Kobayashi_biomass Direct Chemical Looping Process Perspective

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 5 2e1 2 6 2

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Biomass direct chemical looping process: A perspective

Nobusuke Kobayashi a,*, Liang-Shih Fan b

aDepartment of Chemical Engineering, Nagoya University, Furo-Cho, Chikusa-ku, Nagoya 464-8603, JapanbWilliam G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus,

OH 43210, USA

a r t i c l e i n f o

Article history:

Received 2 July 2009

Received in revised form

13 December 2010

Accepted 15 December 2010

Available online 12 January 2011

Keywords:

Biomass energy

Chemical looping process

Carbon capture

Hydrogen production

Economic analysis

Energy conversion efficiency

* Corresponding author. Tel.: þ81 52 789 337E-mail address: [email protected]

0961-9534/$ e see front matter ª 2010 Elsevdoi:10.1016/j.biombioe.2010.12.019

a b s t r a c t

This paper provides a perspective on the biomass direct chemical looping (BDCL) scheme

which directly converts biomass into hydrogen and/or electricity with CO2 capture. The

discussion involves the preliminary design, potential challenges, and feasibility assessments

in the BDCL process. Description of the BDCL process and the characteristics of biomass are

provided in order to discuss potential advantage and disadvantage that may occur in this

system.The feasibilityof employing theBDCLprocess basedonpreliminary studies relative to

competing biomass technologies and conventional fossil fuel conversion schemes is also

discussed based on the perspective of energy conversion efficiencies, and economic analysis.

Biomass has proven to be less efficient andmore cost intensive compared to the current fossil

fuel processes, due to its relatively lowenergy density, highmoisture content anddistributed-

resource. However, the BDCL process has the potential to compete against conventional

energy generation systems via its high conversion efficiency. A preliminary economic anal-

ysis demonstrates that BDCL system integratedwithCCS to receive carbon credit is capable of

producing electricity at a competitive price to the current fossil fuel processes.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction as their feedstock. Due to its relatively high energy density and

The carbon dioxide emission reduction is a major concern for

Annex I countries. They are expected to meet the new envi-

ronmental requirementsof greenhousegas (GHG)emission for

the 2008e2012 period as set by the Kyoto Protocol. Emission of

GHG,especiallyCO2,has continuedanupward trendsince1970

[1,2]. The R&D on clean, safe, and cost-effective power

conversion processes and the adoption of renewable energy

sources, such as hydro, wind solar, geothermal and biomass,

hasbeenpromotedandsupportedby thegovernmentsof these

countries in an attempt to reduce CO2 emission.

The CO2 can be emitted from a number of different human

activities. One of the major sources of this GHG is from the

combustion of fossil fuels used in power generation, trans-

portation, and industrial processes [3]. Thermo-chemical

power production systems generally utilize coal or natural gas

9; fax: þ81 52 789 3271..jp (N. Kobayashi).ier Ltd. All rights reserve

well understood processing techniques, fossil fuels are

considered dominant candidates for supplying the world

electricity requirements. However, combustion of fossil fuels,

especially coal, significantly contributes to carbon dioxide

emissions. Several steps are being taken to reduce its release

into the atmosphere [4].

The following section will review the conventional CO2

reduction scheme via high efficiency process operation and

the current carbon capturing technologies.

1.1. Conventional GHG emissions reduction from fossilfuel conversion plants

There are several methods currently being developed to

reduce the CO2 emissions from conventional power plants.

The reduction of fossil fuel/coal consumption by improving

d.

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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 5 2e1 2 6 2 1253

the energy conversion efficiency represents a first step

approach to reducing GHG emissions. The development of

efficient power generation systems, such as IGCC and ultra-

high temperature alloys for steam turbine applications, has

contributed significantly to this cause [5]. Additionally, CO2

capture and storage (CCS) methods are being developed to

further stabilize its concentration in the atmosphere from

fossil fuel processing impacts [6,7].The CCS involves carbon

dioxide capture, transportation, and sequestration [8,9]. Here,

transportation has been proven feasible with pipeline

construction. Geological sequestration refers to storing a large

amount of CO2 underground [10,11]. This method is currently

under development in several countries such as Norway and

Canada. However, methods of sequestration have not yet

been proven reliable and safe for long-term storage.

There are a number of methods for CO2 capture. These

techniquesvarydependingon itsapplication to thepowerplant

of interest. Overall, each method must be capable of capturing

the carbon in a high purity stream to be pressurized and

transported to a sequestration site. Though numerous, carbon

capture techniques can be divided into three categories; post-

combustion pre-combustion and oxy-fuel combustion

capturing systems [4,12e14]. Here, the post-combustion

capture system refers to separating carbon from the exhaust

stream after its combustion with air. The use of a chemical

solvent to scrub the CO2 from the flue gas is an established

method in coal-fired power plants [15e17]. More cost-efficient

solvents are continuously under development [18,19]. Alterna-

tively, membrane and solid sorbent separation techniques are

also being studied for this application [13,20,21]. In the pre-

combustion capture system, CO2 is removed before the

combustion step. Natural gas reforming process is an example

of the technique [22]. Finally, the oxy-fuel combustion system

uses high purity oxygen instead of air to combust the primary

fuel to produce a flue gas comprisingmainly ofwater vapor and

CO2.

Many of the carbon capture systems currently developed

are capable of reducing CO2 emissions from power generation

plants [4,23,24]. However, a significant amount of parasitic

energy is required for its removal [4,7,13,16,24]. Thus, inte-

grating carbon capture with coal combustion plants reduces

the overall efficiency of the industry lowering its profitability.

Further research is required to achieve better system integra-

tion, thereby increasing the energy conversion efficiency and

hence reducing the CO2 capturing costs. The coal thermo-

chemical processes are well developed fields for energy

production. Coupling these systems with carbon capture

technologies will potentially allow the respective countries to

meet the GHG emissions requirement, but reduce the overall

profitability of the power plant. Additionally, the use of coal

continues our dependence of fossil fuels, a depleting resource.

An energy source that is both renewable and environmental

would be a highly attractive alternative to the current situation.

1.2. Biomass energy conversion overview

Biomass is considered a fuel source to partially replace the use

of fossil fuels in many thermo-chemical processes [25e29].

This renewable resource consists of bio-residues from plants

such as lignocellulosicmaterials. Biomass is considered carbon

neutral, because the amount of carbon it can release is

equivalent to the amount it absorbed during its life time. There

is no net increase of carbon to the environment in the long-

term when combusting the lignocellulosic materials. There-

fore, biomass is expected to have a significant contribution to

the world energy and environment demand in the foreseeable

future [3,7]. Replacing some of the current coal feedstock in

thermo-chemical processes with biomass will result in an

atmospheric carbon reduction. Additionally, if this system is

capable of utilizing carbon capture technologies, a net reduc-

tion of CO2 is achieved. A power plant that integrates both

biomass and CCS technology will receive carbon credit making

this process potentially more economically attractive.

Several systems are being developed to apply biomass for

energy production. Fig. 1 summarizes the application

processes. The biomass systems can be divided into two fields

of study; biochemical and thermo-chemical processes.

Biochemical processes for biomass application consists of the

use of a cell culture to produce a desired product. Glucose is

derived from the hydrolysis of biomass as the feed for the

fermentor [30]. There are threemain thermo-chemical systems

currently under research; direct combustion, gasification, and

pyrolysis. Biomass combustion research is similar to coal

combustion technologies. The biomass directly reacts with air

to produce heat in the form of steam. The product stream will

be used to drive a steam turbine [31]. Ideally, this technology

can serve as a substitute to coal in the conventional combus-

tion system. However, due to the high moisture content, rela-

tively low energy density and highly distributed-resourcewhen

compared to coal, replacing this fossil fuel with the renewable

fuel significantly reduces the overall efficiency of the power

plant. In order to offset the process inefficiencies, a co-

combustion system is proposed where a combination of

biomass, waste plastic and coal is used as the feedstock to the

combustor [32]. This technology is being proven to be cost-

effective and practical, because it uses the currently established

power plant systems to combust the renewable fuel. Several

countries have begun to integrate co-combustion process with

their respective power plants [33e36]. The use of biomass for

our energy demand is gradually increasing every year [37].

Pyrolysis refers to the heating of a substance in the absence

of an oxidizing agent [38]. Using biomass in this process yields

char, liquid and gas. Bio-oils with a wide range of heavy and

light hydrocarbon chains [39,40] is able to produce by

manipulating pressure, residence time, temperature and

heating up rate. This technology has the potential of

producing valuable liquid fuels that may replace the conven-

tional petroleum stations. However, this extensive and highly

complicated process requires further research before indus-

trialization can be considered.

With lower operating temperatures compared to

combustion systems, gasification process are becoming

a potential option for improving the biomass energy conver-

sion efficiency. Furthermore, this technology is capable of

producing a variety of chemical products, such as methanol

and other hydrocarbons, allowing for flexible marketability

[32,41e43]. The biomass gasification system has been

successfully demonstrated under fluidized and circulating

fluidized bed operation. Secondary tar cracking catalyst and

steam-reforming techniques are established down stream



Biomass

Thermochemical Process Biochemical Process

Combustion

Gasification

Pyrolysis

Fermentation

Hydrolysis

Anaerobic Digestion

Liquefaction Biophotolysis

Chemical Looping

Biofuels:hydrogen, methane, alcohols,

FTD,DME

Biomass

Thermochemical Process Biochemical Process

Combustion

Gasification

Pyrolysis

Fermentation

Hydrolysis

Anaerobic Digestion

Liquefaction Biophotolysis

Chemical Looping

Biofuels:hydrogen, methane, alcohols,

FTD,DME

Heat &

Electricity

Fig. 1 e Current biomass process applications.


processing methods that have shown promising results in

this system [44e47]. Additionally, a biomass and coal co-

gasification may offset the biomass low energy density issue

allowing for the production of both valuable fuels and power

[48,49].

Biomass energy is potentially capable of reducing fossil fuel

consumption and CO2 emission. However, the biomass energy

conversion systems are relatively more inefficient and cost

intensive when compared their coal counterpart processes.

Table 1 compares the efficiencies of coal systems to biomass

for the thermo-chemical processes [50]. Furthermore, coupling

CCS methods to biomass would reduce the process efficien-

cies. The low efficiencies compared to coal hinder the further

development of renewable resource. A highly efficient process

is needed in order to replace our current coal dependence.

The thermo-chemical conversion technology based on the

chemical looping process (CLP) has attracted much attention

as it requires little energy for CO2 separation [51,52]. This

system differs from other processes in that the GHG separates

through reaction path design. The CLP technology utilizes

metal oxides insteadof gaseousoxidants as the oxygen carrier.

Table 1 e Energy conversion efficiencies in selectedthermo-chemical process [50].

Process Products Energy conversion efficiency (%)

Biomass Coal

Direct

combustion

Electricity 25 35

Rapid pyrolysis Pyrolytic oil,

gas, char

37e45 40e50

Gasification Fuel gas 50e60 55e65

In onesectionof theCLP, this chemical intermediate is reduced

with carbonaceous fuels to metal producing CO2 and H2O

[53e56]. Condensing the steamfromthis product streamyields

a sequesterable CO2 stream. The reduced metal can then be

oxidized to produce electric power and/or chemical.

This paper will provide a perspective on the biomass direct

chemical looping (BDCL) scheme which directly converts

biomass into hydrogen and/or electricity. The discussion will

involve the preliminary design, feasibility, and potential

challenges with using biomass in this application.

2. Biomass direct chemical looping with CO2

capture

The biomass direct chemical looping process is a strong

candidate for using this renewable resource to achieve

a higher energy conversion efficiency as well as CO2 capture.

This process utilizes an iron oxide composite particle to

directly react with the biomass in an oxidation/reduction

reaction scheme. Li et al. [57] calculated the energy conversion

efficiency of the BDCL process, using ASPEN plus, to be over

38% on the higher heating value (HHV) basis for power

generation with 99% CO2 capture in a 100 MW facility.

Furthermore, for the lower valued hydrogen production the

efficiency was found to be 74% (HHV) for the BDCL system.

These results show that this novel technology has the

potential to penetrate the current economic barriers and

provide a feasible biomass energy conversion process. By

capturing the CO2 released, carbon credit can be claimed

making this process even more economically advantageous.

The following sectionswill provide a detailed BDCL process

description and an overview of its technical and economical

advantages. Further description of the characteristics of




biomass will also be provided in order to discuss preliminary

challenges that may occur with using it in this system.

2.1. BDCL process description

The schematic flow diagram of the biomass direct chemical

looping process is illustrated in Fig. 2. This system consists of

three reactors; a reducer, oxidizer and combustor. Here,

biomass is injected into the reducer where it is partially

gasified into carbon monoxide, hydrogen and other various

carbonaceous volatiles. The gaseous fuels reduce the Fe2O3

particles to Fe/FeO producing a mixed stream of CO2 and H2O.

Condensing the steam from the flue gas yields a highly pure

stream of CO2. A counter-current moving bed reactor is used

for the reducer. The biomass feedstock will be injected at

roughly the midpoint to create a two-stage move bed reactor.

The volatile material within the feedstock will react with the

metal oxide above the injection point while the remaining

carbonaceous solids reduce the particles below. Additionally,

the CO2 produced may be partially recycled back into the

reducer to enhance to gasification of the biomass.

The reduced particles are then sent into the oxidizer. This

unit is also a moving bed reactor. Steam is injected into this

section oxidizing the Fe/FeO particles into Fe3O4 while yielding

a high purity hydrogen stream. The partially oxidized particles

then enter the combustor, an entrained bed. Air is used to

regenerate the particles back into Fe2O3 while generating heat.

Additionally, the combustor will convey the particles back into

the reducer to complete the loop. From this process design

scheme, theH2, CO2, andheat streamsareproduced in separate

reactors. Therefore, no separation steps are required reducing

the systems costs thereby increase the process efficiency.

Proper design allows this system to be self-sustaining. The heat

generated from the combustor can be used to perform the

slightly endothermic reactions that occur in the reducer. The

particles can serve as the heat carrier for this step.

Fig. 2 e Process flow of biomass direct chem

The overall reaction scheme for the BDCL process in the

reducer, oxidizer and combustor is provided below.

aCþ bH2Oþ ða� b=2ÞO2/aCO2 þ bH2 þ DH (1)

For simplicity, ‘a’ represents the number of carbon compo-

nents in biomass, while ‘b’ is the components of water used.

This formula illustrates the flexibility of the chemical looping

system. By varying the carbon to steam ratio, a higher

concentration of either hydrogen or heat can be yielded. This

can be performed by placing a split stream to directly send

a portion of the particles from the reducer to the combustor

[58,59]. Direct combustion of the reduced particles produces

more heat. This energy can be used to drive a steam turbine

for electricity. Additionally, the oxidizer can be completely

eliminated if the only desired product is power. Fig. 3 depicts

BDCL process for electricity generation.

The BDCL process was first conceived with coal as the

feedstock, coal direct chemical looping (CDCL). The BDCL

system parallels the CDCL design. Further details regarding

the CDCL process and the Fe2O3 particle can be found in

previous works [55,56].

2.2. Biomass characteristics

Before discussing the technical challenges in the BDCL

process, the biomass characteristics will first be discussed.

Biomass is readily abundant, but exists widely in various

forms depending on the season and location. For example,

woody plants grow profusely in Scandinavian countries,

Canada and Japan, while herbaceous plants, such as switch

grass, are more available in the United States [60e64].

Municipal waste, raw sludge, and sewage sludge are also

defined as biomass. However, the high moisture and high ash

content within the material makes them not suitable for

thermo-chemical energy conversion process until an efficient

ical looping for hydrogen generation.



Fig. 3 e Process flow of biomass direct chemical looping combustion for power generation.


dewatering process is developed. Therefore, only woody and

herbaceous plants, generally called lignocellulosic biomass,

will be considered in this article.

Lignocellulosic biomass consistsmainly of cellulose, hemi-

cellulose, and lignin with a minor portion composed of

protein, ash and other compounds. Table 2 summarizes the

compositions of three types of biomass. Additionally, it also

illustrates how the chemical composition varies significantly

depending on the species of the lignocellulosic material. For

thermo-chemical processes, this equates to fluctuating reac-

tion behavior dependent of the type of plant residue being

used. Table 3 then compares the elemental compositions of

these plants with coal. A high concentration of carbon is seen

in the coal while oxygen is more abundant in biomass due to

the higher degree of oxidation of cellulose and hemi-cellulose.

Furthermore, the plant residues also have a lower concen-

tration of fixed carbons. Therefore, the cellulosic material is

more abundant with carbonaceous volatiles when compared

to coal. Table 3 shows that biomass is composed of about

75e85% volatile matter. This characteristic is advantageous to

the BDCL process because the reactions between the metal

oxide and the volatiles are much faster than with the solid

fuel. The use of the enhancer gas in the reducer is to promote

this gasification affect as described in the previous section.

Table 4 illustrates how the ash content is much lower in

biomass compared to coal. Depending on soil incorporation,

the cellulosic material is composed of 0.5e3% ash on the dry

mass basis. Additionally, the heavy metals within, such as

cadmium, lead, and mercury, are below the detection limit

making the ash treatment much less energy intensive when

Table 2 e Cellulose, hemi-cellulose and lignin content ofbiomass [28].

Biomass wt % on dry basis(Average) Cellulose

Hemi-cellulose

Lignin

Wheat straw 36.5 22.5 17.5

Switch grass 40 25 12.5

compared to fossil fuels. However, the alkali metals within

biomass, such as potassium, will volatilize in the high

combustor operating temperatures only to precipitate in the

down stream pipelines causing slagging and fouling.

Furthermore, wood with paint or repellent spray may contain

a significant amount of heavy metals.

The moisture content of forest biomass is much higher

when compared to coal and varies depending of plant type

and amount of processing. Generally, fresh cut wood has

a 50e60% moisture content on the wet mass basis. This can

equilibrate to 15e40% of moisture content after several weeks

of sun drying. The wood may be losing the moisture to 5e20%

by the heat produced in pulverization process. Additionally,

fresh cut switch grass may have 70% moisture content.

The nitrogen and sulfurwithin biomass are below 1% of the

total mass friction. From the BDCL thermodynamic calcula-

tions, the nitrogen will exit the system as N2 while the sulfur

will be released as SO2 from the reducer, because of its

concentration within the biomass [57]. Therefore, the sulfur

contamination to thehydrogenproduct streamcanbeavoided.

Due to a low carbon and high oxygen and water content,

the biomass heating value is relatively low. Several factors can

affect the heating value of the plant material [65,66]. For

example, the lignin content is strongly correlated to the

energy density of biomass. The higher the lignin content

corresponds to an increase in heating value. Additionally,

Fig. 4 shows how themoisture content influences the effective

heating value [67]. The energy density of biomass decreases

significantly with an increase in moisture content. The bulk

density of the lignocellulosic material is considerably low

because of its hollow structure characteristics. The bulk

density and heating value properties equate to the energy

consumption required for processing, transportation, and

storage material of interest.

2.3. Potential challenges for BDCL process

For the BDCL process development technical feasibility

assessment, a number of hurdles have to be overcome. Fig. 5



Table 3 e Proximate and ultimate analysis of biomass and bituminous coal.

Biomass Proximate analysis Ultimate analysis Heating value

wt % on dry basis wt % on dry basis (d.a.f) MJ kg�1 (HHV)

VM FC Ash C H Oc N S

Wood pelleta 80.4 14.5 0.2 45.5 6.6 47.7 <0.1 <0.1 18.6

Wheat strawa 77.9 21.5 6.8 56.7 6.7 48.8 1.0 0.2 15.3

Switch grassb 88.5 7.8 3.7 48.3 6.0 44.6 0.5 0.6 17.3

Bituminous coala 32.3 48.1 14.7 65.7 5.6 7.7 1.2 0.5 28.3

a Khan et al. [65].

b Robinson et al. [66].

c by difference.


shows a general pathway of biomass thermo-chemical energy

conversion process. A reduction or elimination of any step

equates to an increase in overall efficiencywhich correlates to

a greater profitability.

Pretreatment is the preparation of the lignocellulosic fuel

before entering the conversion process. Specifically, for the

BDCL system, this mainly involves densification (dewatering,

drying, pelletizing) and sizing (pulverizing). Drying is impor-

tant to this system for two reasons: the vaporized steam can 1)

oxidize the reduced iron oxide particles in the reducer and 2)

decrease the temperature of the reactor. Both effects will

result in a decreased conversion rate and purity of the product

stream. Biomass sizing is also an important parameter. It

influences the fuel/particle mixing capability and the pyrol-

ysis/gasification rate of the lignocellulosic material. Conse-

quently, the energy conversion efficiency is affected by the

sizing. Overall, the pretreatment step is an energy consuming

process for the BDCL system. 2.03 MJ of energy is required to

dry 1 kg of wood chips from 50% to 20% of moisture content.

Pulverizing wood to less than 100 mm biomass powder

requires 2.27 MJ kg�1 of dry-wood, an unacceptable energy

consumption. Fig. 6 illustrates how energy consumption for

pulverization increases exponentially as particle size is

reduced [68].

To reduce costs, the surplus heat produced from the

combustor can be used to dry the feedstock. Additionally,

larger biomass grains (1 mm or more) can be tested for reac-

tivity with the iron oxide particles. To reduce transportation

costs and feeder complications, pelletizingmay be required for

reducing the bulk density of the lignocellulosic materials [69].

However, a significant energy demand will be necessary to

pulverize andpelletize thebiomass.Alternatively, torrefication

Table 4 e Ash analysis of biomass and bituminous coal.

Biomass wt % on dry ash basis

SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O

Wood pelleta 4.3 1.3 1.5 8.5 55.9 0.6 16.8

Wheat strawa 53.1 3.6 1.2 3.0 17.7 4.5 30.0

Switch grassb 58.15 2.3 11.51 5.44 11.18 0.38 9.06

Bituminous coala 59.7 20.3 7.0 1.9 1.8 1.0 2.3

a Khan et al. [65].

b Robinson et al. [66].

maybeapotential solution to thehighpretreatment costs. This

method refers to a mild thermal treatment where the biomass

is converted into a coal-like substance [70]. Torrefication can

both reduce the costs for milling and increase the energy

density of the cellulosic plant.

One of the largest technical barriers for biomass gasification

processes is the formation of tar. It causes low carbon conver-

sion and choking in the system. More than 60% of the biomass

volatiles become first stage tar components at around 850 K in

fast pyrolysis processes [41,71e74]. High temperature gas

cleaning or catalytic reforming is usually required to remove

highly viscous material from the process [72,75], reducing the

energy conversion efficiency and increasing the operating cost.

Since the tar components are exposed to over 1400 K in the

BDCL reducer, it may decompose to gaseous components

before exit. Furthermore, preliminary experiments show that

the oxygen carrier is capable of thermally cracking the tar at

1050 K in a N2 environment. Therefore, tar formation may not

be a significant issue for the BDCL process. Further discussion

Fig. 4 e Effect of moisture content and ahs content on

effective heating vale of various biomasses (Effective

heating value is calculated based on the estimation

method suggested by Ince [67]).



Infrastructure

CollectionCultivation Pretreatment Thermo-chemical

Energy conversionAfter treatment Production Distribution

Cutting

Transportation

Storage

Dewatering

Drying

Pulverizing

Pelletizing

Gas clean up

(Tar, Particle matter,

Ash, S, Cl, Alkali)

Reforming

CO2

capture

Upgrading

Fertilizing Syngas conversion

Synthesis

Separation

Adjustment

Storage

Fig. 5 e General pathway of biomass thermo-chemical energy conversion process.


regarding the biomass tar decomposition with the oxygen

carrier particle will be provided in future reports.

Another technical challenge is the biomass ash. Gasifica-

tion processes will most likely have no substantial issues with

this because of its low content in biomass. However, in some

lignocellulosic species, the ash content is relatively large with

a high concentration of alkali metals such as potassium.

These metals reduce the melting temperature of the ash

allowing for fouling, deposition, corrosion, slagging and

agglomeration to occur in the reactor [65,76,77]. Fouling or

other issues related to ashmelting almost always is a problem

in the thermo-chemical process with a solid fuel feed. For the

BDCL process, an ash removal system may be required to

separate it from themetal oxides particles exiting the reducer.

Above technical problems are common challenges faced in

conventional biomass thermo-chemical conversion process.

The technical hurdles needs to be overcome peculiar in the

BDCL process are selection of oxygen carrier and design of

rector [56,78e80]. Life time of the oxygen carrier, including

attrition, deterioration and losses, is one of the biggest

problem as well as conversion ratio of the carrier. These

problems are directly related to the cost of BDCL operation and

the device design. Unlike rapid oxidation reaction in the

combustion process, the reaction in the reducer and the

oxidizer are relatively slow by the limitation of thermody-

namic equilibriums. Gas-solid contacting pattern inside the

reactor is important parameter to promote fuel and solid

1

10

100

1000

10000

10 100 1000 10000

50% particle size (µm)

Po

we

r c

om

su

mp

tio

n (

kW

h/t

)

Vibration mill

Impact mill

Circulating mill

Hammer mill

Fig. 6 e Relationship between power consumption of

woody biomass pulverization and pulverized woody

powder size [68].

conversion. Moreover, circulation of high temperature solid

between the reducer and the oxidizer would be serious

problem for the development of chemical looping system,

especially selection of the reactor material. Optimal design of

the reactor and oxygen carrier as well as overall system

considering above biomass characteristic is important.

Further research will be performed to determine the optimum

design for this system.

2.4. Preliminary feasibility assessment

The following sections discuss the feasibility of employing the

BDCL process based on preliminary studies relative to

competing biomass technologies and conventional fossil fuel

conversion schemes. Two criterions are considered; energy

conversion efficiencies and economic analysis.

2.4.1. Energy conversion efficiency analysisGenerally, energy conversion efficiency for a power plant

increase when scaling up but decrease when scaling down.

One of the reasons of the efficiency drop is due to the avail-

ability of power generator. Fig. 7 depicts the efficiencies of

several generators at various capacities. Except for fuel cells,

power generation efficiency decreases significantly with the

capacity. Though fuel cells provide the best efficiency at all

plant capacities, it is not currently cost-effective to use [81].

Due to the relatively low energy density and highly distrib-

uted-resource, the biomass power plant capacity is much

smaller compared its fossil fuel counterpart. Therefore,

a system integrated with an optimum power generation

scheme is required to make the lignocellulosic conversion

industrially feasible. The efficiency of the BDCL system is

compared to various other thermo-chemical processes [82] in

Power generation capacity (kW)

Po

wer g

en

eratio

n e

fficie

nc

y (%

L

HV

)

1 10 100 1000 10000

0

10

20

30

40

50

Stirling Engine

Gas Engine

Micro Gas Turbine

Middle scale

Gas Engine

Middle scale

Gas Turbine

Steam Turbine

Middle scale

Fuel cell

Fig. 7 e Biomass power plant efficiencies in various power

generators.



Fig. 8 e Power generation efficiency of various energy

conversion processes (The efficiencies for the BIGCC,

biomass combustion and IGCC systems are obtained from

the literature [82]).


Fig. 8. Li et al. [57] calculated the power generation efficiency of

the 100 MW scale BDCL process with carbon capture to have

a 38% (HHV). Scaling down the BDCL system to 15 MW plant,

the power efficiency falls to 32% (HHV). Additionally, the figure

illustrates how a significant amount of energy is required to

scrub the carbon out of the product stream in other gasifica-

tion and combustion process. The preliminary results show

that the BDCL system is capable of having high power gener-

ation efficiency for both large and small scale plants. More-

over, if the end product of this process is hydrogen, then the

efficiencies increases to 74% (HHV) [57], because H2 is

considered a lower grade of energy compared to electricity.

This highly pure product can be utilized to produce other

valuable chemicals or later used in fuel cells if they become

more cost-efficient. Because of the high efficiency drop in

Table 5 e Performance and cost of biomass energyconversion processes.

BDCL IBGCC IBGCC withC capture

Capacity (MW) 100 100 100

Net power generation

efficiency (% HHV)

38.1a 30b 21b

CO2 capture rate

(% input Carbon basis)

99 0 90

Capital costc ($ kW�1) 1630 2190 3650

Non-fuel O&M costd

($ kW�1 year�1)

80 150 230

Electricity coste

(cents kWh�1)

9.5 18 27

a base on ASPEN simulation of BDCL process, [55,57,59].

b Post-CO2 system by scrubber.

c Assume a scaling factor of 0.7, 10% interest over 30 years, costs

were converted to year 2000 USD.

d Estimated based on a percentage of the capital investment [88].

e 10 USD for CO2 transportation cost and CO2 sequestration cost.

power generation, hydrogen production appears to be an

alternative option for the small scale facilities.

2.4.2. Initial economic analysisThe cost and performance of the BDCL process is summarized

in Table 5 along with comparisons to an integrated biomass

gasification combined cycle (IBGCC) system with and without

CO2 capture. All three processes are scaled to 100 MW. The

cost estimations for the IBGCC process are based off coal fed

IGCC plants [83e85]. The power generation efficiency of the

BDCL process is based on ASPEN simulation [57] and the effi-

ciencies for the IGCC systems are obtained from the literature

[86,87]. The preliminary cost estimates for the BDCL system

are obtained from economic studies of biomass gasification

processes [55,59]. The cost components considered include

capital, operation and maintenance (O&M), and fuel costs.

Capital cost was calculated using a scaling factor of 0.7 and

O&M cost was estimated based on a percentage of the capital

investment suggested by Peters et al. [88]. The cost of the

biomass fuel was assumed to be 50 $ t�1 on the dry mass basis

[87]. The CO2 transportation and sequestration cost was esti-

mated to be 10 $ t�1-CO2 [86]. Conversely, by installing CO2

capture facility on the IBGCC process, the capital cost

increased by 66% and power generation efficiency decreased

by 30% (HHV). The significant increase in capital cost coupled

with the severe energy penalty drives up the cost of electricity

by 50% when 90% of the CO2 is captured from the biomass

based IGCC process. Alternatively, the BDCL process achieves

high power generation efficiency (38.1% (HHV)) at relatively

low costs. In addition, 99% of the carbon in biomass is readily

captured from the BDCL process. The resulting cost of elec-

tricity from BDCL is roughly one half of that from the IBGCC

without CO2 capture and a third of that with carbon capture. It

should be noted that a cost of electricity at 95 $ MWh�1 is still

relatively expensive compared to 66 $ MWh�1 of oxy-coal-

combustion process with 97% CO2 capture [86]. However, the

electricity cost falls to around 60 $ MWh�1 when carbon credit

is clamed at 20 $ t�1. Furthermore, re-performing the

economic analysis for optimum hydrogen production yielded

a cost of around 15.3 $ GJ�1 for hydrogen.

3. Conclusion

With regulations pressing for cleaner energy conversion

processes, several technologies are being developed and

employed. A first step to reducing carbon emissions is

increasing the efficiencies of the current fossil fuel fed power

plants. CCS techniques are also being developed to sequester

the GHG before being emitted into the atmosphere. However,

such methods require a certain amount of parasitic energy to

purify the CO2 stream reducing the overall process efficiency.

Furthermore, with current energy conversion systems

dependent on fossil fuels, a depleting resource, an alternative

renewable resource is being sought.

Biomass becomes a potential candidate for providing both

CO2 concentration stabilization and reducing fossil fuel use.

Several studiespredict thatbiomasswill contributesignificantly

to theworld energy demand in the near future. However, due to

its relatively low energy density, high moisture content, and




highly distributed-resource, this lignocellulosic material has

proven to be less efficient andmore cost intensive compared to

the current fossil fuel processes. Therefore, a highly efficient

process is required to penetrate the economic barrier of today’s

energy market to achieve commercialization of the biomass

based energy. The conventional biomass thermo-chemical

processes, such as direct combustion or gasification, have

proven to be technically feasible. However, the power genera-

tion efficiency is limited by the small scale facility making it

difficult to compete against fossil fuel based energy.

The BDCL process has the potential to compete against

conventional energy generation systems via its high conver-

sion efficiency. This technology utilizes a metal oxide

composite particle to react directly with the carbon based fuel

in oxidation/reduction reaction scheme. The design of the

process runs parallel with previous works with coal as the

direct feedstock. Additionally, the BDCL does not require

a significant energy intensive step to capture the CO2 released

because the carbon stream is separated from the other prod-

ucts via reaction path design. The net result is a carbon

negative process. A preliminary economic analysis demon-

strates that BDCL system integrated with CCS to receive

carbon credit is capable of producing electricity at 60 $

MWh�1, a competitive price to the current fossil fuel

processes. However, it is still in the early stages of develop-

ment. Several factors such as fuel particle size, moisture

content, and tar issues have to be considered before

a complete feasibility analysis can be performed.

Acknowledgement

We would like to express our appreciation to Andrew Tong,

Fanxing Li, Hyung Kim and Liang Zeng for their technical and

editorial assistance during the preparation of thismanuscript.

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