Thermodynamic Analysis of Supercritical Water Gasification of Microalgae Biomass for Hydrogen and Syngas Production

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1 553 A publcaton of VOL. 32, 203 CHEMICAL ENGINEERING TRANSACTIONS Chef Edtors: Sauro Perucc, Jří J. Klemeš Copyrght 203, AIDIC Servz S.r.l., ISBN ; ISSN The Italan Assocaton of Chemcal Engneerng Onlne at: Thermodynamc Analyss of Supercrtcal Water Gasfcaton of Mcroalgae Bomass for Hydrogen and Syngas Producton Antono C. D. Fretas, Regnaldo Gurardello* School of Chemcal Engneerng, Unversty of Campnas (UNICAMP), Av. Albert Ensten 500, , Campnas- SP, Brazl gura@feq.uncamp.br Supercrtcal water gasfcaton (SCWG) s an alternatve thermochemcal technology for the converson of wet bomass n a hgh heat value product gases, such as syngas and hydrogen. Algal bomass has been suggested as one promsng opton to produce bomass for use as fuels and chemcal feedstocks wth hgh area-specfc yelds. In ths study, the thermodynamc analyss of SCWG of mcroalgae bomass (C. vulgars and Sprulna Sp.) for hydrogen and syngas producton was presented. The effects of pressure ( bar), ntal temperature ( K), mcroalgae concentraton (5-20 wt%) and coreactant addton (CO 2 and CH 4 ) were evaluated wth respect to producton and fnal characterstcs of the products. The formaton of coke and the energetc characterstcs of the SCWG were dscussed. The analyss was conducted usng two non-stochometrc thermodynamc models, based on Gbbs energy mnmzaton and entropy maxmzaton methods. The vral equaton of state was used to represent the non-dealty of the system. Both problems were formulated as optmzaton problems (non-lnear programng) and the software GAMS n combnaton wth the CONOPT solver, were used to solve them. The results obtaned were compared wth prevously publshed expermental and smulated data obtaned n the lterature, wth good agreement between them. The syngas producton was low for all condtons tested, but the addton of a CO 2 and CH 4 as a co-reactant proved to be effectve way to ncrease the syngas producton wth a H 2 /CO molar rato close to 2 (deal for Fscher-Tropsch applcatons). The SCWG presents exothermc behavor for low ntal temperatures. In the regon of hgh ntal temperatures the systems showed to be less exothermc.. Introducton Bomass can be converted nto gas for further applcaton, such as hydrogen for fuel cell systems or syngas for applcaton n fuel producton through Fscher Tropsch synthess. In a general way, ths s the scope of bomass gasfcaton. Conventonal thermochemcal processes of bomass (for example pyrolyss or gasfcaton) utlze dry bomass as feedstock. Hydrothermal processes (processes n an excess of water at hgh temperature and pressure are used) opens up the opportunty to process bomass wth the natural water content. In hydrothermal gasfcaton processes the bomass does not need to be dred wth a hghenergy expendture. Also, hydrothermal gasfcaton can lead to low tar and char formaton and hgh hydrogen yelds because water serves as both a reactant and reacton medum (Kruse, 2008). In ths context the supercrtcal water gasfcaton (SCWG) of bomass appears, as a promsng technology for the producton of hydrogen and other useful gases from wet bomass, because of the remarkable characterstcs of the supercrtcal water (T C = 647 K, P C = 22 bar) wth respect to ts densty, delectrc constant, on product, vscosty, dffusvty, electrc conductance, and solvent ablty compared to ambent water. These processes present hgh effcences and can be adapted for many bomass sources (Savage, 2000). The lterature s scarce of works that use thermodynamc analyss n SCWG of real bomass compounds. Thus, n ths study, the thermodynamc analyss of SCWG of mcroalgae bomass (C. vulgars and Sprulna Sp.) for hydrogen and syngas producton was presented. The effects of pressure ( bar), temperature ( K), mcroalgae concentraton (5-20 wt%) and co-reactant addton (CO 2 and

2 554 CH 4 ) were evaluated. The formaton of coke and the energetc characterstcs of the SCWG were dscussed too. 2. Methodology 2.. Gbbs energy mnmzaton: Equlbrum at constant pressure (P) and temperature (T) The thermodynamc equlbrum condton for reactve multcomponent closed system, at constant P and T, wth gven ntal composton, can be obtaned by mnmzaton of Gbbs energy (G) of the system, gven by: g g l l s s mng = n μ + n μ + n μ () = = = Whle satsfyng the restrctons of non-negatve number of moles of each component n each phase: g l n,, s n n 0 (2) And the restrcton of mole balances, gven by atom balance for reactve systems: a g l s ( n + n + n ) = m = = a n 0 m m =,..., NE (3) Snce the system analyzed by the present work was at hgh pressure, the vral equatons of state (EoS), truncated at second vral coeffcent, were utlzed to determne the fugacty coeffcent. The second vral coeffcent s calculated by the correlaton of Ptzer and Curl (957), whch was modfed by Tsonopoulos (974). The fugacty coeffcent was determned by the followng relaton: m ln φˆ = 2 y j B j j B P RT 2.2. Entropy maxmzaton: Equlbrum at constant pressure (P) and enthalpy (H) The thermodynamc equlbrum condton for reactve multcomponent closed systems, at constant P and H, wth gven ntal composton, can be obtaned by maxmzaton of the entropy (S) of the system, wth respect to n : k g g l l n S + n S + s s max S = n S (5) = = = Whle satsfyng the same prevous restrctons, gven by equatons (2) and (3). Usually, physcal propertes are gven as functons of composton, pressure and temperature, not enthalpy. Therefore an addtonal restrcton, referent to enthalpy balance, must be satsfed: g g l l s s 0 0 ( n H + n H + n H ) = ( n H ) = = = H 0 (4) (6) 2.3. Model mplementaton The software GAMS 23.2., (General Algebrac Modelng System) wth the CONOPT solver was used n the resoluton of the combned chemcal and phase equlbrum problem. A descrpton of GAMS software can be found n Brooke et al. (998). The sold phase formed was consdered as sold carbon (pure component). These methodologes and consderatons were appled n prevous works of our research group wth good predctve results (Fretas and Gurardello, 202 a, b). Mcroalgae bomass was modelled as a pseudo-component n the system, whose chemcal formula s C a H b O c N d. The coeffcents a, b, c and d were obtaned n the lterature for both mcroalgae s analysed here and the values of these constants were presented n Table. A Total of 6 output compounds were selected as representatve of the man compounds, whch can be found n the output stream of these reactve systems. These compounds were selected based n

3 expermental works found n the lterature. The complete lst of output compounds, along wth ther crtcal parameters, s reported n Table 2. Table. Coeffcents of molecular formula for mcroalgae bomass analysed. Mcroalga a b c d Source Sprulna sp Castello and For (20) Chorella vulgars Chaknala et al. (200) General formula: C a H b O c N d. Table 2. Chemcal compounds consdered n the smulatons and your thermodynamc propertes. Compound Chemcal formula T C (K) P C (bar) V C (m 3 /kmol) ω (-) Water H 2 O Hydrogen H Methane CH Carbon doxde CO Carbon monoxde CO Oxygen O Ethane C 2 H Propane C 3 H Ethylene C 2 H Propylene C 3 H Methanol CH 3 OH Ntrous oxde N 2 O Ammona NH Ntrc oxde NO Ntrogen doxde NO Ntrogen N Source: Polng et al. (2000) Results and dscusson 3.. Gbbs energy mnmzaton 3... Model valdaton The Gbbs energy model predctons were compared wth expermental data obtaned n the work of Chaknala et al. (200) for the SCWG of Chlorella vulgars wth Ru/TO 2 [E] metallc catalyst. The experments were conducted at 600 C (873.5 K), 240 bar, wth 2 mnutes of reacton tme and 7.3 wt% of mcroalgae feed concentraton. The Gbbs energy model was run at the same condtons n whch the experments of Chaknala et al. (200) were performed. The comparson between the smulatons and expermental results were presented n the Fgure (a). Ths fgure shows the agreement between expermental data for Ru/TO 2 [E] catalyst obtaned n the work of Chaknala et al. (200) and the vral and deal models predctons. The deal model consders the dealty of the system, whch, mathematcally, means that the fugacty coeffcent of the system s equal to unt ( ϕ = ). In order to compare the results calculated usng the proposed approach wth the values found n lterature, the mean relatve error (MRE) was used, accordng to equaton 7. NPEE lterature smulated x j, x j, MRE=. (7) lterature NPE. E j x j, Where NPE s the number of expermental ponts, E s the number of components for each smulated lterature expermental pont, x j, s the value calculated by the present work and x j, s the expermental value obtaned n the lterature. The agreement s found to be very good wth MRE of for Vral equaton model and MRE of.086 for deal system consderaton. Fgure (b) shows the comparson between the smulated data from Castello and For (20) and the smulatons performed by the present work usng the proposed vral model. The smulatons were performed at the followng condtons: feed concentraton of 60 wt% of Sprulna sp., pressure of 250 atm

4 556 and temperatures between 700 and 400 K. Those authors used a model of Gbbs energy mnmzaton n combnaton wth the Peng Robnson EoS. The Fgure shows that the model proposed by ths work usng the vral EoS presents good agreement wth the model developed usng the Peng Robnson EoS. Fgure. (a) Comparson between expermental data obtaned n the work from Chaknala et al. (200) for the Ru/TO 2 [E] catalyst and the predctons obtaned usng the proposed Vral model and the deal system consderaton ( ϕ =). (b) Model valdaton wth smulated data from Castello and For (20); sold lne: smulatons of the present work usng the vral model; Symbols: smulatons of Castello and For (20) usng Peng Robnson EoS. In general, t s possble to state that the Gbbs energy mnmzaton technque combned wth Vral equaton of state provde results whch are n good accordance wth expermental and smulated data from lterature, therefore, the proposed model s a valuable tool for predctng the thermodynamc behavor of SCWG reactve systems Pressure and temperature effects Fgure 2 (a) shows the H 2 compostons as a functon of the temperature of the reacton for the SCWG of mcroalgae bomass, at constant pressure (250 bar) and composton of bomass n the feed (5wt%). The ncrease of the reacton temperature plays a favorable role n hydrogen formaton. The hydrogen composton n product stream was greater for C. vulgars mcroalgae bomass. In the Fgure 2 (b) the effect of the pressure ncreasng was presented, the smulatons were performed at constant temperature (873.5 K) and mcroalgae composton (0 wt%). Fgure 2 (b) shows that modfcatons n the system pressure have no sgnfcant effect on the formaton of H 2 n the system. Fgure 2. Effects of temperature (a) and pressure (b) under H 2 formaton n SCWG of mcroalgae bomass. In a general way, condtons of hgh temperature and low pressures results n hgh H 2 formaton, ths behavor was observed n other works of the lterature for model compounds and real bomass materals (Fretas and Gurardello, 202a).

5 3..3. Mcroalgae concentraton and co-reactants addton effects Fgure 3 presents the effect of feed concentraton under dry gas molar composton for SCWG of mcroalgae bomass. Fgure 3 (a) presents the results for SCWG of C. vulgars and Fgure 3 (b) presents the results for SCWG of Sprulna sp., both systems presented smlar behavor as expected. In general, the elevaton of the feed concentraton, results n decreased producton of H 2. The producton of CH 4 sgnfcantly ncreased at the same condtons. The CO 2 concentraton remans practcally constant throughout entre range of ntal compostons analyzed. The ntrogenous compounds formed don t show sgnfcantly concentratons of ntrogenous oxdes. Ntrogen (N 2 ) was the man ntrogen compound produced, however, small amounts of NH 3 were observed too. CO formaton was low throughout all composton range analyzed. 557 Fgure 3. Effect of feed concentraton under dry gas molar composton for SCWG of (a) C. vulgars; (b) Sprulna sp. Symbols: Sold lne Vral equaton model; dashed lne deal formulaton (φ =). Fgures 4 (a) and (b) shows the effect of CO 2 and CH 4 addton as a co-reactant n SCWG, under number of moles of H 2 produced and under H 2 /CO molar rato obtaned n the product, as a functon of the feed concentraton of mcroalgae. The co-reactants (CO 2 or CH 4 ) were added wth a molar rato of 2: n relaton of the mcroalgae. Ths molar rato was selected based on prevous analyzes. The addton of CO 2 and CH 4 results n decreased H 2 /CO molar ratos, but H 2 /CO rato closer to 2 (deal molar rato for Fscher Tropsch applcatons) was obtaned only wth CO 2 addton n regons of hgh bomass concentraton (see Fgure 4 (b)). Ths behavor was observed for SCWG of glucose (as model compound) when CO 2 was added n the system, n a prevous publshed work (Fretas and Gurardello, 202a). The addton of CH 4 presents another nterest effect: the formaton of H 2 ncreased sgnfcantly when CH 4 was used n the feed as a co-reactant (see Fgure 4 (a)). In none of the smulatons performed, the formaton of sold carbon was observed. Fgure 4. Effect of CO 2 and CH 4 addton under number of moles of hydrogen (a) and H 2 /CO molar rato (b) n the SCWG of C. vulgars and Sprulna sp Entropy maxmzaton: thermal effects In the Fgures 5 (a) and (b) were presented the results obtaned usng the entropy maxmzaton method. In the Fgure 5 (a), the effect of feed concentraton under the equlbrum temperatures was presented for two ntal temperatures (873.5 and K) for both mcroalgae s. In a general way was observed that n

6 558 all condtons the systems presented exothermc behavor, but for hgh ntal temperature ths trend was less pronounced. Smlar results were observed for the SCWG of glucose and cellulose n Fretas and Gurardello (202a). Fgure 5 (b) shows the effect off the ntal temperature under H 2 formaton for the two mcroalgae s analyzed. As can be seen, the ncrease n the ntal temperature results n an ncrease n the formaton of H 2. In a general way the equlbrum temperatures and H 2 formaton was hgher for SCWG of C. vulgars. Fgure 5. (a) Effect of feed concentraton under the equlbrum temperatures for SCWG of C. vulgars and Sprulna sp. at K (dashed lne) and K (sold lne) of ntal temperature; (b) Effect of ntal temperature under H 2 formaton for SCWG of C. vulgars and Sprulna sp. (5wt%). 4. Concluson The methodologes proposed n ths work showed to be relable for thermodynamc predctons n SCWG reactve systems for real bomass compounds. The predctons presented good agreement wth expermental and smulated data from lterature. The methodologes used and appled n the software GAMS 23.2., and solved wth the solver CONOPT proved to be quck and effectve n the resoluton of the proposed problems, wth computatonal tme nferor to s n all cases analyzed. Mcroalgae bomass proved to be a vable source for hydrogen producton. The syngas producton wth a H 2 /CO molar rato close to 2 was possble only wth co-reactant addtons. All reactons presented a slghtly exothermc behavor. Acknowledgements The authors gratefully acknowledge the fnancal support from FAPESP Fundação de amparo à pesqusa de estado de São Paulo (Process 20/ ) and CNPq Conselho Naconal de Desenvolvmento Centífco e Tecnológco, Brazl. References Brooke A., Kendrck D., Meeraus A., Raman, R., 998, GAMS- A User s Manual, In GAMS Development Corp., Washngton DC, USA. Castello D., For L., 20, Supercrtcal water gasfcaton of bomass: Thermodynamc constrants, Boresource Technology 02, Chaknala, A.G., Brlman D.W.F., van Swaaj W.P.M., Kersten S.R.A., 200, Catalytc and non-catalytc supercrtcal water gasfcaton of mcroalgae and glycerol, Ind. Eng. Chem. Res. 49, Fretas A.C.D., Gurardello R., 202a, Supercrtcal Water Gasfcaton of glucose and cellulose for hydrogen and syngas producton, Chemcal Engneerng Transactons 27, Fretas A.C.D., Gurardello R., 202b, Oxdatve reformng of methane for hydrogen and syngas producton: Thermodynamc Equlbrum analyss, Journal of Natural Gas Chemstry 2, Kruse A., Supercrtcal water gasfcaton, Bofuels, Boproducts & Borefnng 2, Ptzer K. S., Curl R.L., 957, The volumetrc and thermodynamc propertes of fluds. III. Emprcal equaton of the second vral coeffcent, J. Am. Chem. Soc. 79, Polng B.E., Prausntz J.M., O Connell P.J., 2000, The propertes of gases and lquds (5th ed.). McGraw Hll, New York, USA. Savage P. E., 2000, Heterogeneous catalyss n supercrtcal water, Catalyss Today 62, Tsonopoulos C., 974, An emprcal correlaton of second vral coeffcents, AIChE Journal 20,