsweredivided into capital (fixed)

and ordinary (running) expenses. Onsite Power Systems, Inc.

provided cost estimates for a 115 td�1 system based on their

experience building a pilot-scale APS Digester system and costing

systems for industry partners [17]. The capital cost quoted was

normalized for the total reactor volume for all five reactors. The

digester’s capital cost (1.49 $dm�3) included the site development

andpermittingcostsaswellasequipmentandinstallationcostsfor

the reactors,material handling equipment, wastewater treatment

unit (i.e. sequencing batch reactor) and gas treatment equipment

(i.e.H2S scrubber andmoisture removal unit). It didnot include the

cost of MSW sorting and size reduction, since the food and green

wastes were expected to be delivered pre-sorted and the cost of

sorting would be embedded in the reduced tipping fees. The

operating and maintenance cost for the digester included esti-

mates for labor, maintenance, administrative costs and overhead

(i.e. property tax, insurance, and infrastructure expenses) and

came to 8.3% of the capital cost [17].

OtherMSWdigesters have been found to exhibit cost curves

that fit the power equationY¼ aXb, where a and b are constants

with economies of scale occurring when 0< b< 1 and X is the

design capacity in tonsper year [18].MSWdigesters inEuropean

countries cost 1.81 $ (kt y�1)�1 capacity (in 2007 dollars) with

a scaling factor of 0.56 on the digester capacity [18]. Therefore,

a 42 kt y�1 digester would cost 14.7 M$. Assuming a sustained

wetMSW loading rate of 16 kt dm�3 y�1, the specific capital cost

of similarly sized digesters in Europewas 5.60 $ dm�3. Full-scale

MSW digesters have yet to infiltrate the U.S. market; therefore,

the capital costs of full-scale systemshavenot been studied and

may have been underestimated. However, the present digester

system did not include pre-sorting equipment for the MSW,

whichmayaccount for someof the lowercost ascomparedwith

European systems. The cost estimate provided by the vendor

was used here, but the sensitivity of the model to changes in

capital cost was evaluated. Since this model only considered

one size of digester, no scaling factor was included. However,

the generator capital cost was scaled by fitting published cost

data [19] (adjusted by the consumer price index to 2007 dollars)

using a linear regression on the data transformed to fit a power

equation, which resulted in the following equation:

Cgen ¼ 2:267�$ W�1

�W 0:9

G (8)

Generator maintenance costs were also scaled using the

published data, but these costs depend on the total operating

time of the generator as well as generator capacity. Hence, the

annual maintenance cost for the generator was modeled as

follows:

EG ¼ 0:043�$ kWh�1

�W 0:81

G tG (9)

The maintenance cost for the hydrogen sulfide removal unit

was based on the cost of replacing the hydrogen sulfide

absorptive media (5.48 $ kg�1 H2S absorbed), which was in turn

based on the amount of biogas purified and its hydrogen

sulfide content (500 ppm). The PSA unit’s capital cost was 81

$m�3 d�1 treatment capacity. Its operating cost was assumed

to be 5% of the capital cost. The cost of the PSA unit was

assumed to be linear (i.e. a capital cost scaling factor of one

was assumed a priori) within the narrow range of sizes

considered in this model. For this study, it was assumed

that the sequencing batch reactor would provide sufficient

Table 2 e Summary of daily energy balance for the four scenarios evaluated.

Units (per day) Pure electricity Pure CNG CNGþ heat CNGþ heat and electricity

Energy production

Biogas ML 15.7 15.7 15.7 15.7

Electricity MWh 29.2 0 0 6.4

Heat MWh 40.0 0 6.4 8.9

CNG dam3 0 8.84 7.87 6.92

Energy consumption

Electricity (% of prod.) MWh 2.0 (7) 7.5 (NA) 7.0 (NA) 6.4 (100)

Heat (% of prod.) MWh 6.4 (16) 6.4 (NA) 6.4 (100) 6.4 (72)

Parasitic biogas % of prod. 7 0 11 22

Daily Mean Ambient Temperature (K)

250 260 270 280 290 300 310

Dai

ly H

eat R

equi

rem

ent (

MW

h)

0

2

4

6

8

10

12

14

Heat recoverd

from generator

(CNG + heat &

electricity scenario)

Total Heat Requirement

Feed Heat

Heat Loss

Min mean ambient temp

Fig. 1 e Digester heat requirement as a function of the

mean ambient temperature. The heat recovered from the

generator when converting biogas to CNGDheat and

electricity shown as a horizontal line, and the minimum

daily ambient temperature at which the heat recovered

supplies the total heat required is indicated by the vertical

line.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 2 1267

treatment of the wastewater to avoid high disposal costs, and

the residual solids would have value as a compost product.

These assumptions depend on the quality of the food and

green waste feedstock and the local regulations restricting

contaminants in the residuals.

The value of the residual solids (11 $ t�1) was half of the

value ofmature compost inNorthernCalifornia, assuming that

stabilization of the residual solids via aeration would be

accomplished off-site. The electricity price used (0.067

$ kWh�1) was the average 2007 wholesale price for a California

trading hub [20]. The price used for natural gas (0.24 $m�3) was

the November 2007 City Gate price [21]. It is worth noting that

more recently the price of natural gas has declined. The effects

of natural gas price on the overall economics of the project can

be evaluated in the sensitivity analysis. Although heat nomi-

nally had no value, the effect of adding heat sales was tested

assuming heat had a value of 0.027 $ kWh�1. The tipping fee

used in this study was the price negotiated by Onsite Power

Systems, Inc. for separated and ground OFMSW at a potential

site in Southern California (24.25 $ t�1). In California, landfills

received an average of 34 $ t�1 in 2000 with higher fees paid in

Southern California [22]. The 2004 average landfill tipping fee

in the U.S. was 38 $ t�1 [23]. In this study, the assumed tipping

fee was lower than average because the OFMSWwas assumed

to have been sorted prior to delivery at the APS Digester

facility. The true cost of sorting OFMSW from a mixed waste

stream merits further study. No additional revenue sources

(e.g. tax or carbon credits) were included in the analysis.

Land was assumed to be free to the digester in this study

(as would be the case if the digester were co-located with

a landfill or other benefiting facility). Land prices can vary

significantly by site and retain a large salvage value, however,

and should be considered when evaluating specific projects.

Equipment was assumed to have no salvage value after 20

years in this analysis. Transportation costs were not included

explicitly, but they were implicit in the values assigned to

tipping fees and residual solids. Debt borrowing was assumed

to be zero (all capital costs were assumed to be paid with

equity) to focus the analysis on the technology rather than

source of capital.

The income and expenses (including capital expenses

which were assumed to be incurred only in year zero) were

adjusted for taxes to calculate the annual cash flow as follows:

ACF ¼ R� EO � EC � TI (10)

Taxes were applied at the average California state and

federal tax rates (9% and 35%, respectively) to the taxable

income using the MACRS 7-year accelerated depreciation

schedule [24] as follows:

TI ¼ ðTS þ TF � TFTSÞðR� EO �MÞ (11)

Revenues and ordinary expenseswere escalated annually at

the non-seasonally adjusted 10-year average inflation rate of

2.57% y�1 [25]. Cash flows for each of 20 consecutive years

were discounted at a standard rate that includes the cost of

money and a risk premium (10% y�1) and summed to compute

the NPW. The IRR was calculated for the resulting cash flow

using the Excel� IRR function. The revenue requirements

method of Jenkins [26] was used to determine the levelized

COE and COT (i.e. price of energy� electricity or CNG e and

tipping fee required for zero NPW at the given discount rate).

3. Modeling results and discussion

3.1. Mass and energy balance

The full-scale digester had a total working volume of 3.418 dam3.

Thus, eachreactorhada total volume (including theheadspace) of

786m3 with a surface area of 499m2. The mass balance calcula-

tions predicted that 64% of the volatile solids would be converted

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 21268

into biogas, resulting in 17%of the feedstockmassbeing converted

tovapor-saturatedbiogas (containing1.8 t d�1water vapor). Biogas

and methane would be produced at a rate of 4.21 and

2.17m3m�3 d�1, respectively, or 15.7 and 8.8 dam3d�1 in total.

Adjusted for the VS content of the feedstock, the biogas and

methane yields were 166 and 90m3 t�1 wet waste. Based on its

energy content the biogashadapre-conversionpower potential of

4.1 MW, or 848 kWht�1, and a total electrical production capacity

of 1.2 MW or 255 kWht�1 wet waste. The remaining volatile and

fixed solids andwaterwould exit the systemas 36 t d�1 press cake

and 59m3d�1 wastewater (32% and 51% of the feedstock mass,

respectively). The wastewater stream would consists of 5% TS, of

which 62% were VS, prior to treatment in the sequencing batch

reactor.

The digester required 6.4 MWhd�1 of heat, on average, (21%

due toheat lossesand79%due to feedheating) and2.0 MWhd�1

electricity. The compressors required for operating the PSA and

Pure E

lectri

city

Pure C

NG

CNG + H

eat

CNG + H

eat &

Elec

tricit

y

Cap

ital C

ost (

2007

M$)

0

2

4

6

8

10

Digester

Generator

PSA Unit

Contingency

Pure E

lectri

city

Pure C

NG

CNG + H

eat

CNG + H

eat &

Elec

tricit

y

Ann

ual E

xpen

ses

(200

7 M

$)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Digeste

Genera

PSA Un

H2S Re

Utilitie

Conting

Fig. 2 e Financial results for the four biogas usages analyzed w

rate of 10% yL1 and tipping fees of 24 $ tL1.

CNG storage units added 4.4e5.5 MWhd�1 electricity, depend-

ing on the amount of CNG produced (Table 2). Comparing the

two self-sufficient scenarios (pure electricity and CNGþheat

and electricity) the parasitic biogas demand more than tripled

when producing CNG. Both scenarios produced useable heat in

excess of that needed formaintaining thedigester temperature.

However, if the mean daily ambient temperature were to fall

below273 K,heat storagewouldbe required (Fig. 1). Thedigester

itself can provide some heat storage, since the bacteria can

withstand modest temperature fluctuations; however, the

boilermayneedtobeusedduringsustainedsub-freezingwinter

weather. If biogas were used to provide the additional heat,

a more complex analysis would be required to maximize the

financial performance. For thepurposesof thecurrent study the

digester was assumed to provide ample heat storage, thus no

additional heat would be required. The three CNG producing

scenarios traded additional CNG for excess electricity and heat

Pure E

lectri

city

Pure C

NG

CNG + H

eat

CNG + H

eat &

Elec

tricit

y

Ann

ual R

even

ues

(200

7 M

$)

0.0

0.5

1.0

1.5

2.0

2.5

Residual Solids Sales

Tipping Fees

Energy Sales

r O&M

tor O&M

it O&M

moval

s Purchases

ency

Pure E

lectri

city

Pure C

NG

CNG + H

eat

CNG + H

eat &

Elec

tricit

y

Net

Pre

sent

Wor

th (

2007

M$)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

ith total net present worth calculated assuming a discount

Energy price ($ per unit energy)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Rat

e of

ret

urn

(% y

-1)

8

10

12

14

16

18

20

22

24

26

Tipping fee ($ t-1

)

10 20 30 40 50 60

Electricity price ($ kWh-1) for Pure Electricity option

CNG price ($ m-3) for CNG + Heat & Electricity option

Tipping fee ($ t-1

) for both options

Fig. 3 e IRR as a function of energy price (electricity or CNG)

and tipping fees. When one of the three amounts was

varied, the other two were held at their assumed values.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 2 1269

required from outside sources. For CNG production, self-suffi-

ciencycost1.9 dam3 d�1CNGwhichwasa22%reduction in total

production (Table 2).

The mass and energy balance calculations were similar to

those seen for other anaerobic digestion systems treating

OFMSW. For example, methane yields for full-scale MSW

digesters have been reported to range from226 to 400 m3 t�1 VS

for source-separated organics fromMSW [27e29], as compared

with 323 m3 t�1 VS for this system. Similarly, the APS Digester

system’s predicted biogas production rate (4.60 m3m�3 d�1)

was within the 3e5 m3m�3 d�1 range reported for thermo-

philic high-solids anaerobic digesters [30]. The overall mass

balance predicted for the APS Digester systemwas also similar

to the measured mass balances of several full-scale digesters

[28,31].

The energy balance revealed that only a fraction of the heat

produced by the engine generator was needed to provide the

heat required by the digester. Therefore, if heat recovery from

the generator is only needed to meet the heat demand of the

digesters, less expensive heat recovery systems with lower

efficiency than assumed in the model (as low as 10% heat

recovery) would be adequate. However, if a small generator

were used to provide only the parasitic electrical load, the heat

recovery system would then need to be more efficient, espe-

cially in cold climates. Thicker insulation would reduce some

of the heat loss, but the majority of the heat was required for

heating the feedstock. Therefore, preheating the feedstock

with waste heat or using aerobic microorganisms would

greatly reduce the heat demand. A brief aerobic treatment

during the organic separation process (i.e. rotary drum treat-

ment) could increase the temperature and provide access to

lignin in the green waste fraction.

3.2. Economic results and sensitivity analysis

The estimated capital costs and annual revenues were similar

regardlessofbiogasusage (Fig. 2), but theannual expenseswere

higher when energy had to be purchased from the utility (i.e.

pure CNG and CNGþheat). When biogas was used to provide

the electricity and heat for running the system, the expenses

were similar to the CNG producing scenario. Hence, the NPW

was almost equal to that of the CNG producing scenario, indi-

cating that both projects would be financially viable. Since the

capital costs were nearly identical, the internal rate of return

and simple payback for both projects were similar as well: 13%

and 7 years, respectively. Therefore, even though theNPWwas

calculatedusing adiscount rate of 10% y�1, theprojectwouldbe

viable for discount rates of up to 13%. The revenue require-

ments calculator was used to determine the required cost of

energy or waste treatment (e.g. tipping fees) needed to obtain

internal rates of return from 10 to 25% (Fig. 3).

With the discount rate set to 10% y�1, the revenue from

tipping fees and compost sales was sufficient to provide 74%

of the required revenue in the pure electricity scenario. An

additional 0.04 $ kWh�1 from electricity sales would provide

the rest of the revenue required. Alternatively, a tipping fee of

18.46 $ t�1 would provide the required revenue if electricity

were sold at the previously assumed rate. Adding heat sales to

the revenue stream would increase the NPW by 142% and

raise the IRR to 16.4%.

For theCNGþheat and electricity scenario, the COE for CNG

was 0.15 $m�3 and the COT was 18.75 $ t�1. Tipping fees can

fluctuate significantly dependingon location, andenergyprices

could be higher if utilities offered premiums for renewable

energy. Furthermore, tax credits and carbon-offset credits

could also improve the financial performance of anaerobic

digestion projects. However, without external support for the

environmental benefits of anaerobic digestion, the financial

viability of thedigester depends on the accuracy of the cost and

productivity assumptions.

Many of the base assumptions could vary from the values

assumedhere. Therefore, the sensitivity of themodel to relative

changes in the primary assumptions was determined for the

two most financially viable scenarios: pure electricity and

CNGþheat and electricity (Fig. 4). The slopes of the sensitivity

curves were calculated at �30% of the base assumption

for comparing the sensitivities within and between the biogas

usage scenarios. The sensitivities were similar between

scenarios. Themodel wasmost highly sensitive to the capacity

factor because decreasing the capacity factor did not affect the

capital costs which were based on the designed capacity, but it

reduced every revenue stream, including tipping fees which

supplied over half of the income. The capacity factor should

improve over time as system operators and managers become

accustomed to the technology, suggesting that financial

performancewill improve over time. Itwas alsohighly sensitive

to changes in the capital costs, tipping fees, economic life, and

organic loading rate. The maximum sustainable OLR should

also increase over time as themicroorganisms acclimate to the

digester conditions. However, the NPWwas not linearly related

to OLR. Lower loading rates will result in greater decline in

financial variability than increased rates, and the assumed OLR

was higher than many operating systems. The model was

moderately sensitive to changes in the annual expenses,MARR,

electricity price, and biogas methane content. Over time,

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 21270

annual expenses should decline as maintenance procedures

become streamlined.

Although the model was relatively insensitive to changes

in the residual solids value, the uncertainty in this parameter

was particularly high since the residual solids could either be

an asset or a waste product depending on their quality. The

plot of NPW over a wide range of values for the residual solids

revealed that, all other factors being equal, the project would

still generate a 10% y�1 rate of return even if charged up to 7

$ t�1 for their disposal (Fig. 5).

The model was also used to determine whether a project

could be financially viable if biogas were stored as CNG in

order to obtain higher off-peak electricity prices. The price of

Pure Electricity

Change from base value (%)

-60 -40 -20 0 20 40 6

Net

Pre

sent

Wor

th (

M$)

-8

-6

-4

-2

0

2

4

6

8

Slope of sensitivity curve (M$ %-1)

-20 0 2

Digester capital cost

Discount rate

Maintenance expenses

Tax rate

Digester power rating

Residual solids value

Escalation

Gas yield

Methane content

Electricity price

CNG price

Economic life

Tipping fee

Organic loading rate

Capacity factor

Fig. 4 e Sensitivity of financial performance to changes in the u

CNGDheat and electricity (top right) scenarios with slopes calc

electricity required to maintain the net present worth was

sought given that all of the biogas was converted first to CNG

and then to electricity in a generator sized to run 8 h d�1. This

increased the capital costs and annual expenses by 47% and

7%, respectively, and reduced the revenue by 7% due to the

additional parasitic electrical demand of the PSA unit. To

maintain the same NPW, the price of peak electricity would

have to be at least 0.128 $ kWh�1. In the U.S., peak electricity

prices often top 0.25 $ kWh�1; therefore, it may be financially

and technically advantageous to include CO2 removal even

when producing electricity only. However, the increased

capital cost reduced the IRR from 13 to 10% y�1 and increased

the payback period to 8 years.

0

Digester capital cost

Discount rate

Maintenance expenses

Tax rate

Digester power rating

Residual solids value

Escalation

Gas yield

Methane content

Electricity price

CNG price

Economic life

Tipping fee

Organic loading rate

Capacity factor

CNG + Heat & Electricity

Change from base value (%)

-60 -40 -20 0 20 40 60

0

Pure Electricity

CNG + Heat & Electricity

nderlying assumptions for the pure electricity (top left) and

ulated between ±30% of the base value (bottom).

Residual solids value ($ t-1)

-80 -60 -40 -20 0 20 40 60 80

Net

pre

sent

wor

th (

M$)

-6

-4

-2

0

2

4

6

Assumed (11)

Minimum (-7)

Fig. 5 e Effect of residual solids value on the financial

performance, assuming a discount rate of 10% yL1 and

tipping fees of 24 $ tL1 (negative value indicates a disposal

cost rather than revenue for the residual solids).

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 2 6 3e1 2 7 2 1271

4. Conclusions

Converting 115 t d�1 of MSW to energy via anaerobic digestion

was calculated to provide either 1.2 MWe or 6.9 dam3 d�1 as

renewable CNG without requiring any external energy inputs.

Both electricity production and renewable CNG production

were found to be financially viable with similar economic

performance under the base assumptions. A side analysis

revealed that converting the biogas first to CNG and then to

electricity only during peak usage periods would result in the

same NPW if the peak price of electricity were at least 0.13

$ kWh�1. However, financial viability depends greatly on the

needs of the investors (i.e. MARR and payback) and the

availability of financing. The current project was predicted to

provide investors up to 13% return on investment without

considering financing or economic incentives. However, since

this analysis was based on an in-silico scale up from a pilot-

scale facility and no commercial digesters have been built to

treat MSW in the U.S., large uncertainties remain with regards

to cost and performance. The sensitivity analysis indicated

that capacity factor, capital cost, sustained organic loading

rate, tipping fee, and MARR had the largest effect on the NPW.

In this study, a large number of the cost items were esti-

mated for the system as sized here. In the future, the model

should include scaling factors for all capital and operating

costs (including the pre- and post-treatment equipment),

which would allow the performance of the digester to be

calculated for any size system. The model could also be

revised to include other sources of heat transfer, such as

radiation and pipe losses, or simulations of different reactor

geometries and spatial orientations. However, based on the

low impact of heat on NPW found in this model, such efforts

would be more useful for financial forecasting if heat were

given amonetary value. Finally, themodel should be validated

with data from the pilot APS digester system and compared

against the financial performance of the first full-scale

versions of the system as they are developed.

Acknowledgements

This research was supported by a grant from the California

Energy Commission and by Onsite Power Systems, Inc.,

who provided commercial information and engineering

specifications.

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