A Technical and Environmental Evaluation of Six Routes for Industrial Hydrogen Production from Empty Palm Fruit Bunches.

Currently, the production of alternative fuels from renewable sources such as biomass has been increased in order to meet energy policies and reduce the environmental impacts of fossil fuels. This work is focused on hydrogen production from oil palm empty fruit bunches using different biomass gasification methods (direct gasification, indirect gasification, and supercritical water gasification) and purification technologies (selexol-based absorption and pressure swing adsorption). Six routes were selected based on these technologies and simulated using Aspen Plus software. Possible operating process improvements were suggested based on parametric sensitivity analysis by studying the effect of several variables on hydrogen production: gasification temperature, gasifying agent-to-biomass ratio, steam-to-carbon monoxide ratio, temperature of a high-temperature step reactor, and pressure in a hydrogen purification unit. The methodology of waste reduction algorithm was performed to assess the environmental impacts of each route. Results showed that hydrogen production was improved by increasing the gasification reaction temperature to 900 °C, oxygen-to-biomass ratio to 1.5, and pressure of purification stage to 10 atm for all routes. However, routes 1 and 2 presented a slight increase up to 0.7% in hydrogen yield using 1.5 mol O2/mol biomass. The environmental assessment revealed that routes 3 and 4 exhibited the lowest toxicological and atmospheric environmental impacts because of the use of char generated in the gasification reaction for energy production. These results indicated that route 4 exhibited the best performance for producing hydrogen from an environmental viewpoint.

Introduction

The fossil fuel depletion and emerging energy policies have led the development of novel alternatives for clean energy sources that face the global energy demand, which rose by 2.1% in 2017.[1,2] Currently, hydrogen is recognized as a cleaner energy source because of its zero-carbon emission.[3] The advantage of this fuel is assigned to its high energy density (143 MJ/kg) compared with fuels such as gasoline and methane.[4] Hydrogen also shows high energy efficiency, competing with other alternative energy sources including wind, solar, and geothermal energy.[5] It has a low cost of storage and transportation, and the rapid development of production technology facilitates its application.[6] Many efforts have been made for hydrogen production in a more affordable, reliable, and efficient way with minimum environmental impacts.[7] Technologies related to hydrogen production using renewable resources are currently under study in laboratory and pilot plant scales in order to improve technical efficiency and achieving sustainability in energy supply.[8] Residual biomass valorization to produce hydrogen is the most appealing route because of its abundance and cost-effectiveness.[9] These biomaterials are selected as a hydrogen feedstock according to their availability, cost, carbohydrate content, and biodegradability.[10] One of the main advantages of using residual biomass lies in facing environmental and economic problems associated to the uncontrolled discharging of municipal solid wastes.[11]

Residual biomass can be transformed into hydrogen using different processes classified into biochemical and thermochemical.[12] Gasification is recognized to be financially viable, achieving a high hydrogen yield and offering high flexibility in the use of different kinds of feedstock materials.[13,14] Biomass gasification reaction can be performed using methods such as direct, indirect, or supercritical water. Direct gasification is an autothermal process in which the gasifying agent may be composed of air or pure oxygen and the gasifier is internally heated through partial combustion.[15,16] Atnaw et al.[17] studied the production of syngas from oil palm fronds using autothermal gasification in order to evaluate the effect of the reactor temperature on gas composition, calorific value, and gasification efficiency. They obtained that an oxidation zone temperature above 850 °C is appropriate to produce a syngas with high concentration of fuel components. On the other hand, the indirect gasification process uses the unreacted char for producing heat that is transferred to the gasifier.[18] Mayerhofer et al.[19] used an indirect or allothermal gasifier to convert wood into syngas and varied the temperature (750–840 °C), steam-to-biomass (S/B) ratio (0.8–1.2), and pressure (0.1–0.25 MPa) in order to analyze its effects on tar content and gas composition, showing that an increase in temperature reduces the total tar content. The gasification in supercritical water is an emerging process for biomasses with high content of moisture, which has reported to show high conversion rates despite of its dependency with operation conditions, feedstock, and reactor design.[20,21] Sivasangar et al.[22] studied the production of hydrogen-rich gas from empty fruit bunches via supercritical water gasification and obtained improvements in hydrogen concentration with reaction time and biomass-to-water ratio.

Several agroindustrial residues have been employed for alternative fuels production; among these, oil palm residual biomass seems to be a promising source of renewable energy because of its high carbohydrate content.[23] African oil palm (Elaeis guineensis) is a perennial crop widely employed for vegetable oil production generating different residues (e.g., bagasse, shells, leaves, and empty fruit bunches) from plantation and milling activities.[24,25] To date, limited research in the open literature exists to assess the scaling-up of biomass conversion technologies in a complete topology for hydrogen production based on technical and environmental aspects. These knowledge gaps may be filled up by contributions focused on technical sensitivity analysis and environmental evaluation of hydrogen production. For the first time, a technical and environmental comparative study of different routes for hydrogen production from oil palm residual biomass is performed. To this end, three different gasification methods and two purification technologies were selected for a six-route arrangement. Such routes were evaluated from an environmental viewpoint using a WAR (waste reduction) algorithm. A parametric sensitivity analysis was also conducted to identify process variables most affecting process performance.

Materials and Methods

Process Description

Biomass gasification is a high-temperature-dependent process that converts biomass into small quantities of char and ash and gaseous or liquid fuel with high conversion rates.[26] The overall reaction for the gasification process is defined by reaction , where GH and LH refer to gaseous and liquid hydrocarbons, respectively.[27]

The gaseous product stream composition is mainly CO, H2, CO2, CH4, and H2O.[28] Separation of hydrogen from this gas is performed using technologies such as pressure swing adsorption (PSA) and absorption with selexol. The production of hydrogen from empty fruit bunches (EFBs) was performed via six routes considering different technologies for biomass gasification (direct gasification, indirect gasification, and supercritical water gasification) and hydrogen purification (selexol absorption and pressure swing adsorption), which were simulated using Aspen Plus software.

Hydrogen Production through Direct Gasification Coupled with Pressure Swing Adsorption (Route 1) or Selexol Absorption (Route 2)

Direct gasification technology uses air or pure oxygen as a gasifying agent, and part of the fuel is employed to supply the heat required for endothermic gasification reactions.[15] A fluidized bed reactor was selected to conduct the gasification of EFBs because of its feeding flexibility, good mixing properties, high conversions, and scalability.[29] The biomass gasification generates char that needs to be separated from the product gas; hence, a cyclone was assembled to retain such materials. The syngas is cooled and fed into a scrubber with water for further treatment. Then, a water gas shift (WGS) process is carried out in order to convert CO and water into CO2 and H2 using steam and can operate in two stages: high-temperature step (HTS) at 320–360 °C and low-temperature step (LTS) at 190–50 °C, enabling the production of an additional amount of H2 and better steam management.[30,31] A common exothermic reversible reaction during the WGS process is given by reaction .[32]

The remaining water in the shifted syngas is removed, and the hydrogen is purified via pressure swing adsorption or absorption with selexol. For route 1, a pressure swing adsorption (PSA) unit is employed for gas purification through an adsorption process onto the surface of porous materials such as activate carbon, zeolites, among others.[33] For route 2, gas separation is performed by bringing the gas in contact with a liquid solvent such as selexol in a scrubber column. This absorbent is a mixture of ether and poly(ethylene glycol) dimethyl ether commonly commercialized by big chemical companies.[34]Figure depicts the schematic representation of direct gasification coupled with both hydrogen purification technologies.

Figure 1

Block diagram of hydrogen production through route 1 or route 2.

Hydrogen Production through Indirect Gasification Coupled with Pressure Swing Adsorption (Route 3) or Selexol Absorption (Route 4)

The indirect biomass gasification consists of the importation of heat from outside the gasifier reactor by combustion of char as shown in Figure . This technology for biomass conversion is considered a second-generation system in comparison to the direct gasification because it faces the drawback of ashes and carbon accumulation inside the gasifier. The gasification reactions take place into a fluidized bed gasifier. The gasifier converts the EFBs into syngas at 870 °C and 1.6 bar. The unreacted char is combusted in the combustor to generate heat, which is transferred to the gasifier.[18] Purification of hydrogen is performed via the PSA unit or selexol absorption for route 3 or 4, respectively.

Figure 2

Block diagram of hydrogen production through route 3 or route 4.

Hydrogen Production through Supercritical Water Gasification Coupled with Pressure Swing Adsorption (Route 5) or Selexol Absorption (Route 6)

The water properties at supercritical conditions (374 °C and 220 bar) are characterized for a reduction in dielectric constant and thermal conductivity. Owing to its physicochemical properties, supercritical water is an acceptable reaction medium for biomass conversion.[35] According to Calzavara et al.,[36] a lower amount of char is generated in comparison to other conventional gasification technologies. As can be observed from Figure , the hydrogen production is increased by steam reforming of hydrocarbons in the syngas stream as methane. The overall reaction for the methane steam reforming process is described by reaction , which occurs at temperatures between 650 and 850 °C.[30]

Figure 3

Block diagram of hydrogen production through route 5 or route 6.

The type of purification technology also provides two topologies: the first one is the PSA unit (route 5), and the second one is the absorption with selexol (route 6).

Parametric Sensitivity Analysis

A parametric sensitivity analysis allows identifying the relationships between input variables and resulting outputs, which is useful for determining suitable operating conditions.[37] The information provided by this analysis can be characterized quantitatively, such as with a sensitivity metric or empirical derivative, or graphically.[38] In this work, different input variables (gasification temperature, gasifying agent-to-biomass ratio, steam-to-carbon monoxide ratio, temperature of an HTS reactor, and pressure in a hydrogen purification unit) were varied in order to evaluate its effects on the selected output variables (hydrogen production yield, hydrogen production, biomass conversion, and char production). To this end, operating conditions entered to the simulation were updated based on the parameters of sensitivity analysis. After running the simulation, mass flowrates of relevant streams such as hydrogen, char, and CO were compiled in an Excel spreadsheet. Finally, plots of output variables versus sensitivity parameters or input variables were built.

Environmental Analysis

The six routes for hydrogen production from empty fruit bunches were evaluated from an environmental viewpoint using the waste reduction algorithm. The potential environmental impacts (PEI) include several categories of impacts that are classified into atmospheric and toxicological.[39] The total output rate of PEI is defined by eq , where iout(cp), iout(ep), iwe(ep), and iwe(cp) are the rate of PEI leaving the system due to chemical interactions, the rate of PEI out of the system due to energy generation processes, and the rate of PEI out of the system as a result of the release of waste energy due to energy generation and chemical processes, respectively.[40]

The PEI generation rate is represented by eq , indicating the total potential environmental impacts generated within the system.

These output and generation PEI were also normalized to the production rate by eqs and 4.

Results and Discussion

Process Simulation

The simulation schemes for hydrogen production from EFBs via six different routes involve process equipment and streams of raw material, products, and byproducts. The feedstock derived from palm oil industry is represented by EFB stream. The solid wastes are composed of ashes and char formed during gasification reactions, which are labeled as ASH stream. The stream properties were estimated by nonrandom two liquid (NRTL) and UNIFAC thermodynamic models. The chemical properties of hydrogen provided by Aspen Plus software were compared to those reported by Emsley et al.[41] and are summarized in Table .

Table 1

Chemical Properties of Hydrogen Produced from Oil Palm-Derived Biomass Provided by Aspen Plus Software

property	this work	Emsley[41]	accuracy (%)
heat capacity (kJ/(kg·K))	14.1	14.304	98.57
enthalpy of vaporization (kJ/mol)	898.5	900	99.83
thermal conductivity (W/(K·m))	0.1724	0.1815	94.99
molar volume (m3/mol)	24.06	22.42	92.69

The main process stages are pointed out as follows: gasification (green frame), WGS (orange frame), PSA (blue frame), and selexol absorption (gray frame). Operating conditions of process streams for routes 1–6 are summarized in the Appendix section. Figure shows the hydrogen production from EFBs using route 1, which consist of gasifiers (GASF1 and GASF2), water gas shift reactors (HTS and LTS), a scrubber (CLEANER), and a pressure swing adsorption unit (PSA). The oil palm biomass (EFB) was milled and dried before feeding into the gasifiers in order to reduce its particle size and remove excess water. In addition, the water gas shift reactors were operated in an equilibrium model. As shown in Figure , the gaseous product stream (S6) is sent to a cyclone (SEP), in which ashes are removed and the gaseous stream is fed into the scrubber (CLEANER). Then, the syngas reacts with steam inside the SWG reactors (HTS and LTS) producing a H2-rich stream that is purified in the absorption column after removing excess water (SEP3). A desorption step after hydrogen purification was required in order to separate carbon dioxide from the selexol stream. Hence, the rich solvent is heated and fed into a regeneration column (SEP2), which produces separate streams with CO2 and the solvent that is sent back to the column.[33] The solvent stream is labeled as S5, while reboiler for desorption is represented as E6.

Figure 4

Simulation flowsheet for route 1.

Figure 5

Simulation flowsheet for route 2.

Figure shows the simulation flowsheet of the hydrogen production process through route 3, in which steam (STEAM1) is sent to the gasifier (GASF2) for gasification endothermic reaction. The unreacted char (S7) passes through the combustor (COMBST) to be oxidized with air (AIR), and the combustion heat is sent back to the gasifier. Figure shows a similar simulation for route 4; however, the purification technology is selexol absorption instead of PSA.

Figure 6

Simulation flowsheet for route 3.

Figure 7

Simulation flowsheet for route 4.

As shown in Figure , supercritical steam gasification does not require a drying stage; hence, the biomass (EFB) is directly fed into a mill (MILL) to reduce its particle size because the gas yield increases when feedstock particle size decreases.[27] The milled stream (S1) is sent to heat exchangers (E1 and E1-1) for heating until reaching the inlet temperature of the gasifier. This route uses PSA technology and two streams leaving from the adsorption column (HYDROGEN and SUBP). The simulation flowsheet of hydrogen production via supercritical water gasification and purification with selexol absorption is shown in Figure . The steam leaving the scrubber (SEP1) is fed into a steam reformer (REF) to convert methane from the syngas stream into hydrogen and carbon monoxide by reacting with steam.

Figure 8

Simulation flowsheet for route 5.

Figure 9

Simulation flowsheet for route 6.

Parametric Sensitivity Analysis

Gasification Process

Effect of Gasification Reaction Temperature

The temperature is recognized as one of the most significant operating parameters on the biomass conversion throughout gasification reactions.[42] For fluidized bed gasifiers, the limited range of temperature is between 550 and 900 °C due to the presence of high volatile matter content in biomasses.[43]Figure shows the hydrogen production under three different reaction temperatures during the gasification process (500, 700, and 900 °C). As can be observed, the highest hydrogen production was achieved using supercritical water gasification and purification with PSA technology (route 5) due to the selectivity toward hydrogen production under suitable operating conditions of gasification temperature and pressure.[22] It was found that an increment in hydrogen production as temperature increased because of the thermal cracking of molecular hydrogen on a compound structure.[44] Similar results were reported by Mohammed et al.,[27] who studied the gasification of empty fruit bunches using a fluidized bed reactor at a temperature range of 700–100 °C and obtained an increase in hydrogen yield from 10.8 to 38.2%.

Figure 10

Effect of gasification reaction temperature on hydrogen production.

The biomass gasification techniques also limit the hydrogen yield; hence, gasification temperature was varied between 500 and 1100 °C using direct, indirect, and supercritical water gasification. As shown in Figure , the supercritical water gasification exhibited the highest biomass conversion as temperature increased. Lachos-Perez et al.[21] pointed out that biomass in a supercritical environment favors the rapid fractionation of hemicellulose and cellulose due to the properties of water at a high temperature and pressure. Sivasangar et al.[22] obtained conversions around 50% of EFBs into hydrogen using a supercritical water reactor at 240 bar.

Figure 11

Effect of temperature and gasification technique on biomass conversion.

Figure shows the influence of temperature on char formation after the gasification reaction using the selected biomass gasification methods at 10 atm. As can be observed, supercritical water gasification (SWG) reported the lowest mass flowrate of char, which was expected according to the information found in the literature related to the advantages of this technique. Changsuwan et al.[45] reported that supercritical water gasification can reach high gasification efficiency because of the reduction of char formation and the improvement of selectivity for H2.

Figure 12

Effect of gasification reaction temperature on char production.

Effect of Gasifying Agent-to-Biomass Ratio

The gasifying agent plays an important role in the syngas generation process because it affects the gas quality, hydrogen content, and heating value.[46] Different gasifying agents have been employed for biomass gasification such as air, oxygen, and steam. The gasifying agent-to-biomass ratio was varied in 10, 30, and 50% in order to evaluate its influence on hydrogen generation. As shown in Figure , supercritical water achieved the highest hydrogen yield because of the changes in ionic product, polarity, and electrical conductivity of water at a high pressure and temperature.[21] Steam also reported better results compared to air, which is attributed to the absence of nitrogen leading to a shift in chemical equilibrium of the gasification reaction.[47] In addition, it was found that hydrogen production was not enhanced by increasing the oxygen-to-biomass ratio in the direct gasification process. Ogu et al.[26] reported an increase in biomass conversion while the gasifying agent (steam) amount increased for indirect gasification. Sivasangar et al.[22] also pointed out an enhancement in hydrogen production from oil palm wastes as the supercritical water-to-biomass ratio increased.

Figure 13

Effect of gasifying agent-to-biomass ratio on hydrogen production.

Water Gas Shift Process

Effect of Steam-to-Carbon Monoxide Ratio

Steam amount in water gas shift reactions constitutes a relevant parameter for hydrogen production because side reactions may occur when the steam-to-CO ratio is low, generating undesirable products such as carbon and methane.[48] The conversion of CO into CO2 and H2 by reacting with water and hydrogen generation were considered as output variables. As can be observed from Figure , the variation of this ratio from 0.5 to 2.5 mol H2O/mol CO caused an increase in both output variables, enhancing hydrogen formation in the WGS reaction. Similar results were obtained by Lang et al.,[49] who reported that hydrogen production increased from 21 to 34% while varying the steam-to-CO ratio between 1.45 and 2.0 in a water gas shift reactor in the presence of a catalyst.

Figure 14

Effect of steam-to-carbon monoxide ratio on CO conversion and H2 generation.

Effect of WGS Reaction Temperature

The temperature is a key parameter for water gas shift reaction performance. Such parameter was varied between 195 and 495 °C in the high-temperature water gas shift reactor in order to analyze its influence on carbon monoxide conversion. The reaction pressure and steam-to-carbon monoxide ratio were fixed at 1 atm and 1.5 mol H2O/mol CO, respectively. Figure shows the performance of carbon monoxide conversion and hydrogen formation as the WGS reaction temperature increased. It is well known that the second stage of the water gas shift process (high temperature) gives a high reaction rate but a low conversion.[50] Hence, the decrease in hydrogen formation was expected.

Figure 15

Effect of WGS reaction temperature on CO conversion and H2 generation.

Hydrogen Purification Process

Effect of Pressure

Figure shows the influence of operating pressure on hydrogen purification using PSA technology. The pressure was varied between 1 and 10 atm, and the temperature was fixed at 40 °C. The hydrogen purity exhibited slight differences for routes 1, 3, and 5, attributed to the inlet concentration of stream in adsorption columns. Yang et al.[51] studied the hydrogen purification using a PSA unit under different operating conditions and found that the highest purity (96–99%) was achieved at 6–10 atm, similar to the results reported in this work.

Figure 16

Effect of pressure on hydrogen purity using PSA technology.

This parametric sensitivity analysis in the hydrogen purification process was also performed considering the selexol absorption technology. As can be observed from Figure , the highest carbon dioxide uptake was achieved in a pressure range of 50–60 bar. Marroig[52] performed an analysis of natural gas purification using selexol and obtained a reduction in CO2 concentration from 80 to 5 ppm, which indicated that solvents are efficient for removing undesirable gases.

Figure 17

Effect of pressure on hydrogen purity using selexol absorption technology.

Table summarizes an analysis of process improvements derived from simulation results during the parametric sensitivity study by varying specific operating conditions such as the gasification reaction temperature, oxygen-to-biomass ratio, steam-to-carbon monoxide ratio, WGS reaction temperature, and pressure of purification columns.

Table 2

Proposed Improvements for Hydrogen Production Using Parametric Sensitivity Analysis Results

route	product stream (kg/h)	operating condition modification	improvement
route 1	2316.0	to increase the gasification reaction temperature from 700 to 900 °C.	increase in hydrogen production up to 27%
		to increase the oxygen-to-biomass ratio from 10 to 50%	increase in hydrogen production up to 0.7%
		to increase the steam-to-carbon monoxide ratio from 0.5 to 1.5	increase in CO conversion from 48 to 95%
		to increase the WGS reaction temperature from 295 to 395 °C	decrease in hydrogen production from 51 to 48%
		to increase the pressure of adsorption columns from 5 to 10 atm.	increase in hydrogen purity from 96 to 99%
route 2	2313.7	to increase the pressure of selexol absorption column from 40 to 60 bar	increase in hydrogen purity from 94 to 98%
route 3	2667.42	to increase the gasification reaction temperature from 700 to 900 °C	increase in hydrogen production up to 26%
		to increase the oxygen-to-biomass ratio from 10 to 50%	increase in hydrogen production up to 45%
		to increase the steam-to-carbon monoxide ratio from 0.5 to 1.5	increase in CO conversion from 48 to 95%
		to increase the WGS reaction temperature from 295 to 395 °C	decrease in hydrogen production from 51 to 48%
		to increase the pressure of adsorption columns from 5 to 10 atm	increase in hydrogen purity from 96 to 99%
route 4	2667.42	to increase the pressure of selexol absorption column from 40 to 60 bar	increase in hydrogen purity from 94 to 98%
Route 5	2816.13	to increase the gasification reaction temperature from 700 to 900 °C	increase in hydrogen production up to 75%
		to increase the oxygen-to-biomass ratio from 10 to 50%	increase in hydrogen production up to 39%
		to increase the steam-to-carbon monoxide ratio from 0.5 to 1.5	increase in CO conversion from 48 to 95%
		to increase the WGS reaction temperature from 295 to 395 °C	decrease in hydrogen production from 51 to 48%
		to increase the pressure of adsorption columns from 5 to 10 atm	increase in hydrogen purity from 96 to 99%
route 6	2813.31	to increase the pressure of selexol absorption column from 40 to 60 bar	increase in hydrogen purity from 94 to 98%

Computer-Aided Environmental Evaluation

Total Output and Generation of Potential Environmental Impacts

The PEI output index for each of the routes is shown in Figure . Natural gas was selected as the fuel for suppling energy requirements in the hydrogen production process. As can be observed, the routes that exhibited the highest PEI output and generation were routes 1 and 2, attributed to the huge amount of char generated in direct gasification compared to other gasification methods. Environmental impacts of char or carbon corresponds to breathing diseases such as lung cancer.[53] The PEI values are positive for each gasification process due to the use of environmentally friendly feedstocks. Susmozas et al.[54] reported that the impacts of biomass gasification are higher than the gas reforming process.

Figure 18

Potential environmental impacts output and generation rates per kilogram of hydrogen produced in each route evaluated.

Figure depicts global results per PEI/kg and PEI/h in order to study the influence of product mass flowrate on environmental performance. Routes 5 and 6 also reported high PEI values, suggesting that supercritical water gasification increases the environmental impacts of hydrogen production because of the energy consumption to achieve supercritical conditions. Routes 3 and 4 presented the lowest PEI because of the use of char for generating energy, which reduces the requirements of utilities.

Figure 19

Comparison of global potential environmental impacts per units of time and units of product rate for each route evaluated.

The PEI categories considered by the WAR algorithm are divided into two groups: toxicological and atmospheric impacts. The toxicological categories are as follows: human toxicity potential by ingestion (HTPI), human toxicity potential by exposure (HTPE), aquatic toxicity potential (ATP), and terrestrial toxicity potential (TTP). The atmospheric categories are as follows: global warming potential (GWP), ozone depletion potential (ODP), smog formation potential (PCOP), and acidification potential (AP).[55]Figure shows the toxicological impacts for each route. The HTPI category exhibited the highest PEI value, which is attributed to the char generated in gasification reactions. Routes 3 and 4 showed the lowest toxicological impacts due to the use of the waste of gasification to generate energy.

Figure 20

Toxicological potential environmental output impacts per kilogram of hydrogen produced for each route evaluated.

The effect of energy consumption for each biomass gasification route on toxicological environmental impacts is shown in Figure . As can be observed, the PEI values increased as energy consumption increased. Hence, it was expected that route 6 presented the highest environmental impacts.

Figure 21

Toxicological potential environmental impacts associated to energy consumption for each route evaluated.

The atmospheric potential environmental impacts are shown in Figure , which allows identifying the categories that most contribute to each route. The acidification potential exhibited the highest PEI values because of the presence of sulfur on fossil fuels used for satisfying energy requirements. Route 3 presented the lowest atmospheric impacts because of the low energy consumption and the alternative use of char.

Figure 22

Potential atmospheric impacts per product per kilogram of hydrogen produced for each route evaluated.

Figure shows the output and generation PEI index for each atmospheric category using the six routes for hydrogen production. Route 2 reported a high output and generation PEI value for the acidification potential followed by route 1, which is attributed to the high consumption of fuels for biomass gasification.

Figure 23

Comparison of generation and output PEI for each atmospheric category.

Effect of Energy Source

Different fossil fuels were employed to analyze its influence on the potential environmental impacts. As shown in Figure , the highest PEI values were reached using carbon. Similar results were reported by Herrera-Aristizábal,[39] obtaining an increase in environmental impacts using carbon instead of oil and gas.

Figure 24

Effect of energy source on total PEI per kilogram of hydrogen produced for each route evaluated.

Conclusions

This work attempted to evaluate the hydrogen production from oil palm residual biomass via six routes based on different gasification methods and product purification technologies. The parametric sensitivity analysis allowed identifying operating modifications in order to improve hydrogen production. An increase in the gasification reaction temperature from 700 to 900 °C enhanced the hydrogen production up to 27%. The steam-to-carbon monoxide ratio also played a key role in CO conversion. The water gas shift reaction temperature was increased from 295 to 395 °C, affecting the hydrogen production from 51 to 48%. In addition, routes 3 and 4 exhibited the lowest potential environmental impacts due to the combustion of char for providing heat from outside the gasifier reactor. The PEI associated to energy consumption corresponded to routes 5 and 6, attributed to the energy requirements to achieve supercritical conditions. Route 4 was selected as the most suitable from an environmental viewpoint because of the low environmental impact’s generation.

Table A1

Operating Conditions of Process Streams for Route 1

Table A2

Operating Conditions of Process Streams for Route 2

Table A3

Operating Conditions of Process Streams for Route 3

Table A4

Operating Conditions of Process Streams for Route 4

Table A5

Operating Conditions of Process Streams for Route 5

Table A6

Operating Conditions of Process Streams for Route 6