HEAT INTEGRATION ASSESSMENT FOR THE COUPLING OF HYDROTHERMAL PROCESSING OF LIPID EXTRACTED ALGAE...

6th International Conference on Hydrogen Production May 3-6, 2015

UOIT—Oshawa, Ontario, Canada

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HEAT INTEGRATION ASSESSMENT FOR THE COUPLING OF HYDROTHERMAL PROCESSING OF LIPID EXTRACTED ALGAE WITH A DOWNSTREAM REFORMER

FOR A CONCEPTUAL HYDROGEN PRODUCTION PLANT 1*

Magdeldin Mohamed, 1Kohl Thomas,

1Järvinen Mika

1Aalto University, School of Engineering, Department of Energy Technology,

Aalto, FI-00076, Finland

*Corresponding Author E-mail: [email protected]

ABSTRACT

Supercritical water gasification as a mean for hydrothermal processing of biomass, has illustrated the potential to overcome technical barriers that continue to prevent the prevalence of biomass based energy systems. This paper provides a systematic analysis to fill the knowledge gap for upscaling the process from laboratory bench scale to energetically efficient commercial demonstration. Conceptual plant flowsheets for non-catalytic supercritical water gasification of lipid extracted algae were developed on Aspen plus® for hydrogen fuel production. The advantageous reactor system configurations are integrated in the process layout with subsequent gas purification, mechanical power extraction and downstream indirect production through steam reforming to maximize the energetic poly-generation of hydrogen fuel, power and thermal heat. The design and synthesis for the process blocks, components and equipment was based on operational data for referenced pilot or commercial units. Heat integration studies based on the pinch analysis method were conducted for the conceptual designs. The minimum process utility demands were computed, and different energy recovery scenarios as well as alternative design configurations for optimal heat recovery were assessed. At the end, a comparative assessment showed that integrating downstream steam reforming with upstream hydrothermal processing lead to an increase in the overall energetic efficiency from 33.7% to 45.9% and in the fuel-equivalent efficiency from 43.4 to 54.5%.

Keywords: Supercritical water gasification, Lipid extracted algae, Conceptual process design, Pinch analysis and Hydrogen.

INTRODUCTION

The establishment of renewable energy systems continues to grow as a priority globally. The current urgency to break the dependence on fossil fuel based energy production, and curb continued rise of green-house gas emissions while developing sustainable energy supply chains is considered the main motivation within scientific, industrial and governance circles. Biomass feed stocks in particular offers distinctive advantages among the wide variety of available primary energy fuel sources. Holistic life cycle system design allows for biomass to provide neutral carbon dioxide emission production pathways to chemical fuel products. Also it offers continuous energy supply as base load for heat and power generation without the utilization of storage technologies in contrast to intermittent renewable sources such as solar and wind. As a result biomass is predicted to play a key role within the intended future green global economy. [1]

The chemical structure of solid biomass is mainly carbon based compounds but also contains mixtures of hydrogen, oxygen, varied portions of sulphur and nitrogen, and often smaller quantities of minerals and alkali earth metals. Several conversion pathways have been investigated to process the constituents into valuable products, they could be categorized into; first biological approaches through fermentation technologies, second type are chemical transformations through hydrolysis, trans-esterification and others, and finally thermal conversion pathways through combustion, pyrolysis or gasification. This research investigates the latter route, where the gasification process which is considered a thermo-chemical approach is utilized to valorise biomass. The produced fuel gas is known as synthetic gas or syngas and is a mixture of methane, hydrogen, carbon monoxide, carbon dioxide, nitrogen and smaller quantities of heavier hydrocarbons.

The technical barrier that continues to hold back biomass gasification systems is the lowered conversion efficiency compared to other conventional fuel systems. The produced syngas is accompanied with fluid like carbon rich contaminants known as char and tar which eventually lead to mechanical troubles during operations, especially those related to pipe clogging and equipment damages. Another challenge is the high



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moisture content that biomass constitutes, which could exceed levels of 90% for some feed stocks, as sewage sludge and water hyacinth. The approximated load required during treatment of the feedstock is 2240 kJ per kg of moisture [2]. The intensive drying pre-treatment process leads to a significant disproportion of energy generated comparable to that of consumed.

Hydrothermal treatment pathways that utilize the existing moisture content, water as the working fluid for organic conversion have shown immense potential to counter such barriers. The findings of Sanjay et al. [3] in the mid-1970s showed that at increased temperature and pressure levels of supercritical state for water; tar and char levels that are naturally formed in subcritical state were significantly minimized. Yoshida et al. [4] observed that when moisture content increased from 5 to 70%, the conversion efficiency for a thermal gasification process dropped from 61% to a merely 27%. While on the other hand, under hydrothermal treatment methods, the gasification efficiency was recorded at 31% and 51% respectively when moisture increased from 5 to 70%. Kruse et al. [5] reiterated the same findings for a feedstock of 80% moisture, where thermal reforming recorded an energetic efficiency of around 10% comparable to that of 70% for a hydrothermal treatment method such as supercritical water gasification, SCWG.

Thus, despite the well documented endothermic nature of the hydrothermal processes, the potential energetic savings from the drying process elimination as well as the higher organic conversion rates provide a viable process pathway for energy generation from biomass in all its forms, thermal heat, power and chemical fuels. The objective of this work is to systematically analyze and develop higher energetically efficient conceptual flowsheets for hydrogen production from SCWG, as well as evaluate process integration pathways with more established hydrogen production technologies such as steam reforming.

RESEARCH BACKGROUND

Supercritical water gasification, SCWG process

Water in the existing moisture content form or as a process additive is the working fluid in hydrothermal applications; it is a non-toxic, cheap and abundant source. Supercritical conditions represent risen levels of pressure and temperature at 22.09 MPa and 374.29˚C respectively, as shown in Fig. 1 [6]. At such acute conditions, water molecules experience the structural breaking of hydrogen bonding and acquire an intermediate set of thermo-physical properties between those of vapour and liquid. Supercritical water experiences decreased density and viscosity but remain relatively higher than the temperature correspondent superheated steam [7]. The acquired properties play a crucial role in the advantageous nature of the SCWG process and could be summarized in the following; [8]

1- Hydrogen bonding loss results in lowered dielectric constants which lead water to lose its polar characteristics and act as a non-polar solvent for organic compounds in the gasification process.

2- The lowered viscosity allows for high diffusion coefficients and efficient heat transport. The miscibility of solid reactants along with intermediate cyclic aromatic hydrocarbon products responsible for char and tar formation lead to single phase chemical reactions with better mixing and higher conversion rates.

3- Around the pseudo critical point, the higher concentration of [H+] [OH

-] ions leads to a richer

medium for acid- or base- catalysed reactions. 4- When compared to biological treatment that counter high moisture content in biomass, SCWG

offers a faster pace of conversion and significant equipment downsizing.

Significant research activities have been conducted for SCWG on a laboratory scale, with the focus mostly on the influence of reactor/gasifier operating conditions such as pressure, temperature, solid content and residence time, along with catalyst contributions and the effect of equipment design configurations on process performance. Some of the parametric analysis was conducted with real biomass; lignocellulose feed stocks [9-12], algal species [13,14], sewage sludge [15,16] and industrial waste streams [17-19]. Another approach is to depict and extrapolate the chemical transformation of real biomass through simply structured

Fig. 1. Water phase diagram. [6]



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model compound mixtures, such model feeds are; glucose and sucrose to model cellulose [20-22] and phenol to model lignin due to its aromatic structure [23-24].

Experimental findings have shown that SCWG has the potential to valorise higher moisture content solid biomass feeds into valuable syngas. The variation in conversion rates as well as the product gas yield is concluded as a function of reactive conditions and biomass type. Complete equilibrium and considerable higher gasification efficiency were recorded at higher temperature levels, above 550˚C and lower solid content, below 15 wt.%. [9,10,13,17,21,22] Char and tar contaminants were not detected and higher gas yields were recorded at temperature levels as high as 600˚C [9,18,20,22]. Also yield nature was parametrically driven, hydrogen production was favoured by both higher temperature intervals between 550-800˚C and lower solid content in the reactor, below 10 wt.%. While lower temperature intervals, higher solid content and increased pressure favoured a methane and carbon dioxide mixture product gas. [9,12-18,20] In an attempt to understand the influence of biomass constituents, both phenol the model compound for lignin [23, 24] and the presence of protein and amino acids in algal species [13] hindered the gasification efficiency.

Hydrothermal treatment of algal species

The research interest in algal biomass as a renewable energy source, and in particular micro algae crops as spirulina and chlorella, has grown significantly over the years. Micro algae are microscopic organic materials that are capable to grow in both brackish and sea water. When compared to lignocellulose feed stocks, the higher photosynthetic efficiency and specific area energy yields are expedient. Also the apparent higher moisture content, solid concentration mediums of 0.1% and small particulate size, less than 100 mm show suitability and promise for hydrothermal application [25]. Physical separation and dewatering yield a 10-20 wt.% feedstock which falls in line with pumpability limitations for high pressure commercial pumps [26].

Hydrothermal treatment of microalgae in both its liquefaction and gasification application has yet to be realized commercially, albeit continuous operation has been demonstrated on a pilot scale liquefaction facility in the University of Sydney [27]. It is worth to note, that a recorded advantage over alternative algae to biofuel pathways is the higher process yields, which are due to the inclusion of protein and carbohydrates conversion along the lipid ones. Also mixed algae cultures could be utilized as feed stocks, and nutrient recycling could be maximized in an intermediate salt separation step [25].

METHODOLOGY

Process design and synthesis is the formulation of a systematic superstructure for intertwined chemical and physical processes with the objective to develop and realize a product of higher value from raw materials. In our case, low grade lipid extracted algae, LEA feedstock is the raw material which is a byproduct of bio-diesel production from algal species. The polygeneration of high grade energy products in the form of gaseous chemical fuels, electricity and thermal heat are the target. SCWG process is the selected thermochemical conversion pathway to the desired products. Aspen plus® simulation software is utilized for the computer aided calculation of the thermodynamic properties of the subsequent process steps and the generation of a conceptual flow sheet for the different process equipment and material streams.

Conceptual Plant Design

Water plays the role of a solvent, catalyst and reactant for the wet biomass feedstock in the SCWG process. The varying nature of supercritical water properties at different process conditions, allows for manipulation and selectivity of desirable yields. Thus, the development of a satisfactory representative model for the thermodynamic behaviour of the process becomes an essential tool for process design and optimization. In the absence of detailed and accepted kinetics representation for the chemical transformations within the reactor, due to the heterogonous structures for different biomass feed stocks, mathematical or simulation modelling are considered reliable alternative approaches to assess the process. The developed flowsheets on Aspen plus® are divided into upstream and downstream operations, the first operates up to supercritical conditions, while the latter is under subcritical conditions.

SCWG Reactor Equilibrium Model

The selected approach for the upstream SCWG reactor is the non-stoichiometric thermodynamic equilibrium model based on the Gibbs free energy minimization principle. The chemical equilibrium principle states that a multicomponent reacting system is at the most stable composition after a finite time, a condition achieved



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only when the entropy and the chemical change within the system, is maximized, while the Gibbs free energy available to create such change is minimized. The model is independent of the reactor mechanical design, which leads to assumptions of isothermal conditions, homogenous mixing, uniform heat and temperature distribution and no time barrier to reach the equilibrium state. Although such assumptions could logically lead to deviation from the real process, when applied through Aspen plus®, given the right boundary conditions are identified, it has produced experimentally validated results in literature. [28-30]

The LEA feedstock is configured in the simulation as a non-conventional compound where the thermodynamic properties were calculated based on enthalpy and density coal correlations known as HCOALGEN and DCOALIGT respectively, both of which recorded adequate results. The feedstock elemental data was extracted from the Phyllis2 ECN database [31] and is summarized in Table 1 below.

Table 1. LEA Elemental Analysis

Proximate analysis, wt.% Ultimate analysis, wt.%

Ash content 4.63 Carbon 48.63

Moisture 5.62 Hydrogen 7.09

Volatiles 80.01 Oxygen 32.03

Fixed carbon 15.31 Sulphur 0.45

Nitrogen 7.31

The selected simulation package for the thermodynamic properties of fluids in the supercritical state was the integrated Equation of state and non-quadratic Gibbs free energy model, Peng Robinson-Wong Sandler. Records in literature show higher accuracy for phase equilibria calculations by the model for asymmetric or/and polar-nonpolar or/and supercritical mixtures as is the case for SCWG of biomass [32]. Withag et al. [28] reported investigating the process with the package but only results based on other packages were recorded. To the author’s knowledge, the model has yet to be examined as basis for supercritical component properties in the SCWG process.

Pinch Analysis

Heat integration methods are the cornerstone of process synthesis and optimization studies. The pinch analysis along with its application based improvements are widely applied to determine minimum energy targets, to develop or retrofit heat exchanger networks, to assess efficient consumption of process raw materials as well as to generate alternative process pathways. Gundersen [33] summarized the basic step dependant elements of the method; the initiation step is based on the developed conceptual designs, at which the physical and chemical conversion in the material streams act as the principal process performance targets. The process temperature and heat duty profiles are developed based on extracted supply and target temperatures and the specific material flow properties, mass flow and specific heat capacity. The minimum temperature difference, ∆Tmin for heat transfer between mediums was selected to be 10˚C. Mixed integer linear programming approach was used to calculate the cascaded table algorithms, determine the minimum hot and cold utility consumption and identify the step-wise shifted temperature and heat transfer intervals between process hot and cold streams.

Thermodynamic Evaluation

The developed conceptual designs are finally thermodynamically assessed based on conversion efficiencies on a lower heating value, LHV basis from solid biomass fuel to gaseous products and electricity. The energetic efficiency 𝜂 is calculated as following;

𝜂 =(𝑚∗ℎ𝐿𝐻𝑉)ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛+ 𝑊𝑒𝑙

(𝑚∗ℎ𝐿𝐻𝑉)𝐿𝐸𝐴+ (𝑄𝑡ℎ)𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 (1)

Where 𝑚 and ℎ𝐿𝐻𝑉 are the mass flow rates and lower heating value for the product, hydrogen and the LEA

biomass fuel extracted from Aspen plus®. 𝑊𝑒𝑙 is the net electricity generated onsite and (𝑄𝑡ℎ)𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 is the minimum thermal plant utility demand calculated from the pinch analysis.

While the energetic efficiency accounts for the thermodynamic relevance of generated products, it does not take into account the quality of such products. The technical quality of product generation is thus further analysed through the fuel equivalent efficiency shown below;



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𝜂𝑓𝑢𝑒𝑙−𝑒𝑞 =(𝑚∗ℎ𝐿𝐻𝑉)ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛+ (𝑊𝑒𝑙∗(1

𝜂𝑁𝐺𝐶𝐶⁄ ))

(𝑚∗ℎ𝐿𝐻𝑉)𝐿𝐸𝐴+ (𝑄𝑡ℎ)𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 (2)

Where, 𝜂𝑁𝐺𝐶𝐶 = 0.57 represents the efficiency of a natural gas combined cycle. The factor is multiplied by the net electricity generated to account for the amount of fuel consumed to generate the same amount of electricity in the reference technology. Such evaluation has shown to provide adequate results and reliable representation of fuel consumption efficiency in literature [34].

RESULTS & DISCUSSIONS

Upstream SCWG blocks

The upstream operations of the conceptual plant design have the principal function to convert the chemical energy stored within the low grade solid biomass to energetically accessible forms. It consists of three main blocks; feed pre-treatment, reactor system and power generation block as shown in Fig. 2. At first, LEA biomass feed enters the plant to the pre-treatment block where it is mixed with a water supply stream in mixer, MX01 to adjust the solid content to the desirable operational conditions. The selected T-Rex pumps produced by the Zeilfelder Pumpen GmbH [26], offers slurry pumpability up to 18 wt.% solid content. Aside from the considerable higher solid content limit, other significant pump features are; operation limit up till 450˚C which would allow intermediate pressure boosting and the capability to handle large particle sizes up till 100 mm. It is worth to note that, the mixed slurry in stream-1 entering the pump depicts in terms of solid content that of dewatered post-cultivated algae feed.

The pressurized feed at 250 bar then enters the three-step reactor block, configured to manipulate the water properties around the critical point; it includes a subcritical hydrolysis reactor, a pseudo critical salt precipitator and finally a supercritical gasifier. At first the feed stream enters the hydrolysis reactor which is simulated by both preheater, HT01, where it is heated till 350 ˚C and yield reactor, HYDREAC. The non-catalytic hydrolysis process in the reactor bed aims to operate within temperature boundaries that allow the decomposition of the organic solids through pyrolytic reactions to bio-crude while maintaining liquid phase processing. The hydrolysate product is a more viscous hydrophobic phase with less dissolved water than pyrolysis oil, yet less dense energetically [25]. The reactor block is simulated by Aspen Plus® RYIELD reactor block which decomposes the non-conventional biomass to its elemental constituents, the over estimation for the endothermic decomposition is balanced out by the coupling in heat stream REACHEAT with the exothermic heat of formation from the same elemental constituents in the SCWG reactor.

The intermediate step in the reactor block is the salt and solid separator around the critical point, the main advantage of performing the separation step at this process point, aside from eluding equipment clogging in gaseous processing, is salt recovery as nutrient rich brine. Gassner et al. [34] developed a heat transfer

Fig. 2. Developed upstream operation blocks on Aspen plus®



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model for an envisaged separator design. The subcritical hydrolysate enters from the top of the vessel through dangling tubes that are externally heated till around the critical point. The continued heating of the liquid down flow transforms the stream within the vessel into a gaseous like supercritical fluid; a reverse flow upwards takes place and the product organic fluid exit from the top while the less soluble salts precipitate at the bottom and exit as brine. The group estimated a conservative loss of 10% on mass basis of the organic feed, a similar value range of 6.8% loss was estimated for a woody feedstock in other literature as well [35]. The block is simulated by both heater, HT02 as the vessel’s external jacket and separator, SP01 where an estimated organic feed loss of 10% along with solids, ash and minerals exit as brine.

As the final step in the reactor block, the product fluid in stream-7 enters the supercritical reactor, which is simulated by heater, HT03 and Aspen’s Gibbs reactor block, SCWGREAC. Where at first the fluid is heated to the desired reactive temperature, then the reactor block calculates the formation of the nine major products; methane, hydrogen, carbon dioxide, carbon monoxide, water, ethane, ethylene, nitrogen and hydrogen sulphide. The product stream-9 then enters a turbine, SCT01 in the power generation block for the extraction of mechanical power. Due to the higher levels of water content in the stream, between 80-95%, the turbine is simulated as commercial ultra-supercritical, USC steam turbines, the threshold input conditions are 600˚C and 250 bar and the outlet pressure is 65 bar with a recorded isentropic efficiency of 92.5% [36]. The expanded superheated gas stream is then further cooled to 28˚C to liquefy the water content and produce the two phase product fed into the downstream gas purification blocks.

Conceptual Hydrogen Production Plant

The conceptual design for hydrogen production consists of six main process blocks as shown in Fig. 3. The chronological process sequence from the plant entry point at the top left corner; feed pre-treatment, reactor system, power generation, gas purification, heat generation and indirect hydrogenation block. The first three blocks are considered the upstream plant operations discussed in details in the previous sub section, while the latter three represent downstream blocks which are simulated under two design configurations. The first configuration is to maximize hydrogen production through feeding all extracted methane from the SCWG product into a steam reformer in the hydrogenation block, at which the heat generation block operates at minimal capacity. While the other configuration targets in-situ heat generation for energy recovery where the opposite occurs, the hydrogenation block is considered nonexistent and all process depleted and residue streams are fed into a furnace in the heat generation block. The comparison between both configurations allows for an energetic performance assessment of the integrated SCWG - steam reformer process layout in comparison to the stand alone SCWG process layout.

The driver to maximize direct hydrogen yield in the SCWG reactor is lowered solid content in the feed. This also leads to an increase in the organic material conversion. Thus, the two feed streams, water and biomass are mixed homogeneously at a solid content of only 5 wt.%, which accounts for 106 MWLHV of feed capacity. Table 2. shows the respective data for major upstream units.

Table 2. Summary of main assumptions and simulation data for the upstream units in the conceptual hydrogen plant

Feed pump Salt separator USC turbine

Outlet pressure, bar 250 Organic loss, wt.% 10 Outlet temp, ˚C 367

Solid content, wt.% 5 Outlet temp, ˚C 400 Power, MWel 34

Hydrolysis reactor SCWG reactor

Temperature, ˚C 350 Temperature, ˚C 600

Yield outlet, wt.% Gibbs outlet, wt.%

Carbon 2.294 CH4 0.88

Hydrogen 0.355 H2 4.54

Oxygen 1.052 CO2 2.49

Sulphur 0.022 N2 0.22

Nitrogen 0.337 CO 0.05

Ash 0.219 H2S 0.01

Water 95.281 H2O 91.74



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Fig. 3. Block diagram for the SCWG conceptual plant design for hydrogen chemical fuel



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The cooled product stream is then fed into the gas purification block which consists of a two phase water separator system and a Selexol acid gas removal unit. At first, the water separator system is configured as a two-step pressure flash separators; the primary one operates at 60 bar and separates the syngas stream. The dry syngas stream accounts for an organic material conversion rate of 1.16 kg syngas per kg of solid biomass feed in the SCWG process. At the meantime, the liquid stream is then further flashed at atmospheric pressure in a secondary separator to produce a depleted gas stream fed into the heat generation furnace. The flash separators configuration recovers 99.9% water in the cooled product stream; the upstream SCWG process water consumption was recorded to be 0.88 kg per kg of solid biomass.

The dry syngas is then fed into a Selexol acid gas removal unit. The main advantage of Selexol is the capability to operate at high pressure levels, in our case 60 bar, this allows for further mechanical power extraction from the outlet sweet gas stream. The unit utilizes the Methyldiethanolamine, MDEA solvent for selective extraction of CO2 and H2S at 96.5% and 91% recovery rate respectively. MDEA is the preferable solvent due to the higher solubility of water which significantly decreases any further moisture content in the syngas and at the same time, MDEA has lower solubility toward hydrogen which allows for maximum hydrogen recovery. The Selexol solvent recovery circuit consists of flash separators; at first, depleted hydrocarbons are released at 20 bar with 25% of the separated CO2 and fed to the heat generation furnace, while the rest of acid gas is finally released at an atmospheric flash. [37]

The sweet gas from the Selexol unit has 79 mol% H2 purity, and a rounded out composition of 15 mol% CH4, 3.7 mol% N2 and traces of CO2, CO and H2S. The sweet gas is expanded to 20 bar then enters the last upgrading step in the pressure swing adsorption, PSA block. This purification technology is generally used in hydrogen production plants to produce a 99.99% purity stream. It is based on regenerative solid adsorbents that select hydrogen from the rest of the mixture components at high pressures and ambient temperatures. Hydrogen recovery rate in the PSA unit is maximized based on hydrogen concentration in the feed and the controlled pressure ratio between the feed stream and the off-gas. Commercially, the hydrogen inlet concentration is around 76 mol% and a pressure ratio of 4 is needed to obtain a 70% hydrogen recovery rate. [38] Table 3. shows the summary for the PSA units simulation data.

Table 3. Summary of PSA unit simulation data

Stream Inlet Hydrogen Off-gas

Pressure, bar 20 1 5

Composition, mol%

H2 79.08 100 27.43

CH4 15.01 0 52.08

CO2 1.15 0 3.99

CO 0.96 0 3.34

N2 3.78 0 13.33

H2S < 0.01 0 0.02

H2O < 0.01 0 < 0.01

The conceptual plant is configured to run under two scenarios; maximize hydrogen production or in-situ heat generation. The PSA off-gas splitter directs the flow to the furnace in the heat generation block for the latter, where the stream is adiabatically combusted, along with flashed fuel gas from the Selexol unit and depleted streams from the water separation unit. While for the first scenario, the methane rich off-gas stream is fed into a sequence of reforming reactors in the indirect hydrogenation block.

The selected steam reform technology accounts for 50% of commercial hydrogen production globally. The steam reformer is designed as a fixed bed reactor operating at 650°C and 5 bar, where steam is supplied at higher than stoichiometric ratio of the off-gas to suppress coke formation within the reactor. Methane conversion of 98% is recorded with Nickel-Magnesium based catalyst [39]. In order to obtain higher hydrogen conversion within the indirect hydrogenation block, the reformed stream is introduced to a water gas shift reactor to convert CO to H2 and CO2. In industrial applications, the process requires being



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performed with catalysts in two consecutive stages with intermediate cooling, first a high temperature reactor around 350˚C for rapid CO conversion and then a low-temperature one at around 200˚C, the recorded conversion is 97%. [40] The product gas stream of the shift reactors is then cooled and fed into a two phase separator, to remove any liquid water. Finally, the dry stream is fed into a secondary PSA unit to produce the indirect hydrogen stream that accounts for 45.3% of the plant total output capacity.

Heat Integration Assessment

For the first configuration scenario, to maximize hydrogen production by integrating the upstream SCWG reactor with the downstream steam reformer, the total plant output was recorded at 90 MWLHV of hydrogen fuel with 54% directly produced in the SCWG reactor and net electricity generation of 30 MW el. This resulted in the major heat loads in the reactor system such as the salt separator preheater, supercritical reactor preheater and the SCWG reactor, as well as the downstream steam reformer to be accounted for above the pinch, while the only available fuel for integrated recovery in the furnace were the off-gas from the secondary PSA unit and depleted purification block streams, which were significantly smaller. This lead to a high minimum heat requirement of 1.47 MWth per MWLHV feed. At the meantime, the cooling demand was recorded at 0.89 MWth per MWLHV feed. The overall plant energetic efficiency was recorded at 45.9% while the fuel equivalent efficiency was 54.5%.

For the second scenario, to maximize heat recovery by the elimination of the indirect hydrogenation block, the in-situ generated heat accounted for only 46 % of the total heat demand. The process pinch point was also located at the supercritical turbine outlet. The absence of the methane reformer load in the hydrogenation block along with presence of a significant hot combustion gases stream leads to the logically lowered 1.21 MWth per MWLHV feed minimum heating demand. The system recorded an energetic efficiency of 33.71% and fuel equivalent efficiency efficiency of 43.71%, both of which are lower than the first scenario. The heat assessment for both configurations showed that the presence of the hydrogenation block not only doubles the total hydrogen output but the plant recorded higher overall efficiencies.

An alternative design is configured based on increasing the LEA feed into the plant to the pumping limitation of 18 wt.% while optimizing the off-gas splitter to meet the minimum heat requirement by the furnace load for energy self-sufficiency while operating the indirect hydrogenation block. The alternative design recorded an output of 105.5 MWLHV of hydrogen at which 53% is produced directly in the SCWG reactor, and a net electricity of 30 MWel. The PSA unit off-gas split to the hydrogenation block was 19.5%, where the furnace load was recorded at 0.496 MWth per MWLHV feed. The design had no process pinch point while the plant minimum cooling demand was recorded at 0.284 MW th per MWLHV feed. The plant design recorded an energetic efficiency of 35.78% and a fuel equivalent efficiency of 41.7%. Table 3 shows a comparative assessment between the developed hydrogen cases.

Although the alternative hydrogen case was developed to enable self-sufficient energy configuration, the plant recorded lower energetic and fuel equivalent efficiencies than the reference first case scenario. This is mainly attributed to the fact that increased solid content in the SCWG leads to lower direct hydrogen production and lower organic material conversion rates. It is worth to note that the plant total hydrogen output increased as well as the heat sink capacity. The development of a detailed heat exchanger network in future work, will allow the assessment of integrated thermal heat production which potentially could lead to optimizing the overall plant efficiencies.

Table 4. Comparative assessment of the developed SCWG designs for hydrogen production

Design Cases Base Case

1st Scenario

Base Case

2nd

Scenario

Alternative Case

Biomass feed, MWLHV 106 106 381

Hydrogen production, MWLHV 90 49.2 105.5

Fraction of direct hydrogen in SCWG 0.541 1.00 0.531

Min. heating demand, MWth/MWLHV feed 1.47 1.21 0

Min. cooling demand, MWth/MWLHV feed 0.89 0.92 0.248

Energetic efficiency 45.9% 33.7% 35.8%

Fuel equivalent efficiency 54.5% 43.4% 41.7%



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CONCLUSIONS

The research presented a systematic energetic assessment for the hydrothermal processing of lipid extracted algae feedstocks. Conceptual designs were developed on Aspen plus® for hydrogen production based on pilot and commercial scale data for process components and equipment collected from literature. The developed flowsheets showed that hydrothermal processing and in particular SCWG has the potential to valorize solid biomass with considerably high organic material conversion rates into chemical fuels such as hydrogen. The upstream SCWG reactor system recorded an organic conversion rate of 1.16 kg syngas per kg of solid biomass feed with a water consumption rate of 0.88 kg per kg of solid biomass.

Stand-alone hydrothermal plant performance with a 106 MWLHV feed capacity was improved by process integration with downstream steam reforming technology; the plant recorded increased output, from 49.2 to 90 MWLHV of hydrogen fuel. The plant recorded an increase in energetic efficiency from 33.7% to 45.9%, and the fuel-equivalent efficiency increased from 43.4% to 54.5%. An alternative design for energy self-sufficient operation was developed with increased solid biomass content, a 381 MWLHV feed capacity. Lower direct hydrogen production in the SCWG reactor was offset with the increased capacity of the downstream reformer. The plant output increased to 105.5 MWLHV while the overall energetic and fuel-equivalent efficiencies were recorded at 35.8% and 41.7% respectively.

The conceptual designs could be further optimized through the integration of district heating networks to utilize the significant heat sink capacities recorded at 0.89 and 0.25 MWth per kg of solid biomass for the base design 1

st scenario and alternative design respectively. Future work will include the design of a heat

exchanger network based on the performed heat integration assessment as well as an analysis of the plant design economics.

ACKNOWLEDGEMENT

Academy of Finland is gratefully acknowledged for funding this work. (Project grants no. 268222, 258319 and 26495)

ABBREVIATIONS

LEA Lipid extracted algae MDEA Methyldiethanolamine PSA Pressure swing adsorption SCWG Supercritical water gasification

NOMENCLATURE

Latin Letters

h Specific enthalpy, kJ/kg m Mass flow rate, kg/s Qth Thermal heat, MW W Electrical power, MW

Greek Letters

𝜂 Efficiency

Δ Difference

Subscripts

el Electricity external Outside system boundaries fuel-eq Fuel equivalent LHV Lower heating value min Minimum NGCC Natural gas combined cycle th Thermal



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