Life cycle assessment of hydrogen production from biomass gasification systems

Life cycle assessment of hydrogen production from biomass gasification systems

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Life cycle assessment of hydrogen production from biomass gasification systems Yildiz Kalinci a, Arif Hepbasli b,*, Ibrahim Dincer c a

Department of Technical Programs, Izmir Vocational High School, Dokuz Eylul University, Education Campus Buca, Izmir, Turkey Department of Energy Systems Engineering, Faculty of Engineering, Yas‚ar University, 35100 Bornova, Izmir, Turkey c Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ont., Canada L1H 7K4 b

article info

abstract

Article history:

In this study, a Life Cycle Assessment (LCA) of biomass-based hydrogen production is

Received 6 January 2012

performed for a period from biomass production to the use of the produced hydrogen in

Received in revised form

Proton Exchange Membrane (PEM) fuel cell vehicles. The system considered is divided into

2 June 2012

three subsections as pre-treatment of biomass, hydrogen production plant and usage of

Accepted 5 June 2012

hydrogen produced. Two different gasification systems, a Downdraft Gasifier (DG) and

Available online 27 July 2012

a Circulating Fluidized Bed Gasifier (CFBG), are considered and analyzed for hydrogen production using actual data taken from the literature. Fossil energy consumption rate and

Keywords:

Green House Gas Emissions (GHG) are defined and indicated first. Next, the LCA results of

LCA

DG and CFBG systems are compared for 1 MJ/s hydrogen production to compare with each

Hydrogen

other as well as with other hydrogen production systems. While the fossil energy

GHG

consumption rate and emissions are calculated as 0.088 MJ/s and 6.27 CO2 eqv. g/s in the

Gasification

DG system, they are 0.175 MJ/s and 17.13 CO2 eqv. g/s in the CFBG system, respectively. The

Efficiency

Coefficient of Hydrogen Production Performance (CHPP) (newly defined as a ratio of energy

Energy

content of hydrogen produced from the system to the total energy content of fossil fuels used) of the CFBG and DG systems are then determined to be 5.71 and 11.36, respectively. Thus, the effects of some parameters, such as energy efficiency, ratio of cost of hydrogen, on natural gas and capital investments efficiency are investigated. Finally, the costs of GHG emissions reduction are calculated to be 0.0172 and 0.24 $/g for the DG and CFBG systems, respectively. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Energy resources have been essential in satisfying human needs and improving quality of life, but may generally and sequentially lead to some crucial environmental consequences. We are now in a critical era and need to address energetic and environmental issues to overcome global problems [1e3]. We apparently need sustainable solutions.

Limitations on supplies of energy resources are problematic and energy use contributes not only to global warming, but also to such environmental concerns as air pollution, acid precipitation, stratospheric ozone depletion, forest destruction, and emission of radioactive substances [4,5]. Various analysis methods have been used for examining the environment impacts. LCA is one of the widely used methods. It is a methodology for this type of assessment, and

* Corresponding author. Tel.: þ90 232 411 5000; fax: þ90 232 374 5474. E-mail addresses: [email protected], [email protected] (A. Hepbasli). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.015

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represents a systematic set of procedures for compiling and examining the inputs and outputs of materials and energy, and the associated environmental impacts, directly attributable to a product or service throughout its life cycle. A life cycle is the interlinked stages of a product or service system, from the extraction of natural resources to their final disposal [6,7]. LCA first received attention in 1960s. Concerns over the limitations of raw materials and energy resources sparked interested in finding ways to cumulatively account for energy use and to project future resource supplies and use. In one of the first publications of its kind, Harold Smith reported his calculation of cumulative energy requirements for the production of chemical intermediates and products at the World energy conference in 1963. Later in the 1960s, global modeling studies resulted in predictions of the effects of the world’s changing populations on the demand for finite raw materials and energy resources. In 1969, researchers in a US-based commercial company initiated an internal study using LCA. The process of quantifying the resource use and environmental releases of products became known as a Resource and Environmental Profile Analysis (REPA), as practiced in the USA. In Europe, it was called an eco-balance. From 1975 through the early 1980s, as interest in these comprehensive studies waned because of the fading influence of the oil crisis, environmental concerns shifted to issues of hazardous and household waste management. During this time, European interest grew with the establishment of an environment directorate (DGX1) by the European Commission. When solid waste became a worldwide issue in 1988, LCA again emerged as a tool for analyzing environmental problems [8,9]. In 1991, 11 state attorneys general in the USA denouncing the use of LCA results to promote products until uniform methodology and consensus reached on how environmental comparison can be advertised non-deceptively. In 1992, International Organization for Standardization (ISO) 14,000 family grew out of ISO’s commitment to support the objective of sustainable development discussed at the United Nations conference on environment and development, in Rio de Janeiro. In 1993, ISO launched the new technical committee, ISO/TC 207, environmental management and ISO developed the LCA standards in 1997e2006 [10]. LCA is defined by the ISO 14000 series international standards: ISO 14040, life cycle assessment e principles and framework, ISO 14041, life cycle assessment e goal and scope definition and inventory analysis, ISO 14042, life cycle assessmentelife cycle impact assessment, ISO 14043, life cycle assessment e life cycle interpretation, ISO 14044, life cycle assessment e requirements and guidelines [6,11e15]. The main reason of mentioned above environment pollutions is to use fossil fuel. Scientific resources on renewable sources denote that some of them are competitive with fossil fuels. At this point hydrogen energy systems stick out. 80e85% of demand on hydrogen in the world is manufactured by Steam Methane Reforming (SMR) [16]. As an alternative, biomass-based hydrogen production systems have been developed. When it is considered that biomass is the fourth largest source of energy sources in the world, biomass-based hydrogen production systems promise. While there are various studies on energy analyses of biomass hydrogen systems in the literature, LCA of the systems is rarely. Some of them are briefly summarized below.

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Corti and Lombardi [17] conducted a study on the performance and LCA of a biomass Integrated Gasification Combined Cycle (IGCC) with reduced CO2 emissions. They simulated an IGCC with carbon dioxide chemical absorption form the syngas using Aspen Plus. Their results in term of CO2-specific emissions were defined as competitive (167 kg CO2/MWh) with respect to conventional coal IGCC (700e800 kg CO2/MWh) and natural gas combined cycle (380 kg CO2/MWh). Koroneos et al. [18] investigated the environmental aspects of hydrogen production. They compared hydrogen production methods with SMR and five different renewable ways, which were solar energy using photovoltaic for direct conversion, solar thermal energy, wind power, hydropower and biomass. They cited that the use of wind, hydropower and solar thermal energy for the production of hydrogen were the most environmental benign methods. Higo and Dowaki [19] conducted an LCA on biomass Di-methyl Ether (Bio-DME) production system considering the species of biomass feedstock in Japan and Papua New Guinea. They selected Papua New Guinea (PNG), which had good potential for supply of an energy crop (a short rotation forestry), and Japan where wood remnants were available, as model areas. Also, they referred to 9 species of biomass feedstock of PNG, and to 8 species in Japan. Consequently, CO2 intensities in the whole system were 16.3e47.2 g-CO2/MJ-DME in the Japan case, and 12.2e36.7 g-CO2/MJ-DME in the PNG one, respectively. Shie et al. [20] studied an LCA of rice straw bioenergy derived from potential gasification technologies. Four alternative energy production processes, radio-frequency plasma systems, microwave-induced systems, downdraft gasifier systems and plasma torch systems, were assessed. The most energy required was defined for the transportation and pre-treatment steps in the energy LCA. Hassan et al. [21] examined the life cycle GHG emissions from Malaysian oil palm bioenergy development and the impact on transportation sector’s energy security. Kimming et al. [22] analyzed the environmental impact of three different small-scale combined heat and power systems based on organically produced agricultural biomass from agriculture, and a scenario based on natural gas for production of power and heat for a rural village. The results showed that the biomass-based scenarios reduced greenhouse gas emissions considerably compared to the scenario based on fossil fuel, but had higher acidifying emissions. Also, there have been studies in the literature based on LCA of other hydrogen production methods. Granovskii et al. [23] reported an LCA of hydrogen and gasoline vehicles, including fuel production and utilization in vehicles powered by fuel cells and internal combustion engines to evaluate and compare their efficiencies and environmental impacts. They also extended their study to an exergetic LCA [24]. Utgikar and Thiesen [25] undertook an LCA of high temperature electrolysis for hydrogen production via nuclear energy. They presented their results in terms of the Global Warming Potential (GWP) and the Acidification Potential (AP) of the system. The GWP for the system was 2000 g carbon dioxide equivalent and the AP, 0.15 g equivalents of hydrogen ion equivalent per kilogram of hydrogen produced. Varun et al. [26] made an LCA of renewable energy for electricity generation systems. They defined that conventional systems, which were coal fired, oil fired, gas fired and nuclear had CO2 emissions as 975.3, 742.1, 607.6 and 24.2 g-CO2/kWh, respectively. Also, they showed

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that renewable sources, which were wind, solar-PV, biomass, solarethermal and hydro, had 9.7e123.7, 53.4e250, 35e178, 13.6e202 and 3.7e237 g-CO2/kWh, respectively. Ozbilen et al. [15] conducted a comparative environmental study of the CueCl water-splitting cycle with various other hydrogen production methods. They reported that the thermochemical cycles had lower environmental impacts while steam reforming of natural gas had the highest. They also investigated variations of environmental impacts with lifetime and production capacity in their other study [27]. This study aims at contributing to the area of biomassbased hydrogen production through an LCA. In this article, DG and CFBG systems are compared for 1 MJ/s hydrogen production using this method. In this regard, the main objectives of this study are to (i) define main steps to analyze biomass-based hydrogen production from the forest to PEM fuel cell vehicles, (ii) calculate fossil energy consumption rates and emissions for main sections and subsections of them, (iii) investigate the effects of some parameters, such as energy efficiency ratio, ratio of cost of hydrogen, on natural gas and capital investments efficiency, and (iv) examine the costs of GHG emissions reduction.

then they are put into the gasifier and gasified with oxygen rich air and steam. The exhausted gas from the gasifier has a high energy and it can be used to produce steam before disappearing in the gas cleaning unit. So, we added a heat exchanger to the system before gas cleaning. After gas cleaning, the cooled gas is compressed. The gas pressure at the compressor outlet varies according to the plant to which it is sent. In the power plant, a pressure of 20 bar is sufficient for gas turbine feeding while a pressure of 35 bar is required for the hydrogen plant [30]. In the study we have chosen 35 bar for the compressor exit. Besides, the CO shift reaction has an important influence on the composition of the raw syngas and operating conditions of the CO shift reactors are defined according to Hulteberg and Karlsson [28]. Then, the hydrogen-rich gas enters into the PSA equipment for purified H2 production. While the produced hydrogen is at 25  C and 32 bar because of pressure drops in CO shift reactors, off-gas is at 25  C and 1 bar. The off-gas is sent to a gas engine for power generation while steam is produced through the waste heat of the steam turbine-based system. A schematic representation of this modified system is shown in Fig. 2.

2.

3.

System description

In this study, we compare two different gasifier systems (DG and CFBG) for hydrogen production from biomass utilizing the data taken from the literature.

2.1.

CFBG system

The process includes a steam/oxygen blown gasifier. Due to the process size range, a pressurized (30 bar) CFBG is used. Before gasification, wood biomass needed for the gasification is milled into smaller particles (5 mm). The wood is then dried in a rotary dryer at 120  C. Oxygen is taken from Air Separate Unit (ASU) equipment. Syngas, which is produced at 30 bar and 850  C, passes through a tar cracker for both thermal tar decomposition and a catalytic tar decomposition, and follows the sulphur removal and wateregas-shift reactors. After the wateregas-shift, the gas is quenched to the near ambient temperature whereby water is condensed and removed. The gaseous components are lead to Pressure Swing Adsorption (PSA) equipment. The PSA is assumed to give a high enough purity, 99.99%, at a hydrogen recovery rate in the high 70% range. The waste heat taken from heat exchangers in the system is used to generate steam. The steam is generated at 50 bar and is first expanded down to 30 bar through a primary turbine for power generation. The steam flow is then split and a part of it goes to the gasifier and the rest is condensed through a secondary turbine for additional power generation. Further details on the system operation are available in Ref. [28] while a flow diagram of the integrated process is illustrated in Fig. 1.

2.2.

DG system

A DG system has been modified by Lv et al. [29]. Biomass (pine wood) is first pretreated by cutting into proper size and

Life cycle assessment

LCA is a systematic process for identifying, quantifying and assessing environmental impacts throughout the life cycle of a product, process or activity. It considers energy and material uses and releases to the environment from “cradleto-grave” (i.e., from raw material extraction through manufacturing, transportation, use and disposal). LCA can be used to help ensure that cross-media and multimedia environmental impacts are considered in design and implementation decisions, identify “hot spots” of potential environmental impact, compare one or more aspects of specific products or processes and establish baselines for further research. LCA is often used in conjunction with other environmental management tools such as risk assessment and environmental impact assessment. A life cycle approach does not necessarily embody every methodological aspect called for in a traditional LCA, but it does use a cradle-to-grave systems perspective to evaluate the full life cycle impacts of a product or process. Various industries, the military and governments have been using life cycle approaches e and often LCAs e to increase the role of science in decisions on product and process designs [31]. The stages of LCA are outlined by ISO 14040 and they are described by different authors [27,32,33]. There are four major stages of the LCA:    

goal and scope definition, inventory analysis, impact assessment, and improvement analysis

As shown in Fig. 3, there are interactions between the stages. The results of an LCA can be used for developing and improving of a product, strategic planning of a company, constitution of public policy and marketing.

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Biomass Pre-treatment

Diesel

Pine Wood Growing

Diessel

Electric

Chipping

Transportation

Drying

Emission

Emission

Steam

Embodied Energy

Hydrogen Production Plant 28 6

4

27

25

16

14

12

10

8

17 Hydrogen

7

1. Oxygen HX1

2. Biomass 3. Steam CFBG

Tar Cracker

13

11 HTS

De-Sulpherization

HX5

HX4

HX3 9

HX2

MTS

18

15 PSA

LTS 24

5. Oxygen

23

26

Water PSA Off-Gas

29 Dryer 22

31

21

HX6 20 19. Air

30

32

Off-Gas Combustion Wet Biomass

33

Gas Line Steam Line

HPT

CT

Using Hydrogen

Hydrogen

Electric

Diessel

Hydrogen Compressor

Transportation

Emission

PEM Fuel Cell

Steam

Emission

Fig. 1 e LCA boundaries of the CFBG system.

3.1.

Goal and scope definition

The first phase of an LCA is goal and scope definition. In the phase, according to ISO 14040, environmental effects, assumptions, functional unit, boundary of the system, distribution methods and needs of the data quality should be defined clearly [14]. The scope of the system also has to define the function of the system. A functional unit (e.g., 1 MJ/s of hydrogen produced), which is a measure of the performance, must be

determined. The primary aim of the functional unit is to provide a reference for relating the inputs and outputs [11,27]. Also, the functional unit helps compare different systems. In this study, an LCA involves three main sections, which are pre-treatment of biomass, hydrogen production and using hydrogen produced. Boundaries of the system are shown in Figs. 1 and 2. Also, the functional unit of the LCA is 1 MJ/s hydrogen production. The aim of this study is to compare with each other during the life time of CFBG and DG systems for 1 MJ/s hydrogen production.

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Biomass Pre-treatment

Diesel

Pine Wood Growing

Diessel

Electric

Chipping

Transportation

Drying

Emission

Emission

Steam

Embodied Energy

Hydrogen Production Plant

21 6

19 15 Hydrogen

13

11

9

7 HX2

4

1. Biomass 2. Oxygen 3. Steam

HX3

HX1 5

HX5

HX4 12

10

8

16 Off-Gas

14

Syngas Compressor HTS

Gas Cleaning

DG

MTS

PSA

LTS

22

18 20

23

17 Water

W Q Steam Turbine

Gas Engine

24 Gas Line Steam Line

Using Hydrogen

Hydrogen

Electric

Diessel

Hydrogen Compressor

Transportation

Steam

PEM Fuel Cell

Emission

Emission

Fig. 2 e LCA boundaries of the DG system.

Goal and scope definition

Inventory analysis

Impact assessment

Interpretation of results

3.2.

Direct use areas: Product developing and improving Strategic planning Constitution of public policy Marketing Other

Fig. 3 e Life cycle assessment framework (modified from Refs. [14,33]).

Inventory analysis

In this phase, data are collected to quantify inputs and outputs of the system being studied to meet the goals of the defined study. The types of data include energy, raw materials, and other physical input; products, co-products, and wastes; releases to air, water, and soil; and other environmental aspects. Generally, a flow model (or flow chart), consistent with the system boundaries defined in the goal scope and definitions, is constructed. The flow model shows the activities in the system (e.g., processes, transportation and waste management) and the input and output flows among them throughout the life cycle. Input and output data (e.g., raw

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 0 2 6 e1 4 0 3 9

materials, energy, products, solid waste, emissions to air and water) are collected for all the processes in the system. The calculations are then performed to estimate the total amounts of resources used and pollution emissions in relation to the functional unit. The results consist of an inventory of the environmental input and output data of the system being studied. Incorrect data, old data or incomplete data will affect the results [31,34].

3.3.

Impact assessment

This step assesses the results of the inventory analysis (the quantified inputs and outputs) to understand their environmental significance. It embodies a number of concepts that have evolved over several years. These concepts and the terms used to denote them continue to evolve as various organizations in many countries contribute to the development of this component of LCA. There is no standard or universally agreed-upon set of environmental impact categories. However, commonly identified impact categories include acidification, eutrophication, climate change, stratospheric ozone depletion, aquatic toxicity, human toxicity, fossil fuel depletion, water depletion, and land use. Sometimes, the term “stressor category” is used instead of “impact category”. Stressor categories fall under one of three broad impact categories: human health, ecological health, and resource depletion [31].

3.4.

Improvement analysis

The last step, the interpretation, is where the consistency between the initial aim and the content of the assessment (e.g., system boundaries, data quality, nature of the data and the impact models) must be checked. The two steps, namely inventory analysis and impact assessment, provide with data about environmental releases and impacts. To use these results for process, product or design changes or for other purposes, decision makers need an understanding of the reliability and validity of the information [31,34]. The results of the analysis can be used to define critical points of systems and improve.

3.5.

Parametric study

E_ i;LFC ¼ E_ i;dir þ DE_ i;dir þ DE_ i;ind

energy rate, which is consumed through being embodied in construction materials and equipment and during installation, operation, maintenance, decommissioning, etc. In this study, electrical energy and Diesel fuels are used as fossil fuel. The fossil fuels are not converted to hydrogen directly, but they are used at transportation and equipment operation steps. So, E_ i;dir is zero in this study. The embodied energy adequately reflects the environmental impact of the material extraction and material and device production stages, but it is inconsistent with the economic cost of these products. Note that construction materials are also produced from mineral sources (ores, limestone, etc.) which, like fossil fuels, have value; their energy contents are much lower than their real economic values. The Energy Equivalent Construction Materials and Devices (EEQ) and the Operation Energy, i.e., the fossil fuel energy required for installation, construction, operation, maintenance, decommissioning, etc., of equipment (EOP) and Life Time (LFT) are used to calculate DE_ i;ind as follows:

(1)

where E_ i;LFC is the life cycle fossil fuel energy consumption rate, E_ i;dir is the fuel energy rate, which is directly transformed into final products, DE_ i;dir is the fuel energy rate, which is consumed to perform the transformation and DE_ i;ind is the fuel

P

DE_ ind ¼

EEQ þ EOP LFT

(2)

The Intensities of Embodied Energies (cost of construction materials and devices per consumed fossil fuel energy to produce them) (IEE) are obtained as below: IEE ¼

IPM EMB

(3)

where IPM denotes Investments to Produce Construction Materials or devices and EMB denotes Energy Embodied in construction materials and devices. The EEQ is calculated by EEQ ¼

IEE$EMB IPM ¼ Cng Cng

(4)

In this study, we assume that electric energy is produced from natural gas and Cng is the industrial cost of natural gas. Also, the density of hydrogen under standard conditions is low. To assist in storage and utilization as a fuel, the density is increased via compression. The direct fossil fuel energy consumption DE_ dir to compress isothermally 1 mol of hydrogen can be expressed, assuming ideal gas behavior, as DE_ dir ¼

Mass, energy and environmental impacts are calculated related to steps of process, which are construction, operation, disassemble in an LCA. Definition of all of inputs and outputs is rather complex, so some simplify and assumptions can be made. LCA of a system enclosing varied technological steps investigates materials, energy streams (e.g., fossil fuel) for ith step and associated environmental impacts. The energy consumption rate corresponding to fossil fuel use can be evaluated with the following expression [23,35,36]:

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  RT0 Pmax ln hcmp hgt Pmin

(5)

where T0 and R are the standard environmental temperature and the universal gas constant as 298 K and 8.314 kJ/mol K, respectively. Also, hcmp denotes isothermal compression efficiency and hgt gas turbine power plant efficiency as 0.65 and 0.33, respectively [23]. Also, the CHPP is newly defined as a ratio of energy content of hydrogen produced from the system to the total energy content of fossil fuels used in the rate form as follows: CHPP ¼

E_ H2 E_ dir þ DE_

(6)

where fossil fuels are not converted to hydrogen energy directly, so E_ i;dir is taken to be zero. DE_ is a sum of embodied energy for equipment and utilization of indirect energy. E_ H2 represents the hydrogen energy rate produced.

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The fossil fuel and renewable technologies for hydrogen production are generally distinguished by (i) source of energy consumed, (ii) efficiency of hydrogen production per unit of energy consumed, and (iii) capital investments made per unit of hydrogen produced. To account for all of these differences, a new indicator, the capital investments effectiveness g, is introduced as a measure of economic efficiency. This indicator is proportional to the relationship between the gain and investments and is equal to: g¼

E_ H2 ða  1=CHPPÞ DE_ ind

(7)

where a is the ratio in costs of hydrogen (CH2 ) and natural gas (Cng): a¼

CH2 Cng

(8)

Finally, a positive environmental impact (i.e., a reduction in GHG emissions in the present case) as a result of the introduction of a renewable technology depends on the replaced technology. The effectiveness of such an introduction is reflected in part by the cost of GHG emissions reductions per kg (CGHG), which can be determined as [35]: CGHG ¼

1000 ðCR  CF Þ GHGF  GHGR

(9)

where GHGF and GHGR represent GHG releases for 1 MJ/s hydrogen production from fossil fuel and renewable sources, respectively. CGHG is the cost of GHG emissions reductions while CR and CF indicate the cost of 1 MJ hydrogen produced from renewable and fossil sources, respectively.

4.

Results and discussion

In this section, LCA of biomass-based hydrogen production systems (CFBG and DG) and environmental effects are evaluated. The boundaries of LCA and system details of CFBG and DG are shown in Figs. 1 and 2, respectively. The goal of the study is to analyze and investigate environmental effects, duration of collection of pine wood waste from forest, chipping to appropriate size, transportation to hydrogen production plant and using the hydrogen in PEM fuel cell. In this study, the unit function is preferred as 1 MJ/s hydrogen production to compare with each other and other hydrogen production systems. The second step of the LCA is inventory analysis, which is the most difficult section. Energy rates and emissions of every equipment and stream are defined in the boundary of LCA and are given for CFBG and DG, separately.

Among of them, agricultural wastes and energy forestry (e.g., growing of willow, poplar) are expansive. Irrigation, fertilize and usage of pesticide need to grow of the biomasses. In this study, pine wood grow in forest itself and irrigation is made with rain water, naturally. So, consumption of any fossil fuel is not taken into accounts for the step. Also, it absorbs CO2 from atmosphere when it grows so, it gives positive effect as environmental. The biomass includes 50.4% carbon and 22.6% moisture with a mass flow rate of 10.1 kg/s. Before the gasifier equipment, chipping of the biomass increases the gasification efficiency. Especially, biomass must be chopped lower 5 mm for the CFBG. The chipping process for one tone biomass uses 13.6 kWh electric energy and 1.23 l Diesel fuel [19]. CO2 emissions caused by utilization of the fuels are given in Table 1. Here, for a biomass mass flow rate of 10.1 kg/s, 0.49 MJ/s electric power and 0.44 MJ/s Diesel fuel are used. Using Table 1, CO2 emissions to the atmosphere of the energy usages are calculated as 60.81 and 32.73 CO2 eqv. g/s for electric power and Diesel fuels, respectively. Although consumption rates of the two fuels are close to each other, CO2 emission of electric power is double. In the step, total fossil energy consumption rate and CO2 emission are calculated as 0.934 MJ/s and 93.54 CO2 eqv. g/s, respectively. Next, biomass moist and chopped must be transported to the hydrogen production plant. It is assumed that the plant is installed near the forest and the distance is taken to be 50 km in this study. Also, diesel trucks with a capacity of 10 ton each are used for transportation of the biomass. Energy consumption rates of the trucks vary based on biomass mass flow rates and distances of transportation. Here, demand on biomass in the plant is considered on the annual basis. Biomass demand of the plant is calculated as 290,880 ton/year based on an operation time of 8000 h/year. Using 1 L Diesel fuel, 10 ton Diesel trucks cover a distance of 3.5 km [37]. When it is considered that 10 ton biomasses are transported to the plant 50 km and empty trucks come back, 2,908,800 km includes the distance for 290,880 ton/year biomass. In this scenario, effect of being empty of the trucks in return on energy consumptions is ignored. Diesel fuel consumption in the step is calculated as 831085.71 l. Taking Lower Heating Value (LHV) of Diesel fuel as 35.5 MJ/l, energy rate of the fuel used is defined as 29503542.71 MJ/year or 1.024 MJ/s. CO2 emission of the energy consumption rate is calculated as 76.21 CO2 eqv. g/s. The next step is drying. Moisture of biomass is demanded to be lower 20% at the inlet of the gasifier, usually. This value can change based on gasifier types and operation conditions. While mass flow rate and moisture content of biomass are 10.1 kg/s and 22.6% before drying, these values are 8.5 kg/s and

Table 1 e Data of fuel energy and CO2 emissions.

4.1.

LCA results of the CFBG system

The system includes three subsections, which are biomass pre-treatment, hydrogen production plant and usage of the hydrogen energy, as can be seen in Fig. 1. Biomass pre-treatment section is divided into subsections, which are pine wood growing, chipping, transportation and drying. At the present time, there are a lot of kinds of biomass.

Fuel

Energy (LHV)

Diesel

35.5 MJ/L

Electric

3.6 MJ/kWh

Bunker C

42.5 MJ/kg

CO2

Note

74.4 g-CO2/MJ Chipping, transportation, harvesting 123.1 g-CO2/MJ Auxiliary power, drying, chipping 76.9 g-CO2/MJ Marine transportation

Source: Modified from Higo and Dowaki [19].

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8% after drying, respectively. So, 1.6 kg/s steam is taken from biomass. The necessary heat is to be met via combustion of the PSA off-gas. So, it is assumed that there is no fossil fuel consumption in the step. At the hydrogen production step, utilization of direct and indirect fossil energy consumption rates are explored. CFBG systems are preferred at high capacity plants. Pre-treatment biomass, steam and oxygen produced are given to the CFBG. Syngas produced passes through de-sulpherization equipment and goes to the water gas shift reactors. The reactors use catalyst, but we ignored embodied energy of the catalysts in this study. After the equipment, syngas is separated into hydrogen and off-gas in the PSA equipment. The energy consumption rate of the ASU equipment is assumed to be 0.5 kWh/Nm3 O2 as taken from Ref. [29]. According to these data, the electric power consumption rate of the ASU is taken as 9.73 MWe. In addition, 2.8 MWe is assumed for auxiliary equipment. Also, the system is feed with 16.731 kg/s water to produce steam. Because the gasification system is pressurized, water pressure is increased to 50 bar via a pump and all of waste heat of the system is used to produce steam. Energy of the steam is converted to electric energy passing in a high pressurized turbine and then condensing turbine equipment, with power production rates of 2.14 MWe and 11.61 MWe, respectively. According to these data, the system produces 1.22 MWe extra power. Also, the embodied energy utilization contains installation of the plant, manufacturing of equipment, assembling, and maintenance. Construction materials are produced from mineral sources (e.g.. concrete, steel, aluminum). Their LCA contains fossil energy consumption rates through the processes. Quantities of the construction materials taken from Ref. [23] are modified in this study. Indirect energy and emissions are given in Table 2 for 62.52 MJ/s hydrogen productions. Embodied energy consumption rates are defined according to 15 years life time and 8000 h/year operation time. In the table, IEE and EEQ are calculated using Eqs. (3) and (4). Unit cost of natural gas is taken from Izmir Gas Inc. [38] as 0.0084 $/MJ. Finally, total of EEQ and emissions are calculated as 0.757 MJ/s and 11.71 CO2 eqv. g/s, respectively. Hydrogen utilization section contains compressing of hydrogen, transportation of liquid hydrogen and using hydrogen in PEM Fuel cells. The density of hydrogen under standard conditions is low. To assist in storage and utilization as a fuel, the density is

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increased via compression. The direct energy consumed and direct greenhouse gas emissions to compress hydrogen are evaluated. Electric energy consumed is assumed to take from natural gas plant with 33% plant efficiency. Also, compressor efficiency is taken as 65% [23]. In this study, 0.521 kg/s hydrogen is taken from the PSA equipment at 30 bar and 298.15 K. The pressure is increased to 202 bar via the compressor. The fossil energy consumption rate is calculated as 5.74 MJ/s using Eq. (5). In this regard, the system has extra power of 1.22 MWe, as we can use this for the next hydrogen compression step. So, only a power rate of 4.52 MWe is needed to purchase from the grid and its emission value is defined as 556.41 CO2 eqv. g/s. In the scenario for the hydrogen transportation step, the liquid hydrogen is transported to filling stations by Diesel trucks. The distance is assumed as 300 km. Direct energy utilization rate for 1 MJ/s hydrogen is taken 0.041 MJ/s [39]. According to these data, 2.56 MJ/s Diesel fuel is defined and its emission value is calculated as 190.71 CO2 eqv. g/s. Then, the hydrogen produced is used in the PEM fuel cell. While its mechanical efficiency varies between 40% and 60%, the efficiency of internal combustion engine ranges from 20% to 30% [23]. Steam is produced from the PEM fuel cell. Fugitive emissions of hydrogen from the fueling station are assumed negligible. So, fossil energy consumption rate is not considered in the step. In conclusion, the fossil energy consumption rates and emissions are indicated in Figs. 4 and 5 for 62.52 MJ/s hydrogen productions, respectively. The most fossil energy consumption rates belong to hydrogen compressor and hydrogen transportation steps as 4.52 MJ/s and 2.56 MJ/s, respectively. Parallel results are taken for CO2 emissions as well. The total CO2 emission is calculated as 928.58 CO2 eqv. g/s for the system.

4.2.

LCA results of the DG system

As can be seen from Fig. 2, the subsections of the system are similar to those of the CFBG plant, but DG systems are preferred in smaller sizes. Biomass mass flow rate and moisture are 0.088 kg/s and 22.6%, respectively. The pine wood grows in forest itself. Biomass chipping step consumes 0.0081 MJ/s fossil energy rate and emissions release as 0.814 CO2 eqv. g/s. Same concessions are valid for transportation distances and transportation trucks as mentioned in the CFBG plant. Annual quantity of biomass needed is calculated as

Table 2 e Energy equivalents and greenhouse gas emissions for CFBG plant (62.52 MJ/s H2 production). Material

Amount required (ton)

Embodied energy (GJ/ton)

Embodied energy consumption rate (for life time) (MJ/s)

IEE ($/MJ)

EEQ (MJ/s)

CO2-es‚ (kg/ton)

CO2-es‚. (g/s)

3748.57 1197.55 9.88 14.64

1.4 34.4 201.4 23.5

0.0121 0.095 0.0046 0.00079

0.1188 0.049 0.0504 0.0494

0.171 0.554 0.0276 0.00464 0.757

520 2471.2 12824.64 1687.17

4.51 6.85 0.293 0.0571 11.71

Concrete Steel Aluminum Iron Total 1 $ ¼ 1.54 TL [41].

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Fig. 4 e Fossil energy consumption rates for 62.52 MJ/s hydrogen production of the CFBG system.

Fig. 5 e CO2 emissions for 62.52 MJ/s hydrogen production of the CFBG system.

2530.94 ton/year. Fossil energy consumption rate and emissions are calculated as 0.0089 MJ/s and 0.663 CO2 eqv. g/s for biomass transportation step, respectively. Biomass brought to the DG plant passes through dryer and the moisture content decreases to 8% and then the biomass is given to the gasifier. While flow of biomass goes from up to down, biomass is combusted with oxygen as partial in the throat section and the reaction is exothermic. The temperature of the dryer section of the DG is 150e300  C. Using the heat, the system produces steam needed to gasification, itself. But oxygen needed is taken from the ASU equipment, which consumes 0.0315 MWe. Also, the produced syngas after cleaning is compressed to the PSA pressure 35 bar and spent 0.06 MWe. Consequently, hydrogen is produced at 32 bar and 298.15 K as 0.004 kg/s, taking into account pressure drops. The mass flow rate of the off-gas is 0.09 kg/s at environmental pressure and temperature. LHV and energy content rate of the off-gas are calculated as 6.22 MJ/Nm3 and 451.64 kW. Using the

off-gas in a gas engine (efficiency is assumed as 39.1%), 0.177 MWe can be produced. Waste heat of the gas engine is used for drying and producing steam as all of other processes heat. Using the steam, 0.058 MWe is produced in a steam turbine. When looking at the total energy rate of the plant, one can say that the system supplies all of energy needed and produces 0.144 MWe extra. Addition, there is embodied energy consumption rate in the DG plant. The calculated values are given in Table 3. As can be seen in the table, total energy rate equivalents and emissions are calculated as 0.584 102 MJ/s and 8.99 102 CO2 eqv. g/s, respectively. Then, the produced hydrogen is compressed from 32 bar to 202 bar with a compressor. Needed energy rate is calculated as 0.043 MJ/s using Eq. (5). It is considered that the energy is met from the extra system energy produced. So, in the step, fossil energy consumption rate and CO2 emissions are taken zero. Addition, the values are calculated as 0.0198 MJ/s and 1.48 CO2 eqv. g/s for transportation of the liquid hydrogen. Also, fossil energy consumption rate is not considered for utilizing the hydrogen in the PEM fuel cell step. Fossil energy consumption rates and CO2 emissions for the DG plant are displayed in Figs. 6 and 7, respectively. As can be seen in these figures, the most fossil energy consumption rate and CO2 emission belong to the hydrogen transportation step. The data are modified to compare the processes with each other for 1 MJ/s hydrogen production. The most fossil energy consumption rates are 0.091 MJ/s and 0.041 MJ/s for the hydrogen compressor and hydrogen transportation steps in the CFBG system, respectively. It becomes in total 0.175 MJ/s fossil energy consumption rate with 17.13 CO2 eqv. g/s emission for 1 MJ/s hydrogen production. Biomass mass flow rate must be 0.18 kg/s to produce 1 MJ/s hydrogen in the DG system. The most fossil energy consumption rate and emissions belong to the liquid hydrogen transportation step as 0.041 MJ/s and 3.05 CO2 eqv. g/s, respectively. It totally amounts to 6.27 CO2 eqv. g/s for 0.088 MJ/s fossil energy consumption rate in the DG system and the comparative results are given in Figs. 8 and 9. Finally, one can say that the DG system is more environmental system, as not forgetting that it is preferred for small capacity systems.

4.3.

Parametric study

The two systems are examined energetically using LCA. In this section, the effects of some parameters (i.e., CHPP, ratio of

Table 3 e Energy equivalents and greenhouse gas emissions for DG plant (0.484 MJ/s H2 production). Material

Concrete Steel Aluminum Iron Total

Amount Embodied required (ton) energy (GJ/ton) 28.78 9.19 0.076 0.1124

1 $ ¼ 1.54 TL [41].

1.4 34.4 201.4 23.5

Embodied energy IEE ($/MJ) EEQ (MJ/s) 102 CO2-es‚ (kg/ton) CO2-es‚. (g/s) 102 consumption rate (for life time) (MJ/s) 9.32 $ 7.32 $ 3.54 $ 6.11 $

105 104 105 106

0.1188 0.049 0.0504 0.0494

0.132 0.427 0.021 0.0036 0.584

520 2471.2 12824.64 1687.17

3.46 5.26 0.23 0.044 8.99

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Fig. 6 e Fossil energy consumption rates for 0.484 MJ/s hydrogen production of the DG system.

Fig. 8 e Fossil energy consumption rates for 1 MJ/s hydrogen production of the DG and CFBG systems.

cost of hydrogen) on natural gas and capital investments efficiency are investigated. In this study, the CHPP values are calculated to be 5.71 and 11.36 for both CFBG and DG systems using Eq. (6), respectively and given in Fig.10. As can be seen in this figure, the CHPP is bigger than one. It means that renewable energy sources enter the systems. For example, in the DG system, 1 MJ/s hydrogen is produced using 0.088 MJ/s fossil energy consumption rate. Hydrogen production from fossil energy and renewable energy sources shows disparities according to sources, energy efficiencies and capital investments made per unit of hydrogen produced. To account for all of these differences, the capital investments effectiveness g is investigated. Ratio of cost of hydrogen to natural gas is calculated first using Eq. (8) and given in Fig. 11. In this study, unit hydrogen costs are taken to be 0.0096 $/MJ and 0.028 $/MJ modifying from Ref. [40] for both DG and CFBG systems, respectively. Also, the unit cost of natural gas is taken from Izmir Gas Inc. [38]. as 0.0084 $/MJ. According to this figure, a values are calculated as 1.14 and 3.33 for the DG and CFBG systems, respectively. Next, g values are determined using Eq. (7) as 87.66 and 262.9 for 1 MJ/s hydrogen production in the DG and CFBG systems, respectively. In this study, building components (e.g., concrete, steel, aluminum, iron) are taken into account as DE_ ind . When components of equipment are considered, due to increasing of DE_ ind , g values will decrease. Also, parameters affecting g values are investigated and displayed in Figs. 12e14. While increasing indirect energy consumption rate decreases g values, an increase ratio in hydrogen price increases g values. So, an expensive hydrogen production method reflects to the factor and increases it, as can be seen in Fig. 13. The coefficient of hydrogen production performance affects the factor less (Fig. 14). Addition, a reduction in GHG emissions as a result of the introduction of a renewable technology depends on the

replaced technology. 99.17 CO2 eqv. g/MJ emission releases for hydrogen production from natural gas [15,25]. It is considered that DG and CFBG systems replace with this technology in this study. GHG emissions are calculated as 6.27 and 17.13 CO2 eqv. g/s for DG and CFBG systems, respectively. According to these data, 92.9 and 82.04 g/MJ GHG emissions reduction are defined for DG and CFBG systems, respectively. The cost of GHG emissions reduction depends on the cost of hydrogen production from natural gas and renewable sources. The cost of hydrogen production from natural gas is taken as 0.008 $/MJ from Corradetti and Desideri [30]. When hydrogen production costs are taken as 0.0096 and 0.028 $/MJ for the DG and CFBG systems, the costs of GHG emissions reduction are calculated as 0.0172 and 0.24 $/g using Eq. (9), respectively. Because, the unit cost of hydrogen produced in the CFBG system is more expensive than that in the DG system and the cost of GHG emissions reduction is higher, too. The effects of two parameters, cost of hydrogen production from natural gas and ratio of hydrogen price, on the costs of GHG emissions reduction, are investigated and shown in Figs. 15 and 16. As can be seen in these figures, when hydrogen production costs are close to each other from natural gas and renewable sources, the costs of GHG emissions reduction decreases. Similarly, when CR/CF increases, the costs of GHG emissions reduction increase. In other words, hydrogen production cost from renewable sources is more expensive than that from fossil energy sources, so the method is not competitive with fossil sources only. Finally, the results are compared with previously conducted studies and given in Fig. 17. Data for 1 kg hydrogen production taken from Refs. [15,25] are modified for 1 MJ hydrogen production. As can be seen in this figure, the lowest emission values belong to the nuclear-based SeI cycle as 3.403 CO2 eqv. g, while the DG and CFBG systems release6.27 and

Fig. 7 e CO2 emissions for 0.484 MJ/s hydrogen production of the DG system.

Fig. 9 e CO2 emissions for 1 MJ/s hydrogen production of the DG and CFBG systems.

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Fig. 10 e CHPP of the CFBG and DG systems. Fig. 14 e Variation of the capital investments efficiency according to CHPP.

0.3 CGHG ($/g)

0.25 0.2 0.15

DG

CFBG

0.1 0.05

Fig. 11 e Ratios of cost of hydrogen to natural gas for the CFBG and DG systems.

0 0.005

0.006

0.007 0.008 CF ($/MJ)

0.009

Fig. 15 e Variation of the cost of GHG emissions reduction according to the cost of hydrogen produced using natural gas.

Fig. 12 e Variation of the capital investments efficiency according to indirect energy consumption rate.

17.13 CO2 eqv. g., respectively. Also, we can take an observation about emissions of biohydrogen production, roughly. The emissions vary from 31 to 77 g CO2 for 1 MJ hydrogen production; further details on the system operations are available in Refs. [42,43]. Consequently, we may conclude that biomass-based hydrogen production methods are more environmentally benign than steam natural gas reforming releasing 99.173 CO2 eqv. g.

0.6 0.5 CGHG ($/g)

DG

CFBG

0.4 0.3 0.2 0.1 0 1

2

3

4

5

6

CR/CF Fig. 13 e Variation of the capital investments efficiency according to ratio of prices.

Fig. 16 e Variation of the cost of GHG emissions reduction according to the cost rate of hydrogen energy.

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For a future work, it is planned to analysis hydrogen production systems based on biomass and solar energy through energy, exergy and life cycle cost methods. The results of this study are expected to be beneficial to the researchers and engineers working in the area.

Acknowledgement The authors gratefully acknowledge the support provided by Dokuz Eylul University, Yas‚ar University and University of Ontario Institute of Technology as well as the Natural Sciences and Engineering Research Council.

Nomenclature Fig. 17 e Quantity of CO2 emissions for 1 MJ hydrogen production with different methods.

5.

Conclusions

The present study examines biomass-based hydrogen production systems through LCA method. Two gasification systems, DG and CFBG, are then compared with each other to assess their LCA performances. Fossil energy consumption rates and greenhouse gas emissions are calculated. Some environmentally critical steps are defined. The effects of some parameters on fossil energy consumption rate and the costs of GHG emissions reduction are also investigated. The following concluding remarks are drawn from the present study:  While the most energy consumption rates are seemed in the hydrogen compression and hydrogen transportation steps for the CFBG system, it belongs to the hydrogen transportation step in the DG. In total, biomass pre-treatment and hydrogen transportation steps are mutual. But fossil energy consumption rate can be different according to processes and equipment used.  To compare the two gasification systems, all values are modified for 1 MJ/s hydrogen production. Fossil energy consumption rate and emissions are determined as 0.088 MJ/s and 0.175 MJ/s, 6.27 CO2 eqv. g/s and 17.13 CO2 eqv. g/s for the DG and CFBG, respectively. As can be seen from the results, fossil energy consumption rate in the CFBG is higher than that in the DG.  Also, CHPP of CFBG and DG systems are calculated as 5.71 and 11.36, respectively. In the DG system, one produces 1 MJ/s hydrogen using 0.088 MJ/s fossil energy consumption rate, and hence, energy produced is 11.36 multiple of fossil energy consumption rate.  The capital investments effectiveness reflects a collective impact of indirect energy utilization, fossil energy efficiency ratio and unit hydrogen cost from renewable sources and natural gas. While the parameter decreases with increasing indirect energy consumption rate, it increases via racing of prices of renewable hydrogen energy.

C E_ P Q_ R T _ W

cost, $/MJ or $/g energy rate, MJ/s or MW pressure, kPa or bar heat rate, MW universal gas constant, kJ/kmol K temperature,  C or K work rate, MW

Greek letters g capital investment effectiveness h energy efficiency, % a ratio of cost of hydrogen to natural gas

Subscripts cmp compressor dir direct e electric F fossil fuel GHG green house gas emissions gt gas turbine i per component ind indirect LFC life cycle fossil consumption max maximum min minimum ng natural gas R renewable fuel 0 reference index

Superscripts Over dot quantity per unit time

Abbreviations ANL Argonne National Laboratory AP Acidification Potential ASU Air Separate Unit BTL Biomass to Liquid CFBG Circulating Fluidized Bed Gasifier CHPP Coefficient of Hydrogen Production Performance

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CT DG DME EEQ EMB EOP EPA GHG GWP HPT HTS HX IEE IGCC IPM ISO LCA LFT LHV LTS MTS PEM PNG PSA PV REPA SMR TC TCMB

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Condensing Turbine Downdraft Gasifier Di-methyl Ether Energy Equivalent of Construction Materials and Devices Embodied Energy Operation Energy to install, operate, maintain, etc. equipment Environmental Protection Agency Green House Gas Emissions Global Warming Potential High Pressure Turbine High Temperature Shift Heat Exchanger Intensity of Embodied Energy Integrated Gasification Combined Cycle Investments to Produce Construction Materials and Devices International Organization for Standardization Life Cycle Assessment Life Time Lower Heating Value Low Temperature Shift Medium Temperature Shift Proton Exchange Membrane Papua New Guinea Pressure Swing Adsorption Photovoltaic Resource and Environmental Profile Analysis Steam Methane Reforming Technical Committee Central Bank of Republic of Turkey

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