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Exergy analysis of synthetic natural gas production method from biomass

Martin Juraščík, Ana Sues, Krzysztof J. Ptasinski

Eindhoven University of Technology, Chemical Engineering Department,

P.O. Box 513, Helix STW 1.22, 5600 MB Eindhoven, The Netherlands m.jurascik@tue.nl, a.sues@tue.nl, k.j.ptasinski@tue.nl

ABSTRACT: The paper presents the exergy analysis results concerning a biomass to synthetic natural gas (SNG) conversion process. The presented study is based on wood gasification integrated with CH4 synthesis. The analysed temperature of gasification was 700°C and the pressure was changed from 1 to 15 bar. The main process units of biomass-to-SNG conversion technology are gasifier, gas cleaning, synthesis gas compression, CH4 synthesis and final SNG condition. The results showed that the largest exergy losses take place in the biomass gasifier, CH4 synthesis part and CO2 capture unit. The overall exergetic efficiency of the biomass-to-SNG process was estimated in the range of about 62.8 – 63.9 %.

Keywords: exergy analysis, renewable energy, biomass, SNG

1. INTRODUCTION

Most of the world energy consumption is supplied by non-renewable energy sources such as oil, coal and natural gas. In many countries the main energy source is natural gas. In Europe, there are two kinds of natural gas: high calorific and low calorific gas. The high calorific gas consists mainly of CH4 and often contains some other hydrocarbons. The low calorific gas contains less CH4 and contains considerable amounts of N2 or CO2. For instance, in the Netherlands, about half of the energy input comes from the natural gas, of which a significant part is the Groningen gas (low calorific gas).

In order to solve the problems of depletion of fossil fuels and their destructive influence on

the environment, synthetic natural gas (SNG) is suggested as the important future energy carrier. Sustainable SNG produced from biomass can provide an attractive option for renewable biofuels.

The conventional route for SNG production is based on gasification of biomass to produce

synthesis gas and the subsequent methanation of the synthesis gas to SNG. Biomass gasifiers typically produce a synthesis gas containing CO, H2, and CH4 as the main components that carry the majority of energy in addition to remaining components as CO2, H2O and N2, and also a variety of potential contaminants like tars, ammonia, alkalis, etc. First of all, gas cleaning is needed. Secondly, the subsequent chemical processing of the synthesis gas requires a specific gas condition, such as desired H2/CO ratio, temperature and pressure. Then the gas enters the methanation synthesis step. The methanation is a catalytic reaction and there is a substantial risk of catalyst overheating or deactivation due to carbon formation. Finally the product gas from the methanation synthesis is processed to meet the requirements for SNG.

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2. OBJECTIVES

The objective of this paper is to determine the exergetic efficiency of the biomass-to-SNG conversion process. In the investigated technology a woody stream was chosen as a feedstock to produce SNG, which has to meet the local quality requirements.

This work is focused on the north part of the Netherlands, where the Groningen gas is

distributed. The main requirements for the SNG produced are: gross calorific value (HHV) 31.6 – 38.7 MJ/Nm3 and Wobbe index 43.4 – 44.4 MJ/Nm3 [1].

3. PROCESS DESCRIPTION

Figure 1 shows a block diagram of a biomass gasification process integrated with CH4

synthesis studied in this work. Woody matter is considered as a feedstock, with the composition shown in Table I.

Table I: Composition of woody biomass. Moisture Content [wt % wet] 13.8

Carbon [wt % dry] 49.03 Hydrogen [wt % dry] 5.74 Oxygen [wt % dry] 39.41 Nitrogen [wt % dry] 1.62 Sulphur [wt % dry] 0.08 Chlorine [wt % dry] 0.10

Ash [wt % dry] 4.02 HHV [MJ/kgdry] 19.7

The biomass feedstock (at the wet biomass flowrate of 10 kg/s) was gasified in a direct

gasifier. Steam was used as a gasification agent at the temperature of 227 °C and at the pressure of gasifier. An external heat was used to control the gasification temperature of 700°C and the pressure range was changed from 1 to 15 bar. The gasifier was operated at the carbon boundary line, which determines the optimal conditions for operating the biomass gasifier from the thermodynamics point of view [2]. The flowrate of the gasification agent was adapted to keep the gasifier at the carbon boundary conditions.

After gasification the synthesis gas was cooled to 30°C and condensed water was removed

at 1 bar. Subsequently, the synthesis gas was pressurized to 28.5 bar using a three-stage compressor with an intercooling heat exchanger. The compressed synthesis gas was heated to the temperature of 398°C and it passed through a gas cleaning section. It was decided to use dry high temperature adsorption methods for gas cleaning since the synthesis gas from woody biomass gasification contains low amounts of chlorine and sulphur components [3]. However, this section was not simulated in details; to calculate the exergy losses a pressure drop of 0.5 bar was assumed.

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Subsequently, the synthesis gas entered the CH4 synthesis section at the pressure of 28 bar.

The CH4 synthesis is a catalytic exothermal process. In this study a nickel based catalyst was applied. It was assumed that the catalyst has a water-gas-shift activity. To simulate the CH4, synthesis the steam-moderated ICI high-temperature once-through methanation process was chosen [4]. This section consists of 3 methanation rectors and 2 heat exchangers placed between the methanation rectors in order to control the temperature of gas entering the 2nd and 3rd methanation reactors. The inlet gas temperatures of the methanation reactors were kept constant according to the ICI process (1st 398°C, 2nd 325°C and 3rd 300°C). Steam (398°C, 28 bar) was added to the methanation section to avoid carbon formation. Since a low temperature has a positive influence on the methanation process and there is a risk of overheating of the catalyst, the temperature in methanation reactors was controlled in order not to exceed the desired temperatures (729°C for the 1st reactor, 590°C for the 2nd reactor, 425°C for the 3rd reactor). Hence, according to the synthesis gas composition (influenced by gasification conditions) methanation reactors were operated adiabatically or cooled. This is discussed later in the results section of this paper.

After methanation section the product gas was cooled to temperature of 40°C and condensed water was separated from the gas at the pressure of 28 bar. The cooled product gas was compressed to the pressure of 40 bar and subsequently entered a CO2 removal section. Also the condensed water, which contains dissolved CH4, entered the CO2 removal section where the dissolved gas was released. A physical absorption method based on dimethylether of polyethylene-glycol solvent (commercially called SELEXOL method) was chosen as a CO2 removal technology. A detailed description of the physical absorption CO2 removal section can be found in the work of Lampert and Ziebik [5], which concept was adopted for this work. The CO2 removal absorption column worked at the temperature of 30°C and the pressure of 40 bar. A CO2 rich gas stream leaving the CO2 removal section was considered in this study as a waste. Finally, the produced SNG was pressed to 66 bar and cooled to 25°C.

Figure 1: A block diagram of a biomass gasification process integrated with methane synthesis.

Gasifier G-L flash Gas compressor

Gas heating & cleaning

Methanation section

Gas cooling

CO2removal

SNG compressor

SNG cooling

1 - biomass

2 - steam 3 - heat

heat (waste)

ash (waste)

cooling water

4 - steam

7 - steam (reactant)

liquid (waste)

5 - work waste

cooling water

10 - steam

cooling water

8 - steam

CO2 rich gas (waste)

12 - work 14 - SNG

13 - work

cooling water

hot water

heat (waste)

liquid

gasSNG

compressor

11 - work

9 - heat

6 - heat

Gas cooling

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3. METHODS The biomass-to-SNG process is simulated with a computer model using the flow-sheeting

program Aspen Plus. Process simulations for various gasification pressures were carried out. The equilibrium (Gibbs) model was used to simulate the gasifier and methanation synthesis, thus tars were not produced in the gasifier. However, in the real process tars are present in the synthesis gas depending on the conditions used in the gasifier.

3.1 Exergy analysis

The development of efficient technologies for biomass gasification and synthesis of biofuels requires a correct use of thermodynamics. Usually, a process analysis is based on energy and mass balances. However, this type of analysis only shows the mass and energy flows of the process and does not take into account how the quality of the energy and materials degrades through the process due to dissipation. Exergy analysis is a relatively new method of thermodynamic analysis that has recently been applied in power and heat technology, chemical technology, and other fields of engineering and science. The exergy method takes into account not only the quantity of materials and energy flows, but also the quality of materials and energy flows as well. This uniform approach to materials and energy flows is the key to the application of this new concept.

The exergy concept is based on both the first and second law of thermodynamics. The main reason of exergy analysis is to detect and evaluate quantitatively the losses that occur in thermal and chemical processes. The majority of the causes of thermodynamic imperfection of thermal and chemical processes can be detected by analyzing the process performance using exergy analysis what is not possible by means of energy or enthalpy balances. It is also important to point out that although exergy analysis can give us information about the possibilities of improving thermal and chemical processes, it cannot state whether or not the possibilities are practically and economically viable. Such kind of questions can be answered only by an economic analysis, which is not discussed in this review.

Exergy is defined as the maximum amount of work that can be obtained from a material stream, heat stream or work interaction by bringing this stream to environmental conditions. The environmental state is a crucial concept in exergy analysis. The term environment is regarded as a medium composed of common substances existing in abundance within the Earth’s atmosphere, oceans, and crust. The reference state is usually taken to be at standard temperature (To = 25°C) and pressure (Po = 1 atm).

Among the different forms of exergy, three forms are the major contributors to total exergy and they are: thermal exergy, work exergy, and exergy of material. The exergetic value of a heat flow that is its quality of energy, is the maximum amount of work that could be obtained from it by using the environment as a reservoir of zero-grade thermal energy. The quality of thermal energy is determined by the Carnot factor τ, defined as (T-To)/T, where To is the environmental temperature, and T is the temperature of a heat stream. The exergy content of the heat stream Q, called as a thermal exergy ΕQ, is calculated as:

)1(TTQQ oQ −==Ε τ (1)

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Work interaction, such as mechanical Wx, or electrical work We, is a completely ordered form of energy and therefore the exergy value ΕW equals to the amount of work done.

exW WW ==Ε (2)

The quality of a material stream can be expressed using its physical and chemical exergy.

chpht εεε += (3)

where εt is the total material exergy, εph is the physical exergy and εch is the chemical exergy.

The physical exergy εph is equal to the maximum amount of work obtainable when a compound or mixture is brought from its temperature T and pressure P to environmental conditions, characterized by environmental temperature T0 and P0. The physical exergy is calculated using enthalpy and entropy data for the given system:

( ) ( )00 ssThh oph −−−=ε (4)

where h and s are enthalpy and entropy of a system at given temperature and pressure, and ho and so are the values of these functions at the environmental temperature and pressure.

The standard chemical exergy of a pure chemical compound εch is equal to the maximum amount of work obtainable when a compound is brought from the environmental state, characterized by the environmental temperature T0 (25°C) and environmental pressure P0 (1 atm), to the dead state, characterized by the same environmental conditions of temperature and pressure, but also by the concentration of reference substances in standard environment. For instance, the standard reference substance for carbon is CO2 at the concentration of about 300 ppm, as it represents the lowest thermodynamic value of all carbon containing compounds.

The exergy balance of a process can be represented in the following form using exergy values of all streams entering and leaving the process:

∑ ∑∑∑∑ ++Ε=++ΕOUT OUT

Qml

IN

Wk

IN

Qj

INi IEEE (5)

where ∑ΕIN

j and ∑ΕOUT

k are exergy flow of all entering and leaving material streams,

respectively, ∑ΕIN

Qj and ∑Ε

OUT

Qm are the sums of all thermal exergy entering and leaving a

process, respectively, and ∑ΕIN

Wk is the sums of all work entering a process. The difference

between the concept of exergy and those of mass and energy is that exergy is not conserved but subjected to dissipation. It means that the exergy leaving any process step will always be less than the exergy in. The difference between all entering exergy streams and that of leaving streams is called irreversibility I.

Irreversibility represents the internal exergy loss in process as the loss of quality of materials and energy due to dissipation and it relates to entropy production Π in the system:

Π= oTI (6)

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The exergetic efficiency is defined as the ratio between useful exergy outputs from the process to the necessary exergy input to this process. In this study three definitions of rational exergetic efficiencies were applied as follows:

∑ ∑∑ ++=ψ

IN IN

Wk

IN

Qji

SNG1 EEE

E (7)

∑ ∑∑∑

++

+=ψ

IN IN

Wk

IN

Qji

OUTnsteamSNG

2 EEE

EE ,

(8)

∑ ∑∑∑∑

++

++=ψ

IN IN

Wk

IN

Qji

OUT

Qmprod

OUTnsteamSNG

3 EEE

EEE ,,

(9)

where: Ψ1, Ψ2, Ψ3 are the exergetic efficiencies, ESNG is the exergy flowrate of the product SNG stream, ∑Ε

OUTnsteam , is the sum exergy flowrate of product steam streams, ∑

OUT

QmprodE , is the

sums of all product thermal exergy of a process, ∑ΕIN

i is the exergy flow of all entering

material streams, ∑IN

QjE and ∑

IN

WkE are the sums of all thermal exergy and work entering a

process, respectively.

In this work, exergy of streams (biomass, gases, liquid, heat and work) was calculated using

the method of Szargut at al. [6]. Flowrate, temperature, pressure and exergy flowrate of main process streams for the gasification condition of 700°C and 1 bar are listed in Table II. Table II Flowrate, temperature, pressure and exergy flowrate of main streams in the biomass-to-SNG process for standard gasification condition 700 °C and 1 bar (see Fig. 1).

No. Stream

input / product

T [°C]

P [bar]

mass flow[kg/s]

exergy flowrate [MW]

1 biomass input 25 1 10 178.5 2 steam input 227 1 3.45 2.13 3 heat input - - - 32.42 4 steam product 100 1 6.9 3.91 5 work input - - - 11.84 6 heat input - - - 2.35 7 steam input 398 28 5.75 7.16 8 steam product 212 20 9.46 9.46 9 heat product - - - 7.25 10 steam product 100 1 11.21 6.35 11 work input - - - 0.41 12 work input - - - 10.72 13 work input - - - 0.36 14 SNG product 25 66 3.28 133.26

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4 RESULTS AND DISSCUSION Influence of gasification conditions on irreversibility

Fig. 2 shows process irreversibilities (I, internal exergy losses) of the technological units for

the biomass-to-SNG process operating the gasifier at the temperature of 700°C at four different gasification pressures (1, 5, 10 and 15 bar, respectively) based on the wet biomass flow rate of 10 kg/s. As can be seen the largest process irreversibilities take place in the gasifier, methanation section and CO2 capture unit.

On the other hand, process irreversibilities of gas cleaning, product gas compression and

final SNG compression step (0.06, 0.14 and 0.12 MW, respectively) are rather small that they can be almost negligible.

After methanation section, about 91 % of CO2 present in the product gas has to be removed

to reach the quality requirements for the SNG. The final composition, the gross calorific value (HHV) and the Wobbe index of the produced SNG are listed in Table III.

All the three methanation reactors were cooled when the gasifier was operated at the lowest

pressure used (1 bar). Using the other gasification pressures (5, 10 and 15 bar, respectively), the methanation reactors operate adiabatically. The temperatures of the rectors are indicated in Table III. The explanation of this fact could be that at higher gasification pressures a significant amount of CH4 was already produced in the gasifier, resulting in less production of CH4 by the exothermic CH4 synthesis in the methanation reactors. Hence, the reactors may work adiabatically without overheating.

At the lower gasification pressures used (1 and 5 bar) the produced SNG contains 12.5 and

12.3 mol.% of H2, respectively. It is indicated that the maximum allowed amount of H2 is 10 mol.% [7]. However, when mixing with the natural gas is applied, the H2 content of the final gas can be decreased. Higher gasification pressures (10 and 15 bar) results in the final SNG, of which the Wobbe index is slightly higher than the requirement is.

0

5

10

15

20

25

gasifie

r

cooli

ng I

SynG fla

sh

SynG co

mpres

SynG he

ating

clean

. Up I

methan

ation

cooli

ng II a

fter m

eth +fla

sh

R SNG compres

CO2 cap

ture

SNG compre

+ coolin

g

I [M

W]

1 bar5 bar10 bar15 bar

Figure 2 Internal exergy losses per process sections of the biomass-to-SNG technology at different pressures.

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Influence of gasification conditions on exergetic efficiency Figs. 3 shows a comparison of the exergetic efficiencies Ψ1, Ψ2 and Ψ3 according to

Eqs. (7-9) of the biomass-to-SNG process at different gasification pressures. Gasification pressure has a positive influence on the exergetic efficiency to SNG produced in the process.

The exergetic efficiency Ψ1 (see Eq. 7) considering only SNG as the process product ranges

between 53.2 to 55.8 %, see Fig. 3. If the produced steam is considered as the additional product, then the efficiency of the process (Ψ2, Eq. 8) increases by adding in average about 8 %, see Fig. 3. Moreover, if the heat released from cooled methanation reactors is considered also as a product then the efficiency of the process (Ψ3, Eq. 9) increases, see Fig. 3. Obviously, this increase occurs only when the methanation reactors are cooled (operate none adiabatically, at the gasification pressure of 1 bar). In Table III it is indicated when the methanation reactors were cooled.

However, it should be noticed that these results were obtained using equilibrium model for

the gasifier, which does not take into account tars formation in the gasifier. Generally, at lower gasification temperatures, higher amount of tars is presented in the synthesis gas than in the cases when higher gasification temperatures are used. Hence, there is need to used efficient tar breaking catalyst in the gasifier and/or well operated tar cleaning.

0 2 4 6 8 10 12 14 16

53

54

55

56

61

62

63

64

Ψ [

%]

Gasifier pressure [bar]

Ψ1, Ψ2, Ψ3

Figure 3 Exergetic efficiency (Ψ3) of the biomass-to-SNG technology at various gasification conditions

5 CONCLUSION This paper shows the results of the exergetic analysis of biomass-to-SNG process. The main process units of this technology are gasifier, gas cleaning, synthesis gas compression, CH4 synthesis and final SNG condition. The study was focused on the influence of the gasification pressure on the whole process. The analysed gasification temperature was set to 700°C and the pressure range was changed from 1 to 15 bar. The results showed that the largest exergy losses take place in the biomass gasifier, CH4 synthesis part and CO2 capture unit.

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Moreover, it was demonstrated that increasing gasification pressure has a positive influence on the exergetic efficiency to SNG and produced steam in the biomass-to-SNG process. The efficiency may be improved by considering heat stream, which was released from the process, as a product. Then, the positive influence of the gasification pressure does not apply strictly. However, the highest exergy efficiency of about 63.9 % was achieved using the lowest and the highest gasification pressures (1 and 15 bar).

Table IIII Influence of the different gasification conditions on the main characteristic of the biomass-to-SNG process.

Gasification pressure [bar] 1 5 10 15 SNG produced [kg/s] 3.28 3.29 3.31 3.30 Composition of SNG: H2 [mol %] 12.5 12.3 7.5 5.9 CO [mol %] 0.6 0.5 0.2 0.1 CO2 [mol %] 7.6 7.6 8.0 8.0 CH4 [mol %] 76.8 77.0 81.7 83.4 N2 [mol %] 2.5 2.5 2.6 2.6 Quality parameters of SNG: HHV [MJ/Nm3] 32.3 32.4 33.6 34.0 WOBBE index [MJ/Nm3] 43.4 43.4 44.0 44.3 CH4 synthesis temperature: 1st reactor [°C] 729 701* 652* 623* 2nd reactor [°C] 590 578* 520* 488* 3rd reactor [°C] 428 426* 377* 356* Percentage of the total amount of CH4 produced in the following parts of the process: Gasifier [%] 13.1 39.5 52.5 59.6 1st reactor [%] 45.1 25.9 21.7 19.2 2nd reactor [%] 28.4 22.7 18.2 15.6 3rd reactor [%] 13.4 11.9 7.6 5.6 * these methanation reactors worked adiabatically

ACKNOWLEDGEMENTS The project is sponsored by the Cartesius Institute, in Leeuwarden, Friesland, The Netherlands. REFERENCES [1] Mozaffarian M., Zwart, R.W.R. Feasibility of biomass/waste-related SNG production

technologies. ECN Report No. ECN-C-03-066, Petten, The Netherlands, 2003. Aviable at:

www.ecn.nl/publications

[2] Prins J.J., Ptasinski K.J, Janssen H.J.J.G. Thermodynamics of gas-char reactions: first and second law analysis. Chem.Eng.Science 2003; 58(3-6): 1003-1011.

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[3] van Paasen S.V.B., Cieplik M.K., Phokawat N.P., Gasification of Non-woody Biomass. ECN Report No. ECN-C-06-032, Petten, The Netherlands, 2006. Aviable at:

www.ecn.nl/publications

[4] Twigg M.V., Catalyst Handbook. Wolfe Publishing Ltd. London, 1989.

[5] Lampert K, Ziebik A., Comparative analysis of energy requirements of CO2 removal from metallurgical fuel gases. Energy 2007; 32(4): 521-527.

[6] Szargut J., Morris D.R., Steward F.R. Exergy analysis of thermal, chemical and metallurgical processes. Hemisphere Publishing Corporation, New York, 1988.

[7] Mozaffarian M., Bracht M., den Uli H., v.d. Woude R. Hydrogen conversion in substitute natural gas by biomass hydrogasification. ECN, Petten, The Netherlands, Aviable at:

www.ecn.nl/publications