Life cycle assessment of green diesel production from microalgae

Renewable Energy 86 (2016) 623e632

Contents lists available at ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Life cycle assessment of green diesel production from microalgae Namita Pragya, Krishan K. Pandey*, 1 College of Management & Economic Studies, Acres, Vill. Kandoli, PO Bidholi Via Preamnagar, University of Petroleum & Energy Studies, Dehradun 248007, Uttarakhand, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 September 2013 Received in revised form 31 July 2015 Accepted 25 August 2015 Available online xxx

Many LCA based viability studies have already been done for the production of green diesel from microalgae, still a comprehensive LCA, has not yet been undertaken. Current study aims to find out if the Net Energy Balance(NEB) can further be increased by using a combination of many available agronomical practices & the techniques of production of green diesel from microalgae. The results show that neither open raceway pond nor Photobioreactor routes (Wet and dry routes) yield positive energy balance. The production of green diesel via open raceway pond, both in dry as well as wet route, have less negative NEB and comparatively higher NER than the photobioreactors. Comparison says that open raceway pond dry route has slightly higher value for NER than the wet route. Even with the best possible route (open raceway pond dry route), the total energy use is almost 5 times more than the energy produced, with a negative NEB of 4.07 MJ & very low NER value of 0.20. Study concludes that R & D in the area of green diesel production from microalgae has yet to go a long way & has a huge scope to further lower its input energy demand for biofuel production. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Life cycle Energy balance Net energy ratio Greenhouse gas Pyrolysis Liquefaction

1. Introduction

2. Life cycle energy balance

Depletion of fossil fuels and climate change concerns [1], have led to increasing interest in new sustainable alternatives [2] and clean energy sources [1]. Among the various forms of alternative energy currently under study, biodiesel exhibits particular promise [2]. In recent years, ‘Algae for fuel’ concept has gained renewed interest [3]. Microalgae store energy in their cells in the form of lipid droplets [4]. The algal oil industry, though presently in its infancy, has tremendous potential to provide future liquid transportation fuels that can improve national energy security by providing less dependence on imported oil. Photosynthetic activity of microalgae is highly efficient [5]. Advantages of algae over other energy crops includes rapid growth rate [2], higher CO2 fixation [6] and high lipid content [7]. Moreover, the production of microalgae does not require high quality arable land, and therefore does not compete with food crops [5].

2.1. Goal, scope and system boundaries

* Corresponding author. E-mail addresses: [email protected], (K.K. Pandey). 1 Website: www.krishan.hpage.com. http://dx.doi.org/10.1016/j.renene.2015.08.064 0960-1481/© 2015 Elsevier Ltd. All rights reserved.

[email protected]

Every activity involved in the production of biofuels from microalgae is energy intensive and also produces greenhouse gases. Many LCA (Life cycle assessment) based viability studies, with net energy balance (difference of energy input and energy output) or net energy ratio (ratio of energy output to energy input) as the viability indicators, have already been attempted for the production of liquid fuels from microalgae [6,8]. In an LCA study of Nannochloropsis sp., though Jorquera et al. found a positive energy demand for biomass production, but the study did not consider harvest, post-harvest and conversion to fuel stage [6]. Clarens et al. in a similar kind of study did not include post biomass production stages. Razon et al. did net energy analysis of the production of biodiesel and biogas from Haematococcus and Nannochloropsis sp., wherein his results showed large energy deficit for both the species. The scope of the study included only two harvesting techniques, i.e. gravitational settling and microfiltration, of many available techniques. For oil processing only transesterification was considered [9]. Even Lardon et al., in his life cycle assessment study of biodiesel production from microalgae found a very low value for energy balance. The production system included only open raceway pond for culture, and trans-

624

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

esterification process for oil processing. The study did not include oil cake processing, but did recommend further processing of oilcake to reduce external energy demand [8]. In another LCA study, Khoo et al. calculated a total energy demand of 4.44. MJ per MJ biodiesel produced. The scope of the study included an integrated photobioreactor raceway system for algae culture, transesterification for oil processing and did not take oil cake into account [10]. It is clear, from the above review of various LCA studies on microalgae, for liquid fuel production, that most of the studies have either found a very low value [8] or negative values for energy balance [9,10]. A few though have found a positive energy balance; have not included the entire value chain [6]. Moreover, almost all of them have considered only a few of the many available methods and techniques of biofuel production from microalgae [8e10]. The net energy ratio depends on the type of techniques used for cultivation, harvesting and oil processing (conversion) [11]. Through a combination of different techniques several different routes can be formed for green diesel production from microalgae. Considering the same, the current study aims to find out if the net energy balance can further be increased by using a combination of many available agronomical practices and the techniques of biofuel production from microalgae. The functional unit of the study is one hectare. Fig. 1 shows the system boundary with various possible combinations and routes for green diesel production from microalgae. Energy requirements for items with many years of useful life have not been included in the study.

2.2. Open pond description The design specifications of the raceway ponds considered for the current study comprise of 10 units of open ponds, each 100 m

long, 10 m wide and 30 cm in depth. 2.3. Photobioreactor description Flat -plate airlift photobioreactors have been considered for the current study. Two cases have been studied: Case 1: The design specifications comprise of 20 flat reactors, each with 105 units. Each unit is 4.53 m long, 1 m high and 10 cm thick. Reactors are 1 m apart from each other to avoid shading. A part of the light energy required for illumination was from sunlight, and the rest was from the fluorescent tubes on both sides of the reactors. Case 2: The design specifications comprise 39 flat reactors each with 105 units. Each unit is 4.53 m long, 1 m high and 10 cm thick. The reactors were illuminated on both the sides using fluorescent tubes. The culture conditions for flat plate reactors were kept similar to that by Pruvost et al., 2011 [12]. For all reactors the temperature was controlled at 25
C and pH was set at 7.5 by air and CO2 injections. The incident photon flux density was 270 mmol m2 s1. The number of fluorescent tubes was adjusted accordingly to provide the required photon flux density. In addition, algae was grown under stressful condition of nitrogen depletion, which increased the total lipid content from 20% to 23%. 2.4. Microalgae (Chlorella vulgaris) Chlorella vulgaris is a freshwater species, and is also known for lipid accumulation [12]. Chlorella is a single celled, spherical nonmotile green alga 2.0e10.0 mm in diameter. Chlorella occurs in both fresh and marine water. Due to its occurrence in various different habitats it is also called ubiquitous [13]. According to various studies on Chlorella vulgaris, it is an ideal

Nutrients

Electricity

Culture

Harvesting & Drying

Open raceway

Flocculation & Belt Drying

Lipid Extraction

Photobioreactor

Flocculation, centrifugation & Belt Drying

Centrifugation & Belt Drying

Flocculation & Filtration

Bligh & Dyer Solvent Extraction

Conversion Process

Hydrogenation

Lipid Pyrolysis Biomas

Dry Route

Wet Route

Hydrothermal Liquefaction

Pyrolysis Hydrotreatin

Green Diesel

HTL

Fig. 1. System boundary, showing the various possible combinations and routes for green diesel production from Microalgae.

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

microalga for the production of biodiesel. The features which make it an ideal strain for biodiesel production are [14]: 1. High amount of saturated fatty acids like palmitic and stearic acid, give the biodiesel good cetane number and high oxidation stability. 2. Length of the hydrocarbon chain is between C10 and C18. 3. It has good biomass productivity. Moreover, it has been significantly studied and quantitative estimates of productivities and composition in various conditions are available. 2.5. Methodology Life cycle assessment (LCA) has been used to calculate energy balance and greenhouse gas emissions from the energy use of microalgae (Chlorella vulgaris) based biofuel (Biodiesel and biogas) production, and their uses in the internal combustion engine for transportation. In LCA, a product is followed from its cradle to grave. Both natural resource use and pollutant emission are described in quantitative terms [15]. This study includes all the major activities and various inputs/outputs in every stage of microalgae green diesel production. The whole life cycle of Chlorella vulgaris has been divided into four major stages: a. b. c. d.

Microalgae culture & harvesting Oil extraction Oil & Biomass residue processing End combustion of green diesel

Since microalgae biofuel industry is still in infancy, data on large scale algae production is lacking. For the accurate LCA study large scale production data are required. Therefore, laboratory data has been extrapolated to meet the requirements of the current study. Moreover, the entire study has been modelled as per Indian climatic condition. 2.5.1. Method of data collection Laboratory observations and published data of known industrial processes have been used and extrapolated. 2.5.2. Assumptions a. Only CO2 emissions from fossil fuel electricity and fertilizer use have been considered. While N2O emissions have only been considered from fertilizer uses. b. According to Indian Centre for Science and Environment in 2007e2008, 78% of India's urea production comes from natural gas as the feedstock and rest from fuel oil and naphtha contributing 11% each [16]. Since the maximum urea in India is produced from natural gas, so have assumed 100% urea production from natural gas. c. Considering that Sulfur is produced as a byproduct in petroleum and steel industry [17], specific energy consumption for sulfur, as a component in Aluminium sulfate, has not been taken into account. d. As CO2 is formed as a by-product during electrical power generation [18], and the same has been assumed to provide CO2 requirements of the microalgae during its culture in the current study, therefore, specific energy consumption for CO2 production has not been considered. e. Initial temperature of biomass and water has been considered to be at 20
C.

625

f. Specific heat of microalgae has been considered to be the same as that of cellulose because it is the main component of plant cell wall [19]. g. Energy required for maintaining high pressure in certain processes has not been considered. h. 100% solvent recovery after Bligh and Dyer process has been assumed. i. Hydrogenation is a part of hydro-treating. Therefore, the energy consumption for hydrogenation has been assumed to be the same as that for hydro-treating. j. The heating value of HTL is twice that of pyrolysis oil. Therefore, it has been assumed that the amount of hydrogen required to upgrade per kg of HTL will be half as that required for upgradation of pyrolysis oil. k. Off-gas contains 80% of propane [20], therefore, the calorific value of off-gas was assumed to be equivalent to that of propane.

2.5.3. Calculations and study parameters The various study parameters considered are given in Table 1 and calculation methods are shown in Table 2. 2.6. Microalgae culture & harvesting 2.6.1. Microalgae culture Based on the reactor type and configuration considered for the present study, Table 3 shows the annual biomass production. The biomass concentration for raceway pond was assumed to be 0.5 kg/ m3 [45], while for flat-plate photo-bioreactor it was assumed to be 1.9 kg/m3 [12]. Dilution rate for raceway pond was assumed to be 0.1 d1 [6]. The specific growth rate for flat-plate photobioreactor was calculated with the help of the equation (1) [46] and values for the various variables were taken from the work done by Pruvost et al., 2011 [12].

m ¼ 1=t ln½Xm =X0 

(1)

where Xm and X0 are the concentrations of biomass at the end and at the beginning of a batch run, respectively, and t is the duration of the run. As the specific growth rate under stationary condition is equal to the dilution rate, therefore, dilution rate for flat-plate reactor was assumed to be equal to the specific growth rate calculated from equation (1). Volumetric productivity was calculated using equation (2) [6].

Pv ¼ mX

(2)

where Pv is volumetric productivity, m is specific growth rate (which here is equal to dilution as growth rate has been considered to be stationary) and X is biomass concentration. Flow rate was calculated using equation (3) [6].

F ¼ D*V

(3)

where F is flow rate, D is dilution rate and V is reactor volume. Annual biomass production was calculated using equation (4)

ABP ¼ F*X

(4)

where ABP is annual biomass production, F is flow rate and X is biomass concentration Total illuminated surface area for flat-plate reactor was calculated using equation (5) [12].

TIS ¼ 2*l*h*U*R

(5)

626

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

Table 1 Various parameters considered for study. Parameters

Unit

References

Average energy consumed per ton of fertilizer produced in India (Nitrogen and phosphorous fertilizer) 34.20 Fossil fuel energy per 1 kWh electricity (current electricity generation mix of India) 13.42

Value

GJ/ton of fertilizer MJ

Fossil fuel energy per 1 kWh electricity (from hard coal in India)

16.58

MJ

Fossil fuel energy per 1 kWh electricity (from natural gas in conventional power plants of India)

11.64

MJ

Fossil fuel energy per 1 kWh electricity (from natural gas in combined cycle power plants in India)

7.23

MJ

Specific energy consumption of Indian Aluminium industry CO2 emission from electricity

85 980

GJ/MT gCO2/kWh of electricity produced kWh/ton of CO2 mmol L1(g/L) kWh/m3 of water pumped m kJ MJ J
C1 g1 MJ/kg kJ/kg kJ/kg kWh/kg of Hydrogen Kg/m3 NL (normal litre) wt% of algae oil kWh/kg of oil processed MJ/kg of hydrogenated oil kJ/kg g/cm3 Kg/m3 GJ W/m3 kJ/kg
C

[21] Ecoinvent database Ecoinvent database Ecoinvent database Ecoinvent database [22] [23]

22.2 Energy required for CO2 injection, from a power plant 100 km away Amount of Aluminium sulphate required for flocculation 25 (8.55) Energy consumption for pumping water 0.05 Average annual evaporation losses 2.25 Energy required to process 1 m3 of culture during air sparging assisted coagulation flocculation (ASACF) 8.35 Energy consumed by Belt dryer per kg of evaporated water 3.35 Specific heat of cellulose 1.5 Heating value of Bio-oil 17 Specific latent heat of vaporization of trichloromethane (chloroform) 247 Specific latent heat of vaporization of methyl alcohol (methanol) 1100 Energy consumed in Hydrogen production 50.9 Density of hydrogen at NTP (normal temperature and pressure) 0.0899 Hydrogen required for hydro-treatment/kg pyrolysis oil 400 Hydrogen required for hydrogenation/kg algal oil 1.5 Average energy consumed in hydro-treating/hydrogenation process 0.32 Lower heating value of Hydrogenated oil (LHV) 44 Approx. enthalpy of water at 350
C and at 207 bar pressure 2540 Density of trichloromethane (chloroform) 1.48 Density of methyl alcohol (methanol) 791.8 1 MMBTU of natural gas 1.055 Power required by raceway pond for mixing 4
Specific heat of water at 20 C 4.182 kJ/ kg
C Power required by Flat-plate air reactor for mixing 53 Average sunshine hours/year in India 2750 % DW obtained after centrifugation 16 The concentration of the harvested culture after 19 h of gravitational settling of flocculated microalgae 40 culture Heating value of gas obtained during pyrolysis and liquefaction 12 Calorific value of Naphtha 10500 Calorific value of Propane 10792777.3

where TIS is total illuminated surface area, l is length of each unit of the reactor, h is height of each unit of the reactor, U is number of units per reactor and R is the number of reactors. During culture, water, light and nutrients are required. For uniform distribution of nutrients and light, the culture should be properly mixed. In flat-plate reactors, the mixing was achieved by air injection, while in photobioreactors it was achieved through paddlewheels. Table 4 shows the composition of the biomass obtained from the culture of microalgae. Based on the biomass composition, it was calculated that the amount of carbon required per kg of biomass is 487 g. Thus, the amount of CO2 required to fulfill this requirement is 1787 g (41 mols). The amount of nitrogen required per kg of biomass was calculated to be 15 g, and thus, amount of Di Ammonium phosphate [(NH4)2HPO4] required to provide nitrogen requirement per kg of biomass will be 70.7 g. Di Ammonium phosphate will also provide for the phosphorous requirement of algal biomass. Table 5 shows the nutrient requirement based on the biomass composition. Further, the temperature of the water in open pond was controlled by evaporation. Considering the average annual evaporation losses, total annual water lost through evaporation was estimated to be around 22500 m3. Therefore, in order to maintain the culture concentration, extra 22500 m3 of water was pumped

[18] [24] [25] [26] [10] [9] [19] [27] [28] [29] [11] [30] [31] [20] [11] [32] Steam table

[33] [6] [34]

W/m3 Hours/year % DW times

[35] [36] [25] [24]

MJ/kg kCal/kg kCal/ton

[37] [38] [39]

into the culture medium. Tables 1 and 2 can be referred for estimation of the total energy input during microalgae culture in the various reactors, and emissions from electricity and fertilizer use. Per unit of N2O emission from fertilizer is equivalent to 310 units of Carbon dioxide [48]. 2.6.2. Microalgae harvesting & drying After culture, microalgae need to be harvested and dried for further processing. The dry weight % (DW %) required will depend on the routes followed for further processing of microalgae for biofuel production. There are two routes for biofuel production from microalgae, i.e. I. Dry Route: Dehydration is needed to obtain 90% dry weight (DW) of microalgae because biomass left after lipid extraction will further be processed by pyrolysis method, for which moisture levels of 5e10 wt% are generally considered acceptable [27]. II. Wet Route: Dehydration is needed to obtain 20% DW of microalgae as Bligh and Dyer method for lipid extraction can be applied to any tissue containing water up to 80% [49]. Depending on the two routes, following combinations of harvesting and drying, techniques were used to reach the required dry weight. The amount of Aluminium Sulfate required for flocculating algae culture can be referred from Table 1. After 10 h of gravitational settling the concentration of the harvested culture is found to be 40

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

627

Table 2 Various calculation methods. Parameters

Calculation methods

Calculation for CO2 emissions Hydrogenated oil

Hydrogenated oil has following chemical structure CnH2nþ2 [32], which calculates to be 0.75 kg of carbon/kg of hydrogenated oil CO2 emission ¼ Fuel combusted * carbon content coefficient * Fraction oxidized* (44/12) Where 44 is molecular weight of CO2 and 12 is molecular weight of carbon. If we consider fraction oxidized ¼ 1 (As per IPCC, 2006 guidelines [40]) CO2 emission from 1 kg of Hydrogenated oil ¼ 1*0.75*1*(44/12) ¼ 2.75 kg of CO2 N2O emission ¼ Fertilizer consumption * Emission coefficient Emission Coefficient ¼ 17.68 kg/ton of fertilizer consumed [41]. NEB ¼ Total Energy Output - Total Energy Input NER ¼ Total Energy Output/Total Energy Input Energy (W) ¼ 0.2176* Incident light Intensity (mmole m2 s1)* Surface area (m2) [42] Average temperature in the four seasons of India [43]:

Calculations for direct N2O emissions from fertilizer uses NEB and NER calculation Light energy supplied to the flat-plate reactors Electricity consumption by heat exchanger

Energy use by centrifuge [25]

Energy required for Pyrolysis Heating value of char [44]

Energy required for heating the microalgal culture up to the reaction temperature of 350
C

To simplify the calculation, the increased or decreased temperature of the flat-plate photobioreactors has been assumed to be the same as that of the atmospheric temperature. Power (heat load) consumed by heat exchanger (both for heating and cooling), for temperature control in order to maintain culture temperature at 25
C [34], has been calculated with the help the following formula: Power (kW) ¼ Mass flow (kg/s)*Specific heat (KJ/kg/
C) *Difference between inlet and outlet temperatures on one side (
C). The amount of electricity consumed can further be calculated by multiplying the power with the number of working hours of heat exchanger. Energy (kWh) ¼ [Dry weight of microalgae to be processed (kg)* Power consumed by centrifuge (kW)]/ [Concentration of the algae culture (kg m3)* Throughput capacity of the centrifuge (m3 h1)] For the current study, the specifications considered for the centrifuge were same as that by Xu et al., i.e. throughput capacity of 85 m3 h1 and motor power of 45 kW Energy for pyrolysis (J/g) ¼ Specific heat of biomass* (temperature rise in biomass)* amount of biomass The experimental results have demonstrated that heating value of char increases with increase in temperature. HHV ¼ 0.0069T þ 24.68 [MJ/kg] Heating value of char produced at around 500
C was estimated to be 28 MJ/kg. Energy ¼ Mass of water (kg)* (Enthalpy of water at 350
C and pressure of 3000 psi - enthalpy of water at 20
C) þ Algal biomass (kg) *Specific heat of algal biomass*(350
Ce20
C).

Table 3 Based on the Reactor Type and Configuration, Annual biomass Production. Variables

Units

Raceway pond

Flat-plate photobioreactor Case 1

Case 2

Annual biomass production Volumetric productivity Total illuminated surface area Annual Flow Biomass concentration Dilution rate Space Reactor volume Flow rate

kg/year kg/m3*d m2 m3 kg/m3 d-1 m2 m3 m3/d

54750 0.05 10000 109500 0.5 0.1 10000 3000 300

197917.97 0.57 19026 104167.4 1.9 0.3 10000 951 285

385940 0.57 37100.7 203126.3 1.9 0.3 10000 1855. 556.5

Table 4 Composition of the biomass obtained from culture of microalgae. Biomass fractions

Composition [47]

Biomass fractions in N-deprived medium (%) [12]

Biomass fractions in normal medium (%) [12]

Molar mass (g/mol)

Protein Carbohydrates Lipid

(C6H13.1O1N0.6) (C6H10O5)n (C57H104O6)

20 40 23

60 20 20

109.5 162 884

628

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

Table 5 Energy requirement for various harvesting and drying processes (per megajoule of green energy produced). Dry route

Flocculation followed by belt drying Flocculation followed by centrifugation and belt drying Centrifugation followed by belt drying

Wet route

Open raceway pond

Photobioreactor Case 1

Case 2

89.9 15.53 3.47

23.22 6.14 2.97

23.22 17.16 2.97

times higher than the initial culture [24], and was estimated to give 7.6% DW algae culture. The various combinations are as mentioned below: 1. Combination 1: It includes flocculation, followed by belt drying. Refer Table 1 for estimating energy required for the process. 2. Combination 2: It includes flocculation, followed by centrifugation and belt drying. Refer Tables 1 and 2 for estimating energy required for the process. 3. Combination 3: It includes centrifugation, followed by belt drying. Refer Tables 1 and 2 for estimating energy required for the process. 4. Combination 4: It includes flocculation, followed by filtration. Cell size of chlorella being very small, micro filtration alone won't be effective. Therefore, filtration must be done after flocculation. Based on the energy requirements combination three i.e. centrifugation followed by belt drying was the most energy efficient combination (refer Table 5). Moreover, flocculation alone contributes more energy input than the total energy input in Combination 3. Therefore, Combination 4 is not an energy efficient route. Accordingly, for further calculations the energy requirements of the Combination 3 have been considered.

2.7. Oil extraction Chemical solvent has been reported to be more efficient method of lipid extraction than mechanical press [50]. Lam and Lee compared four different chemical solvents for lipid extraction, ethanol, methanol, n-hexane and Bligh and Dyer method (mixed methanol-chloroform with volume ratio of 2:1). They found Bligh and Dyer method to have highest lipid extraction efficiency and nHexane, which is one of the widely used solvent for lipid extraction, to have lipid extraction efficiency [50]. Bligh and Dyer method of lipid extraction, yields 95% of total lipid, and further to it, this method can be used for any tissue containing water up to 80% [49]. Therefore, for lipid extraction, Bligh and Dyer method has been considered for both dry and wet routes. The critical volume ratios of methanol, trichloromethane (chloroform) and water should be 2:1:1.8 and that of solvent to tissue should be 3:1. After the solvent and culture are mixed, in the given ratio, they are homogenized to form a monophasic system and then re-homogenized with another similar quantity of trichloromethane. Therefore, the overall volume ratio of methanol, trichloromethane and water should be 2:2:1.8, and that of solvent to tissue is {(3 þ 1):1} [49]. Considering the critical ratios, for dry route, since water content is insignificant in comparison to biomass, solvent to tissue ratio of {(3 þ 1):1} should be considered, while for wet route, because of high water content, methanol, chloroform and water, volume ratio of 2:2:1.8 should be considered.

Open raceway pond

Photobioreactor Case 1

Case 2

112.34 5.16 1.74

27 5.16 1.1

27 1.1

The homogenization was done by centrifuge, which also separates the biphasic layer (lipid dissolved in chloroform and methanol dissolved in water) formed during the process. Thereafter, the lipid is separated from trihloromethane (chloroform), and methanol from water by fractional distillation (heat energy was assumed to be provided by burning of natural gas). During this process the solvents are recovered. 2.8. Oil and biomass processing 2.8.1. Oil processing Oil ages and tends to phase separate upon prolonged standing. As a consequence, it is not suitable for direct application in (nonstationary) internal combustion engines and requires upgrading to fulfill the stringent specifications for (bio-) fuels [31]. Out of the total 23% lipid content, TAG was only 14% [12]. Moreover, only TAG can be converted to biodiesel via transesterification process. The rest is polar lipids and are, thus, unwanted in the biodiesel process, as they strongly influence the processing and the quality of the product. Therefore, in order to produce a biofuel with more favorable properties, hydroprocessing is a better process. It converts entire lipid content, even the polar lipids [51], into biodiesel. Hydro-processing produces green diesel, naphtha and offgas in the mass ratio of 78:2:6 [20]. The energy produced by naphtha and offgas can be used to produce electricity required for hydrogenation process and, thus, decreasing the net energy input for the entire process. Table 1 can be used to estimate energy requirements during oil processing via hydrogenation. 2.8.2. Biomass processing 2.8.2.1. Dry route (pyrolysis). It involves heating biomass in the absence of air or oxygen [52] to around 500
C to form liquid fuel (Bio-oil), charcoal and gaseous fraction [53]. Depending on the pyrolysis time or heating rate, it is of two types i.e., slow and fast pyrolysis [54]. Slow pyrolysis forms more of char [54,27], while fast pyrolysis dramatically alters and shifts the reaction to form more of liquid bio-oil. According to National renewable energy laboratory (NREL) report, slow pyrolysis produces liquid, gas and char in the ratio (by mass) of 30:35:35, while fast pyrolysis produces them in the ratio of 75:13:12 [27]. The bio-oil also contains about 15 wt% water [27] and higher oxygen content, which decreases the heating value of bio-oil to about 15e19 MJ/kg. Also, it is not suitable for direct application in diesel engines and need to be further upgraded [31]. One of the technologies used for upgrading is catalytic hydrotreatment, which is carried out at about 200e400
C and 100e200 bar pressure. The Ru/C catalyst was found to be superior, with yield of about 60 wt% upgraded oil [31]. Since the green diesel obtained by up-gradation of pyrolysis oil was assumed to be upgraded via hydro-treatment, its calorific value was assumed to be

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

same as that of the green diesel obtained via hydrogenation i.e. 44 MJ. This heating value of 44 MJ/kg [32] is almost more than twice the heating value of bio-oil obtained after pyrolysis. Feed particle size can significantly affect the balance between char and liquid yields. Larger particles are beneficial in processes targeting char production, and small particles are preferred to maximize liquid yields in fast pyrolysis. Fast pyrolysis is characterized by high heating rates and short vapour residence times. This generally requires a feedstock prepared as small particle sizes and a design that removes the vapours quickly from the presence of the hot solids [55]. 2.8.2.2. Wet route (hydrothermal liquefaction). The remaining wet algae culture, after lipid extraction, can be further processed to obtain oil by hydrothermal liquefaction. Hydrothermal liquefaction converts carbohydrates and proteins, left after lipid extraction, into oil. 10% DW to 20% DW microalgae, i.e. wet algae, is an excellent substrate for hydrothermal liquefaction. No drying or cellular disruption is required [56]. The wet algal slurry is heated to approximately 250
Ce350
C at 1500 to 3000 psi. Water catalyzes the reaction and the algal cells liquefy. Hydrothermal liquid (HTL) obtained is 38 wt% of total biomass feedstock and 15 wt% gas is also formed [56]. The hydrothermal liquid (bio-crude), obtained from hydrothermal liquefaction, is not suitable for direct engine application. That is because of high oxygen and nitrogen content [56]. HTL is not comparable with fossil products, such as, diesel [51]. Therefore, the oil needs to be upgraded [56]. HTL has heating value of 33.3e39.9 MJ/kg, which is higher than the pyrolysis oil [51]. Since the heating value of HTL is twice that of pyrolysis oil, it has been assumed that the amount of hydrogen required to upgrade per kg of HTL will be half as that required for pyrolysis oil up-gradation. After up-gradation 80% renewable diesel is obtained [56]. 2.9. End combustion of green diesel Tables 1 and 2 can be used to estimate the total energy and total carbon dioxide emissions from green diesel, which it would release on combustion. 3. Results & discussion Table 6 and Fig. 2 show the final stage wise energy input/output, NEB and NER for per megajoule of green diesel energy produced, while Table 8 shows the total CO2 emissions from the different

629

50 40 30 20

Culture & HarvesƟng

10 0

Oil ExtracƟon

-10 -20

Oil & biomass processing

-30 -40 -50 Case 1

Case 2

Case 1

Case 2

End combusƟon of green diesel NEB

Open Photobioreactor Open Photobioreactor raceway raceway pond pond DRY ROUTE

WET ROUTE

Fig. 2. Energy flow during various stages of green diesel production from microalgae and NEB.

120 100 80

Pyrolysis/liquefacƟon of biomass

60

Oil Processing

40 Oil ExtracƟon 20 HarvesƟng & Drying

0 Case 1 Open raceway pond

case 2

Photobioreactor

Case 1 Open raceway pond

DRY ROUTE

case 2

Photobioreactor

Culture

WET ROUTE

Fig. 3. Life cycle energy use analysis of the various stages.

routes. It is clear from Table 6 that none of the proposed routes in the current study have positive NEB. Production of green diesel via open raceway pond, both in dry as well as wet route, have less negative NEB and comparatively higher NER than the photobioreactors. As per Table 3, though the photobioreactors produce far more biomass than the open pond in the same time period, yet, this extra energy produced by them is compensated by the high energy requirements by the tube lights and the heat exchangers used to support this high biomass production in the photobioreactors. Further, on comparison between the two open raceway pond routes, dry route has slightly higher value for NER than the

Table 6 Stage wise Energy input/output, NEB and NER (per megajoule of green energy produced). Dry route Open raceway pond

Culture & Harvesting Oil Extraction Oil & biomass processing Total energy input End combustion of green diesel Total energy output (MJ) NEB NER

Culture Harvesting & Drying Extraction Oil Processing Biomass Pyrolysis/liquefaction Hydrogenated algal oil Hydrotreatment of pyrolysis oil/HTL

1.32 3.47 0.05 0.03 0.2 5.07 0.33 0.67 1 4.07 0.20

Wet route Photobioreactor Case 1

Case 2

21.63 2.97 0.05 0.03 0.2 24.88 0.33 0.67 1 23.88 0.04

30.64 2.97 0.05 0.03 0.2 33.89 0.33 0.67 1 32.89 0.03

Open raceway pond

Photobioreactor Case 1

Case 2

1.69 1.74 0.19 0.04 2.68 6.34 0.42 0.58 1 5.34 0.16

27.68 1.09 0.19 0.04 2.68 31.68 0.42 0.58 1 30.68 0.032

39.21 1.09 0.19 0.04 2.68 43.21 0.42 0.58 1 42.21 0.023

630

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

Table 7 Total CO2 Emissions from the Different Routes (per megajoule of green energy produced). Dry route

Culture & Harvesting Oil Extraction Oil & biomass processing

Culture Harvesting & Drying Extraction Oil Processing Pyrolysis/liquefaction of biomass

End combustion of green diesel Total Carbon dioxide emissions (kg)

Wet route

Open raceway pond

Photobioreactor Case 1

Case 2

0.11 0.25 0.003 0.003 0.0002 0.06 0.43

1.59 0.22 0.003 0.003 0.0002 0.06 1.87

2.25 0.22 0.003 0.003 0.0002 0.06 2.53

Open raceway pond

Photobioreactor Case 1

Case 2

0.13 0.13 0.009 0.004 0.20 0.06 0.53

2.03 0.08 0.009 0.004 0.20 0.06 2.39

2.87 0.08 0.009 0.004 0.20 0.06 3.23

Table 8 Sensitivity analysis for showing the effect on NER values due to factors like change in electricity generation source and change in oil yield and input energy during various stages. Dry route Open raceway pond

Base case (current Indian mix energy generation scenario) Coal (Hard coal) Natural gas (conventional power plant) Natural gas (combined cycle power plant) 25% increase in oil yield from algae 25% decrease in harvesting and drying energy input 25% decrease in culture energy input When only green diesel from algal oil is considered

0.20 0.16 0.23 0.36 0.21 0.24 0.21 0.07

wet route. Even the total CO2 emissions for dry route is only slightly more than that of wet route. The reason for this could be attributed to the huge energy requirements during hydrothermal liquefaction process in the wet route, which compensates for the entire energy savings made during harvesting and drying process. Moreover, compared to other conversion technologies, research on pyrolysis of algal biomass is quite extensive and has achieved reliable and promising outcomes that could lead to commercial exploitation [57]. And, despite rising interest in HTL, many of its operational parameters are uncertain [56] and moreover, reactors for thermochemical liquefaction and fuel-feed systems are complex and are, therefore, expensive [57]. Even with the best possible route (open raceway pond dry route), as per the current study, the total energy use is almost five times more than the energy produced with a very low NER value of 0.20. NEB is 4.07 mj, i.e. it utilizes this much more energy to produce per megajoule of energy. Life cycle energy use analysis (see Fig. 3) shows that among the various stages, microalgae culture and harvesting consumes the highest energy, i.e. 94%, followed by oil and biomass processing. Oil extraction stage consumes the least energy. The close reactors consume far more energy than the open pond during culture itself, and the major reason behind is the use of tubelights and heat exchangers. Moreover a lot of energy is also used during the drying process, i.e. to dry the algal biomass up to 90% DW. Though centrifugation followed by belt drying has been shown to have the least energy requirements, yet it is important to develop more efficient dryers so that this energy requirement can further be reduced. If we look at Table 7, microalgae culture and harvesting produces maximum CO2 followed by end combustion of green diesel. The two major CO2 contributors, during culture and harvesting stage, are the use of large quantities of fertilizer and electricity. A sensitivity analysis (refer Table 8) was done to determine the

Wet route Photobioreactor Case 1

Case 2

0.04 0.03 0.05 0.07 0.04 0.04 0.05 0.01

0.03 0.02 0.03 0.05 0.03 0.03 0.04 0.01

Open raceway pond

Photobioreactor Case 1

Case 2

0.16 0.13 0.18 0.28 0.17 0.17 0.2 0.11

0.03 0.026 0.04 0.06 0.03 0.032 0.040 0.01

0.02 0.019 0.03 0.04 0.03 0.023 0.030 0.01

effect on NER of potential changes in the electricity generation source of India. Of the four electricity generation sources, i.e. current Indian generation mix, coal, natural gas (conventional power plant) and natural gas (combined cycle power plant), the most efficient (refer Table 1) is natural gas (combined cycle power plant), which only uses 7.23 mg of energy to produce per kWh of electricity. But still this could not increase the values of NER to more than one. In another scenario when the oil content of algae was increased by 25%, yet the increase in NER was very minimal i.e. from 0.08% to 0.10%. Moreover, it was observed that NER values are more sensitive to changes in input energy during harvesting and drying stage in comparison to the cultural stage. Further, when the oil cake was not considered for energy production and only oil obtained from algae was taken into consideration, the NER values further decreased by more than 25e60% in most of the cases. When compared to various previous LCA studies, mentioned in Section 2.1 under goal, scope and system boundaries, the values of the NER in the current study were found to be much lower than the mentioned LCA studies. There could be two reasons for it, first, as mentioned before, most of the mentioned studies did not include all the important life cycle stages. The second could be the difference in the input energy values considered for the production of various chemicals and electricity generation efficiency. In the present study authors have tried to incorporate the most relevant input energy values from most relevant sources, considering the conversion efficiencies. 4. Conclusion Both raceway pond and flat-plate reactors gave negative values for NEB and NER. Fertilizer uses during culture, and electricity uses during harvesting and drying were the major energy consuming activities. As far as fertilizer uses are concerned, a lot of work has already been done on the use of wastewater from various sources for algae culture. However, microalgal growth rates in waste water

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

have been shown to be very less in comparison to growth rates in water with added fertilizer. Therefore, further research is needed to develop a source of water, which can provide the same growth rate as in the water with added fertilizer. A lot of energy which is used in production of these fertilizers can be saved, and also the environmental hazards caused by this water disposal can be minimized. Further, an efficient harvesting and drying technology is required, which can reduce the electricity uses especially for drying water. Though the wet route doesn't require much energy for drying water, yet the use of excessive amount of chemicals during oil processing reduces its overall efficiency. It can be concluded from the current study that though microalgae has huge potential as an alternative source of renewable energy, yet research and development in the area of green diesel production from microalgae has to go a long way. Such technologies need to be developed, which can further reduce the huge energy requirements during green diesel production from microalgae. References

[22] [23]

[24]

[25]

[26] [27]

[28]

[29] [30]

[1] E.B. Sydney, et al., Screening of microalgae with potential for biodiesel production and nutrient removal from treated domestic sewage, Appl. Energy 88 (10) (2011) 3291e3294. [2] J. Yang, X. Li, H. Hu, X. Zhang, Y. Yu, Y. Chen, Growth and lipid accumulation properties of a fresh water microalga, Chlorella ellipsoidea YJ1, in domestic secondary effluents, Appl. Energy 88 (10) (2011) 3295e3299. [3] A.B. Fulke, et al., Bio-mitigation of CO2, calcite formation and simultaneous biodiesel precursors production using Chlorella sp. Bioresour. Technol. 101 (21) (2010) 8473e8476. [4] J.N. Rosenberg, G.A. Oyler, L. Wilkinson, M.J. Betenbaugh, A green light for engineered algae: redirecting metabolism to fuel a biotechnology revolution, Curr. Opin. Biotechnol. 19 (5) (2008) 430e436. [5] S.M. Heilmann, L.R. Jader, L.A. Harned, M.J. Sadowsky, F.J. Schendel, P.A. Lefebvre, M.G. Keitz, K.J. Valentas, Hydrothermal carbonization of microalgae II. Fatty acid, char, and algal nutrient products, Appl. Energy 88 (2011) 3286e3290. [6] O. Jorquera, A. Kiperstok, E.A. Sales, M. Embirucu, M.L. Ghirardi, Comparative energy life-cycle analyses of microalgal biomass production in open ponds and photobioreactors, Bioresour. Technol. 101 (2010) 1406e1413. [7] Q. Hu, M. Sommerfeld, E. Jarvis, M. Ghirardi, M. Posewitz, M. Seibert, A. Darzins, Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances. Harnessing plant biomass for biofuels and biomaterials, Plant J. 54 (2008) 621e639. [8] L. Lardon, A. Helias, B. Sialve, J.P. Steyer, O. Bernard, Life-Cycle Assessment of Biodiesel production from Microalgae, Environ. Sci. Technol. 43 (2009) 6475e6481. [9] L.F. Razon, R.R. Tan, Net energy analysis of the production of biodiesel and biogas from the microalgae: Haematococcus pluvialis and Nannochloropsis, Appl. Energy 88 (10) (2011) 3507e3514. [10] H.H. Khoo, P.N. Sharratt, P. Das, R.K. Balasubramanian, P.K. Naraharisetti, S. Shaik, Life cycle energy and CO2 analysis of microalgae-to-biodiesel: preliminary results and comparisons, Bioresour. Technol. 102 (2011) 5800e5807. [11] F. Delrue, P.A. Setier, C. Sahut, L. Cournac, A. Roubaud, G. Peltier, A.K. Froment, An economic, sustainability, and energetic model of biodiesel production from microalgae, Bioresour. Technol. 111 (2012) 191e200. [12] J. Pruvost, G.V. Vooren, B.L. Gouic, A. Couzinet-Mossion, J. Legrand, Systematic investigation of Biomass and lipid productivity by microalgae in photobioreactors for biodiesel application, Bioresour. Technol. 102 (1) (2011) 150e158. [13] M.M. Phukan, R.S. Chutia, B.K. Konwar, R. Kataki, Microalgae Chlorella as a potential bio-energy feedstock, Appl. Energy 88 (10) (2011) 3307e3312. [14] S. Rasoul-Amini, N. Montazeri-Najafabady, M.A. Mobasher, S. Hoseini-Alhashemi, Y. Ghasemi, Chlorella sp.: A new strain with highly saturated fatty acid for biodiesel production in bubble -column photobioreactor, Appl. Energy 88 (2011) 3354e3356. [15] H. Baumann, A.M. Tillman, The Hitch Hiker's Guide to LCA: an Orientation in Life Cycle Assessment Methodology and Application, 2004. ISBN 91-4402364-2. [16] Centre for Science and Environment, New Delhi; 2007. Available: http://www. cseindia.org/userfiles/79-90%20Fertilizer(1).pdf. [Accessed 25 July 2010]. [17] Petroleum Conservation Research Association. Available: http://www.pcra. org/English/latest/book/10-Chapter%20-%2010.pdf. [Accessed 9 August 2012]. [18] K.L. Kadam, Environmental implications of power generation via coalmicroalgae cofiring, Energy 27 (2002) 905e922. [19] A. Heredia, Biophysical and biochemical characteristics of cutin,a plant barrier biopolymer, Biochimica Biophysica Acta 1620 (2003) 1e7. [20] R. Davis, A. Aden, P.T. Pienkos, Techno-economic analysis of autotrophic microalgae for fuel production, Appl. Energy 88 (2011) 3524e3531. [21] Ernest Orlando Lawrence Berkeley National Laboratory. Assessment of Energy

[31]

[32]

[33] [34]

[35]

[36]

[37]

[38] [39] [40] [41]

[42]

[43] [44] [45]

[46]

[47]

[48] [49]

[50]

[51] [52]

631

use and Energy Savings Potential in Selected Industrial Sectors in India. Supported by the climate protection division, office of air and radiation, US Environmental Protection Agency through the US Department of Energy under contract no. DE-AC02e05CH11231; 2005. Centre for Science and Environment, New Delhi. Available: http://www. cseindia.org/userfiles/57-66%20Aluminium(1).pdf. [Accessed 9 August 2012]. S. Ghosh, Status of thermal power generation in Indiadperspectives on capacity, generation and carbon dioxide emissions, Energy Policy 38 (11) (2010) 6886e6899. M. Ras, L. Lardon, S. Bruno, N. Bernet, J.P. Steyer, Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris, Bioresour. Technol. 102 (2011) 200e206. L. Xu, D.W.F. Brilman, J.A.M. Withag, G. Brem, S. Kersten, Assessment of a dry and a wet route for the production of biofuels from microalgae: Energy balance analysis, Bioresour. Technol. 102 (2011) 5113e5122. National Institute of Hydrology, Estimation of Evaporation Losses from Water Surface - a Study of Tawa Reservoir, Jalvigyan Bhawan, Roorkee, 1996-97. M. Ringer, V. Putsche, J. Scahill, Large-scale Pyrolysis Oil Production: a Technology Assessment and Economic Analysis, 2006. Technical report NREL/TP510e37779. The Engineering Toolbox. [Online]. Available: http://www. engineeringtoolbox.com/fluids-evaporation-latent-heat-d_147.html. [Accessed 10 August 2012]. Engineering Toolbox. [Online]. Available: http://www.engineeringtoolbox. com/human-body-specific-heat-d_393.html. [Accessed 10 August 2012]. Engineering Toolbox. [Online]. Available: http://www.engineeringtoolbox. com/gas-density-d_158.html. [Accessed 10 August 2012]. J. Wildschut, F.H. Mahfud, R.H. Venderbosch, H.J. Heeres, Hydrotreatment of fast pyrolysis oil using heterogeneous noble-metal catalysts, Ind. Eng. Chem. Res. 48 (2009) 10324e10334. R. Arvidsson, S. Persson, M. Froling, M. Svanstrom, Life cycle assessment of hydrotreated vegetable oil from rape, oil palm and Jatropha, J. Clean. Prod. 19 (2e3) (2011) 129e137. GasTerra. [Online]. Available: http://www.gasterra.com/aardgas/Pages/ woordenlijst.aspx. [Accessed 1 August 2012]. The theory behind heat transfer. [Online]. Available: http://www. distributionchalinox.com/produits/alfa-laval/echangeurs/heat-transferbrochure.pdf. [Accessed 21 August 2012]. E. Sierra, F.G. Acien, J.M. Fernandez, J.L. Garcia, C. Gonzalez, E. Molina, Characterization of a flat plate photobioreactor for the production of microalgae, Chem. Eng. J. 138 (2008) 136e147. N.K. Sharma, P.K. Tiwari, Y.R. Sood, Solar energy in India: Strategies, policies, perspectives and future potential, Renew. Sustain. Energy Rev. 16 (2012) 933e941. T. Malkow, Novel and innovative pyrolysis and gasification technologies for energy efficient and environmentally sound MSW disposal, Waste Manag. 24 (2004) 53e79. Central Electricity Authority, Planning Wing, Report of the Expert Committee on Fuels for Power Generation, Government of India, New Delhi, 2004. P. Energycopia- Energy Conservation, in: P. Dwivedi, Diwan (Eds.) III, Pentagon Energy Press, New Delhi, 2008. Intergovernmental Panel on Climate Change, Chapter 1: Guidelines for National Greenhouse Gas Inventories, 2006. A. Garg, P.R. Shukla, S. Bhattacharya, V.K. Dadhwal, Regional and sectoral assessment of greenhouse gas emissions in India, Atmos. Environ. 35 (15) (2011) 2679e2695. C. Yang, Q. Hua, S. Kazuyuki, Energetics and carbon metabolism during growth of microalgal cells under photoautotrophic, mixotrophic, and cyclic lightautotrophic/dark-heterotrophic conditions, Biochem. Eng. J. 6 (2000) 87e102. Climate of India. [Online]. Available: http://en.wikipedia.org/wiki/Climate_of_ India. [Accessed 21 August 2012]. C. Bulmau, C. Marculescu, A. Badea, Pyrolysis parameters influencing the biochar generation from wooden biomass, U.P.B. Sci. Bull. 72 (1) (2010). P. Collet, A. Helias, L. Lardon, M. Ras, R.-A. Goy, J.-P. Steyer, Life-cycle assessment of microalgae culture coupled to biogas production, Bio-resource Technol. 102 (2011) 207e214. A. Converti, A.A. Casazza, E.Y. Ortiz, P. Perego, M.D. Borghi, Effect of temperature and nitrogen concentration on the growth and lipid content of Nannochloropsis oculata and Chlorella vulgaris for biodiesel production, Chem. Eng. Process. Process Intensif. 48 (6) (2009) 1146e1151. B. Sialve, N. Bernet, O. Bernard, Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable, Biotechnol. Adv. 27 (2009) 409e416. IPCC, “Emissions from waste incineration,” [Online]. Available: http://www. ipcc-nggip.iges.or.jp/public/gp/bgp/5_3_Waste_Incineration.pdf. S.J. Iverson, S.L. Lang, M.H. Cooper, Comparison of the Bligh and Dyer and Folch methods for total lipid determination in a broad range of marine tissue, Lipids 36 (11) (2011) 1283e1287. M.K. Lam, K.T. Lee, Immobilization as a feasible method to simplify the separation of microalgae from water for biodiesel production, Chem. Eng. J. 191 (2012) 263e268. M. Kroger, F. Muller-Langer, Review on possible algal-biofuel production processes, Biofuels 3 (3) (2012) 333e349. A. Demirbas, Progress and recent trends in biodiesel fuels, Energy Convers. Manag. 50 (1) (2009) 14e34.

632

N. Pragya, K.K. Pandey / Renewable Energy 86 (2016) 623e632

[53] S. Amin, Review on biofuel oil and gas production processes from microalgae, Energy Convers. Manag. 50 (2009) 1834e1840. [54] Reed, T. B., and Cowdery, C. D. Heat flux requirement for fast pyrolysis and a new method for generating biomass vapour. [Online]. Available: http://web. anl.gov/PCS/acsfuel/preprint%20archive/Files/32_2_DENVER_04-87_0068.pdf. [Accessed 1 August 2012]. [55] T.M. Brown, P. Duan, P.E. Savage, Hydrothermal Liquefaction and Gasification

of Nannochloropsis sp, Energy Fuels 24 (2010) 3639e3646. [56] E.D. Frank, A. Elgowainy, J. Han, Z. Wang, Life cycle comparison of hydrothermal liquefaction and lipid extraction pathways to renewable diesel from algae, Mitig. Adapt Strateg. Glob. Change 18 (1) (2012) 137e158. [57] L. Brennan, P. Owende, Biofuels from microalgaedA review of technologies for production, processing, and extractions of biofuels and co-products, Renew. Sustain. energy Rev. 14 (2) (2010) 557e577.