Inherent Safety Assessment of Indirect Gasification of Oil Palm Empty Fruit Bunches for Hydrogen Production and Purification by Pressure Swing Adsorption (PSA)

Abstract

Oil palm empty fruit bunches (EFB) are lignocellulosic residues generated in large quantities by the palm oil industry and represent a potential biomass resource for renewable hydrogen production. This study evaluated the inherent safety of an indirect gasification process of empty fruit bunches (EFB) for hydrogen production, coupled with purification via pressure swing adsorption (PSA), using the Inherent Safety Index (ISI) methodology. The assessment considered critical operating variables, including temperature and pressure, and integrated chemical sub-indices (toxicity, flammability, chemical interaction, and corrosivity) with process sub-indices (inventory, temperature, pressure, equipment, and process structure). The analysis yielded a total ISI of 36, exceeding the reference value of 24 used in the original ISI framework and indicating an unfavorable inherent-safety profile under the evaluated conceptual design conditions. The main hazard drivers were the combined flammability, explosiveness, and toxicity contribution governed by CO; the high individual explosiveness of H₂; the toxicity of SO₂; the potential formation of explosive mixtures following air ingress; and the relatively large process inventory. The indirect gasification section, reaching a maximum verified temperature of 900 °C and a maximum operating pressure of 60 bar, governed the temperature and pressure sub-indices, respectively. These findings identify significant inherent-safety challenges that should be addressed during subsequent process development. In particular, inventory minimization, prevention of air ingress, and evaluation of less severe temperature and pressure conditions where technically feasible represent relevant priorities for inherently safer design before industrial-scale implementation.

1. Introduction

The rapid depletion of fossil fuel reserves, combined with the implementation of global energy and environmental policies, has intensified the search for alternative and renewable energy sources capable of reducing greenhouse gas emissions and mitigating climate change. In this context, hydrogen has emerged as one of the most promising energy vectors of the 21st century, owing to its exceptionally high energy density and the fact that its combustion produces only water as a byproduct, making it a genuinely clean fuel. Unlike conventional hydrocarbons, hydrogen does not contribute directly to CO₂ emissions during its use, which positions it as a strategic pillar in the transition toward low-carbon energy systems [1].

Among the various pathways available for hydrogen production, those derived from residual biomass have attracted considerable scientific and industrial interest, particularly in regions with significant agro-industrial activity. Empty fruit bunches (EFB) from oil palm represent a lignocellulosic residue generated in large quantities as a byproduct of palm oil extraction. These residues possess a high carbohydrate content, including cellulose and hemicellulose, which makes them well-suited for thermochemical conversion processes [2]. In producing countries, the valorization of this type of agricultural waste not only contributes to the circular economy but also offers a strategic opportunity to develop locally sourced, renewable hydrogen supply chains [3].

Indirect gasification physically separates the gasification and combustion environments. In configurations based on circulating bed material, char generated during gasification is combusted separately, and the resulting thermal energy is transferred to the endothermic gasification stage through the circulating solid medium. Consequently, the thermal conditions of the gasifier and combustion section must be considered as coupled through an overall energy balance rather than interpreted as independent temperature specifications. In the process evaluated in this work, the gasification stage is represented at 900 °C, consistent with the underlying conceptual simulation. Detailed thermal design of the circulating-solid heat-transfer loop, including determination of the specific combustor temperature, is outside the scope of the present conceptual inherent-safety assessment [4].

Beyond the specific configuration of indirect gasification, the broader body of research on catalytic biomass gasification for hydrogen production has expanded considerably in recent years. Rubinsin et al. [5] conducted a comprehensive review of the factors governing catalytic biomass gasification, encompassing catalyst selection (alkaline earth metals, transition metal oxides, and natural minerals such as dolomite and olivine), reactor configuration (fixed-bed versus fluidized-bed systems), and operating parameters including temperature, equivalence ratio, catalyst loading, and biomass blending ratio. Their analysis confirmed that hydrogen yield and tar reduction are strongly interdependent on these variables, with optimal catalyst loadings typically below 20–30 wt% and gasification temperatures between 600 and 900 °C representing the most favorable operating windows for hydrogen-rich syngas production. The review further incorporated a techno-economic dimension, comparing capital and operating costs across fluidized-bed, entrained-flow, and dual fluidized-bed gasification schemes with and without carbon capture. Notably, while these reviews provide an exhaustive treatment of process performance, catalyst behavior, and economic feasibility, none of them incorporate a systematic evaluation of inherent safety indicators, leaving a persistent gap between process optimization studies and safety-oriented process design for biomass-based hydrogen production routes.

Once the syngas is produced, achieving industrial-grade hydrogen purity requires subsequent purification steps, since the raw gas stream contains significant fractions of CO₂, CH₄, CO, SO₂, and other impurities that must be removed before the hydrogen can be used in high-value applications such as fuel cells or chemical synthesis [6]. PSA is one of the most widely employed technologies for this purpose. PSA operates through cyclic pressurization and depressurization sequences, using highly selective solid adsorbents such as zeolites and activated carbon to separate hydrogen from the remaining gas components [7]. In the present conceptual model, the PSA feed is specified at approximately 45 bar after pressure reduction from the upstream high-pressure process sections. The result is a high-purity hydrogen stream, often exceeding 99.9% purity, suitable for demanding industrial and energy applications.

However, the combination of high-temperature gasification and high-pressure purification in a single integrated process introduces a complex safety landscape that must be rigorously evaluated from the earliest stages of design. The presence of toxic compounds such as CO and SO₂, highly flammable gases including H₂, CH₄, and CO, and the risk of explosive mixture formation in the event of air ingress, represent significant chemical hazards that must be identified and quantified systematically. Furthermore, the large mass flows involved in an industrial-scale plant, along with the extreme thermal conditions in the reactor stages, amplify the potential consequences of any process failure.

The Inherent Safety Index (ISI) provides a structured and quantitative methodology to evaluate both chemical and process-related intrinsic hazard, integrating sub-indices for toxicity, flammability, chemical interaction, temperature, pressure, inventory, and equipment into a single comparative metric. In this framework, the assessment of inherent safety has become an indispensable tool for early-stage process decision-making. Unlike conventional safety approaches that address risks through protective layers added after the design is finalized, inherent safety principles seek to eliminate or reduce hazards at their source by modifying process chemistry, operating conditions, or equipment configuration before detailed engineering begins [8]. A precedent for the application of inherent safety assessment to biomass-based hydrogen production was reported by Meramo-Hurtado et al. [9], who applied the ISI and the Process Route Index (PRI) to a direct biomass gasification route (cassava and rice residues) coupled with membrane-based H₂ purification, reporting an ISI of 42 and a PRI of 2.01 and identifying flammable and toxic syngas components as the dominant risk drivers. While this study confirms the value of ISI/PRI as early-stage safety diagnostic tools for hydrogen production via gasification, it also reveals a gap in the literature: no comparable inherent-safety assessment has been reported for indirect gasification schemes integrated with PSA purification, despite these configurations involving distinct thermal risks (dual-temperature reactor/combustor operation) and pressure-cycling hazards absent from direct, atmospheric-pressure topologies.

A more methodologically advanced treatment of inherent safety in hydrogen production was recently presented by Gao et al. [10], who developed a mechanistic model for the Hydrogen Production Process through Natural Gas Reforming (HPPtNGR). Their approach extended the classical ISI framework by incorporating fuzzy set theory to normalize inherent safety indices across process units of unequal size and complexity, the Analytic Hierarchy Process (AHP) to derive unit-specific weighting factors, and equation-based Bayesian networks to reconcile chemical and process hazard indicators into a single, comparable inherent-safeness score. Applying this framework to two alternative natural gas reforming configurations, the authors demonstrated that process intensification (fewer processing units) does not necessarily translate into improved inherent safety when it is achieved at the expense of harsher operating conditions, such as higher reaction temperatures and compression ratios. This finding underscores a critical methodological limitation of the conventional ISI approach when applied in isolation: raw index sums are not directly comparable across process alternatives with differing numbers of units, a limitation that fuzzy normalization and Bayesian aggregation effectively resolve. However, this refined methodology has, to date, only been demonstrated for hydrocarbon-reforming-based hydrogen production and has not been extended to biomass gasification routes, nor to configurations involving indirect gasification coupled with PSA purification.

Taken together, the reviewed literature reveals three complementary but disconnected lines of research: (i) extensive technical and economic optimization of catalytic biomass gasification for hydrogen production [5], (ii) inherent safety evaluation of direct biomass gasification routes using the conventional ISI and PRI indices, limited to single-value, non-normalized scoring [9], and (iii) methodologically advanced, fuzzy-normalized and Bayesian-reconciled inherent safety assessment, but applied exclusively to fossil-based (natural gas reforming) hydrogen production [10]. No study to date has applied a rigorous inherent safety assessment—whether through the conventional ISI/PRI approach or through more advanced normalization techniques—to an indirect biomass gasification route integrated with PSA purification, despite this configuration presenting distinctive risk profiles that are not captured by direct, atmospheric-pressure gasification topologies. The present study addresses this gap by applying the ISI methodology to a simulated indirect gasification process of palm EFB coupled with PSA-based hydrogen purification, thereby providing, to the best of the authors’ knowledge, the first systematic inherent-safety evaluation of this specific process configuration. This contribution is intended not only to identify the critical risk-driving variables (temperature and pressure) at the conceptual design stage but also to establish a technical basis upon which future work may extend fuzzy-based normalization and multi-criteria reconciliation techniques—following the precedent set by Gao et al.—to biomass-derived hydrogen production routes, an integration that remains unexplored in the current literature [10].

2. Materials and Methods

2.1. Process Description

Figure 1 shows the flow diagram for hydrogen production from EFB of the oil palm via the indirect gasification route and PSA purification. The process is divided into three main stages: biomass pretreatment, gasification, and synthesis gas purification.

The first stage is pretreatment, during which EFB are fed into the system at a rate of 41,068 kg/h. The biomass undergoes a drying process to reduce its moisture content before entering the reactor. Subsequently, the dry biomass is fed into the indirect gasification system. Unlike direct gasification, this system consists of two separate chambers: a gasifier and a combustor. The dehydrated EFB is fed to the indirect gasification train, which is represented in the process simulation by equilibrium-based gasification reactors operating at a maximum temperature of 900 °C and 60 bar. Steam is used as the gasifying agent. At the conceptual-model level, the gasification steam is specified at 60 bar at the point of injection, consistent with the gasifier operating pressure. In an actual installation, the upstream steam-supply pressure would need to be slightly higher to overcome pressure losses through the piping and injection system; these equipment-specific losses are outside the scope of the present conceptual design.

The resulting syngas subsequently leaves the gasification section at a lower temperature before solid separation and downstream conditioning. As a shown in Figure 2 the process configuration also includes a char-combustion section associated with the indirect supply of heat to gasification. A detailed determination of the combustor temperature, circulating-solid flow rate, and associated temperature driving force requires an equipment-level energy balance and is reserved for subsequent process-design stages.

The resulting syngas stream exits the cyclone separator (S8 = 23,309.6 kg/h) with the following composition: CO = 4.13%, H₂O = 33.59%, H₂ = 2.76%, CH₄ = 11.92%, CO₂ = 45.97% and SO₂ = 1.64%. After cooling, the conditioned syngas is directed to the water–gas shift (WGS) section. Additional water/steam is supplied to provide the reactant required for the WGS reaction and to establish the required steam-to-CO ratio. The stream-wise mass balance was explicitly verified across the main conditioning and purification steps. The raw syngas (S8, 23,309.60 kg/h) and scrubbing water (35,000.00 kg/h) give a total inlet flow of 58,309.60 kg/h to the cleaning section, which is exactly balanced by S10 (56,703.75 kg/h) and S11/S17 (1605.85 kg/h). The gas stream S11/S17 subsequently combines with 24,883.90 kg/h of steam, yielding 26,489.75 kg/h at the WGS section. After WGS, the 26,489.75 kg/h stream is separated into a water-rich stream S26 (25,254.46 kg/h) and the PSA feed S18 (1235.29 kg/h). Finally, the PSA feed is divided into 690.07 kg/h of hydrogen product and 545.22 kg/h of tail gas, closing the total mass balance at each of the reported process boundaries. Prior to entering the high-temperature shift reactor, the combined stream is thermally conditioned to the HTS operating range of 320–360 °C. The HTS stage is followed by a low-temperature shift stage operating at 190–250 °C, thereby maximizing the yield of H₂. After passing through both reactors, the shifted gas stream (S15 = 26,489.75 kg/h) shows a notably different composition: CO = 0.01%, H₂O = 91.61%, H₂ = 2.69% and CO₂ = 5.70%, reflecting both the transformation of CO and concurrent methane reforming reactions. Following the WGS stages, water knockout removes the water-rich fraction from the shifted gas before hydrogen purification. Consequently, the gas flow decreases from 26,489.75 kg/h at S15 to 1235.29 kg/h at the PSA feed (S18), which contains CO = 0.23%, H₂O = 0.85%, H₂ = 56.92% and CO₂ = 42.01%.

The downstream process operates at progressively lower pressures than the gasification section. After syngas conditioning and the WGS stages, the gas pressure is reduced from the upstream high-pressure conditions to approximately 45 bar before entering the PSA unit. Therefore, the transition to PSA involves pressure reduction rather than additional syngas compression. At the conceptual-design level considered in this study, the specific pressure-control equipment and associated line pressure drops were not dimensioned; the reported pressures correspond to the operating pressures adopted for the main process sections. The PSA unit was represented by an idealized component-separation block at the conceptual-design level, producing a hydrogen-rich product stream of 690.07 kg/h with an imposed hydrogen recovery of approximately 98%. This representation is intended to close the conceptual material balance and define the composition of the product and tail-gas streams; it does not reproduce the cyclic adsorption/desorption dynamics, bed sizing, adsorbent behavior, or detailed pressure transients of an industrial PSA unit. Consequently, the present ISI assessment does not draw equipment-level safety conclusions that depend on a detailed dynamic PSA model. The governing process pressure remains the 60 bar gasification section rather than the PSA section.

The EFB handling streams S1, S2, S3, and S39 remain at 41,067.76 kg/h and are therefore reported separately from the selected gas, liquid, and solid process streams summarized in

Table 1. Component mass fraction (wt.%) of syngas along the process, from gasification to PSA purification. Small deviations in the summation of mass fractions and component mass flows may occur due to rounding.

Stream Name	Mass Flows (kg/h)	H2%	CO2%	C%	O2%	CO%	CH4%	H2O%	SO2%	ASH%	N2%
STEAM1	4106	0	0	0	0	0	0	100	0	0	0
S36/S6	45,173.76	1.42	23.72	44.24	0	2.13	6.15	17.33	0.84	4.16	0
S7	21,864.16	0	0	91.40	0	0	0	0	0	8.60	0
AIR	120,000	0	0	0	23.29	0	0	0	0	0	76.71
EXAHUST	141,864.16	0	27.10	6.69	0	0	0	0	0	1.33	64.89
S8/S9	23,309.6	2.76	45.97	0	0	4.13	11.92	33.59	1.64	0	0
WATER	35,000	0	0	0	0	0	0	100	0	0	0
S10	56,703.75	0	18.90	0	0	0	4.90	75.53	0.67	0	0
S11/S17	1605.85	40.0	0	0	0	60.0	0	0	0	0	0
STEAM	24,883.9	0	0	0	0	0	0	100	0	0	0
S13/S14	26,489.75	2.68	5.60	0	0	0.07	0	91.64	0	0	0
S15/S16	26,489.75	2.69	5.7	0	0	0.01	0	91.61	0	0	0
S26	25,254.46	0.03	3.92	0	0	0	0	96.05	0	0	0
S18	1235.29	56.92	42.01	0	0	0.23	0	0.85	0	0	0
HYDROGEN	690.07	100	0	0	0	0	0	0	0	0	0
SUBP	545.22	2.39	95.18	0	0	0.52	0	1.92	0	0	0

.

To facilitate the understanding of the methodology adopted in this work, Figure 3 summarizes the workflow followed for the inherent safety assessment. The hydrogen production process by indirect gasification coupled with PSA purification was adopted from the previously reported literature.

Based on the operating conditions and process information extracted from this reference, the chemical and process safety sub-indices were determined according to the ISI methodology. Subsequently, the overall ISI was calculated to classify the process according to its inherent safety level and to identify opportunities for intrinsic hazard reduction through safer design strategies.

2.2. Inherent Safety Index Evaluation

The Inherent Safety Index (ISI) is applied in this work as an early-stage, semi-quantitative measure of the intrinsic hazards associated with the chemical substances, operating conditions, equipment characteristics, inventory, and process structure [11]. The method is intended for conceptual/preliminary design, as the information required for detailed probabilistic risk analysis is not yet available. The total index is obtained from the Chemical Inherent Safety Index (I_Ch) and the Process Inherent Safety Index (I_Ps), as expressed in Equation (1):

$${\mathrm{I}}_{\mathrm{S}\mathrm{I}}={\mathrm{I}}_{\mathrm{P}\mathrm{s}}+{\mathrm{I}}_{\mathrm{C}\mathrm{h}}$$

(1)

The first part is the Chemical Index (I_Ch). Here, the flammability or toxicity of each substance is assessed. For example, this process involves H₂, which is highly flammable, and CO, which is toxic. The second part is the Process Safety Index (I_Ps), which rates the physical hazard contribution of the operation, such as high pressures, very high temperatures, and the type of equipment needed at the plant [11].

2.3. Process Inherent Safety Index

The Process Inherent Safety Index allows for the assessment of a plant’s intrinsic hazard level based on the performance of its equipment and the physical conditions of the operation. This value is obtained by summing five specific factors, as shown in Equation (2):

$${\mathrm{I}}_{\mathrm{P}\mathrm{s}}={\mathrm{I}}_{\mathrm{I}}+{\mathrm{I}}_{\mathrm{T},\mathrm{m}\mathrm{a}\mathrm{x}}+{\mathrm{I}}_{\mathrm{p},\mathrm{m}\mathrm{a}\mathrm{x}}+{\mathrm{I}}_{\mathrm{E}\mathrm{Q},\mathrm{m}\mathrm{a}\mathrm{x}}+{\mathrm{I}}_{\mathrm{S}\mathrm{T},\mathrm{m}\mathrm{a}\mathrm{x}}$$

(2)

Within this framework, the inventory sub-index (I_I) quantifies the intrinsic hazard associated with the tonnage of raw materials and products present in the system, given that handling large volumes of hazardous substances increases the likelihood of incidents. Meanwhile, the I_T,max and I_P,max factors assign a hazard score based on the highest recorded temperature and pressure values, which is critical in this process given that the gasifier operates at around 900 °C and 60 bar pressure. Likewise, the I_EQ,max factor evaluates the intrinsic hazard of each type of equipment used, while the I_ST,max incorporates the safety history and stability of the process structure based on similar industrial operations [11].

Standard numerical scales are used to assign these scores, which evaluate the plant’s capacity in two main sections. The first section, known as “within battery limits” (ISBL), includes all equipment on the main production line; the second, known as “outside battery limits” (OSBL), encompasses the support systems and industrial services necessary for overall operations.

Table 2. Determination of the inventory subindex.

Inventory ISBL	Inventory OSBL	Score
0–1 t	0–10 t	0
1–10 t	10–100 t	1
10–50 t	100–500 t	2
50–200 t	500–2000 t	3
200–500 t	2000–5000 t	4
500–1000 t	5000–10,000 t	5

presents the scores assigned based on the plant’s inventory or operational capacity [11].

At the conceptual design stage, detailed mechanical information, such as vessel dimensions, operating liquid levels, bed voidages, and equipment-specific physical hold-ups, is not yet available. This limitation is explicitly contemplated in the original ISI methodology, which was developed for preliminary process design using the information normally available at this stage. Following Heikkilä, the ISBL inventory was therefore estimated from the representative mass flow associated with each major process vessel and a nominal residence time of one hour [11]. Recycle streams were included where applicable, as established in the original method, and the total ISBL inventory was obtained as the sum of the inventories assigned to the individual process vessels. Therefore, this calculation represents a nominal ISI equipment-inventory estimate rather than a detailed mechanical hold-up calculation. Equipment-specific inventories based on vessel volume, phase density, liquid level, gas compressibility, or bed voidage can only be determined after equipment sizing and detailed engineering have been performed [12].

Since the thermal levels reached directly impact the inherent safety score, the maximum process temperature is used as the governing parameter for thermal severity.

Table 3. Determination of the process temperature subindex.

Temperature	Score
<0 °C	1
0–70 °C	0
70–150 °C	1
150–300 °C	2
300–600 °C	3
>600 °C	4

details the score ranges for the I_T sub-index, assigned based on the highest temperature recorded in any unit of the plant. This variable acts as an indicator of thermal severity; elevated process temperatures increase the likelihood of material degradation, loss of mechanical strength, and thermal stress in equipment walls and seals, all of which raise the probability of containment failure.

Pressure is a measure of the potential energy accumulated in the process, which can seriously compromise plant operations if control system failures occur [13].

Table 4. Determination of the process pressure subindex.

Pressure	Score
0.5–5 bar	0
5–25 bar	1
25–50 bar	2
50–200 bar	3
200–1000 bar	4

shows the established limits for the I_P sub-index, where an intrinsic hazard score is assigned based on the maximum pressure detected in the system. This factor is critical to safety, as high-pressure levels require more resistant materials and constant monitoring to prevent mechanical failures or dangerous leaks. For the present process, the maximum normal operating pressure is 60 bar in the gasification/combustion section. Consequently, this condition governs I_P,max. The downstream WGS and PSA sections operate at lower pressures and therefore do not determine the overall pressure sub-index.

The safety assessment by equipment type is linked to various factors, including both the type of machinery used and its operating conditions. To assign these scores, the Dow E&F index classification is used, which is based on accumulated experience and the history of failures and accidents recorded in the industry for each piece of equipment.

As with the inventory subindex, the I_EQ factor is analyzed by dividing the plant into the ISBL (main area) and OSBL (auxiliary areas) sections, as shown in

Table 5. Scores for the equipment sub-index $I_{EQ,max}.$

Equipment		Score
ISBL	Non-flammable and non-toxic material handling equipment	0
	Heat exchangers, pumps, towers, drums	1
	Air coolers, reactors, high-risk pumps	2
	Compressors, high-risk reactors	3
	Furnaces, heaters	4
OSBL	Non-flammable and non-toxic material handling equipment	0
	Atmospheric storage tanks, pumps	1
	Cooling towers, compressors, purge systems, pressurized or refrigerated storage tanks	2
	Flares, boilers and furnaces	3

. For the final calculation of this parameter, the highest value obtained between the two sections is taken, ensuring that the highest-risk scenario present in the plant is considered.

The I_ST sub-index evaluates operational hazard contribution from a holistic view of the system. This analysis covers how equipment interacts, connects, and is controlled throughout the plant, while also considering how auxiliary systems influence the main production line. Furthermore, this assessment draws on incident history and safety records from similar processes to determine whether there is sufficient information to consider the operation safe or whether it presents recurring intrinsic hazard. The criteria used to assign this score are detailed in

Table 6. Values of the safe process structure subindex.

Safety Level of the Process Structure	Score
Recommended (Standard, safe)	0
Good engineering practice	1
No data or neutral	2
Probably unsafe	3
Minor accidents	4
Serious accidents	5

.

It is worth noting that sub-indices such as I_ST, and to a lesser extent I_EQ, are inherently based on the accumulated operational experience and incident history of existing industrial installations. Since indirect biomass gasification integrated with PSA purification for hydrogen production has not yet been implemented at an industrial scale with a documented safety record, these sub-indexes were estimated by analogy with technologically comparable processes that share similar hazard mechanisms, namely coal gasification and petrochemical reforming/combustion systems, for which extensive incident statistics and engineering practice guidelines are available. This analogy-based estimation is consistent with standard practice for inherent safety assessment of prototype or emerging technologies at the conceptual design stage, where direct operational data are not yet available. To account for the resulting uncertainty, a conservative, worst-case scoring criterion was applied whenever ambiguity arose in the absence of process-specific historical data, consistent with the general evaluation principle described for the ISI methodology.

2.4. Chemical Inherent Safety Index

The Chemical Inherent Safety Index is used to assess the intrinsic hazard of the chemicals involved in the process by analyzing properties such as the flammability, explosiveness, and toxicity of raw materials, intermediates, and products. This index is calculated using Equation (3), which integrates both reaction hazard terms and compound-specific chemical properties:

$${\mathrm{I}}_{\mathrm{C}\mathrm{h}}={\mathrm{I}}_{\mathrm{M}\mathrm{R},\mathrm{m}\mathrm{a}\mathrm{x}}+{\mathrm{I}}_{\mathrm{S}\mathrm{R},\mathrm{m}\mathrm{a}\mathrm{x}}+{\mathrm{I}}_{\mathrm{I}\mathrm{N}\mathrm{T},\mathrm{m}\mathrm{a}\mathrm{x}}+{({\mathrm{I}}_{\mathrm{F}\mathrm{L}}+{\mathrm{I}}_{\mathrm{E}\mathrm{X}}+{\mathrm{I}}_{\mathrm{T}\mathrm{O}\mathrm{X}})}_{\mathrm{m}\mathrm{a}\mathrm{x}}+{\mathrm{I}}_{\mathrm{C}\mathrm{O}\mathrm{R},\mathrm{m}\mathrm{a}\mathrm{x}}$$

(3)

In accordance with the standard Inherent Safety Index (ISI) methodology established by Heikkilä (1999) [14], the term ${({\mathrm{I}}_{\mathrm{F}\mathrm{L}}+{\mathrm{I}}_{\mathrm{E}\mathrm{X}}+{\mathrm{I}}_{\mathrm{T}\mathrm{O}\mathrm{X}})}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ is evaluated as a combined sum for each individual chemical species present in the system, rather than independently summing the maximum flammability, maximum explosivity, and maximum toxicity scores from different compounds. For each species “i”, the compound score is determined, and the controlling substance is defined as that which maximizes this cumulative sum.

In this framework, the factors I_MR and I_SR represent the chemical reactivity of the Main and secondary reactions, respectively, based on the maximum heat generated by each reaction; the classification of the safety impact of the heat generated by the reactions is presented in

Table 7. Scores for Main and secondary reactions sub-index.

Heat of Reaction		Score
Neutral	≤200 J/g	0
Slightly exothermic	<600 J/g	1
Moderately exothermic	<1200 J/g	2
Strongly exothermic	<3000 J/g	3
Extremely exothermic	≥3000 J/g	4

.

The chemical interaction (I_INT) sub-index makes it possible to identify and classify hazardous phenomena that could occur due to contact or cross-reaction between the various substances present in the process areas. This analysis is essential for anticipating unwanted reactions that could compromise the integrity of the plant. Depending on the nature of these interactions and the potential effects they may generate, each phenomenon is classified according to the criteria established in

Table 8. Scores for chemical interaction sub-index.

Chemical Interaction	Score
Heat formation	1–3
Fire	4
Formation of Harmless, non-flammable gas	1
Toxic gas formation	2–3
Flammable gas formation	2–3
Explosion	4
Rapid polymerization	2–3
Soluble toxic chemicals	1

, where an intrinsic hazard level is assigned based on the severity of the interaction.

The flammability (I_FL) sub-index evaluates the ability of chemicals to generate flame, using their flashpoint and boiling points as a technical basis. This factor makes it possible to predict how easily a compound can burn in the event of a leak or exposure in the plant. The scoring criteria for this intrinsic hazard are organized in

Table 9. Scores for flammability sub-index.

Flammability		Score
Non-flammable		0
Combustible	Flashpoint > 55 °C	1
Flammable	Flashpoint < 55 °C	2
Highly flammable	Flashpoint < 21 °C	3
Very flammable	Flashpoint < 0 °C and boiling point < 35 °C	4

, where higher values are assigned to substances with greater ease of combustion.

The explosivity sub-index (I_EXP) evaluates the ability of a gaseous substance to form flammable mixtures when combined with air. This intrinsic hazard is measured using the explosive limits, expressed as a percentage by volume of vapor in air, which defines the concentration range within which a mixture can ignite upon contact with an ignition source. For this analysis, the Lower Explosive Limit (LEL), which indicates the minimum concentration required, and the Upper Explosive Limit (UEL), which marks the maximum point above which the mixture is too rich to burn, are considered, as a shown in

Table 10. Scores for explosivity sub-index.

Explosiveness (UEL–LEL) %v/v	Score
Non-explosive	0
0–20%	1
20–45%	2
45–70%	3
70–100%	4

.

The toxicity sub-index (I_TOX) is used to assess the intrinsic hazard to human health posed by exposure to chemical compounds. This value is determined based on the toxicity threshold of each substance, measured in parts per million (ppm).

Table 11. Scores for toxicity sub-index.

Toxicity Limit (ppm)	Score
TLV > 10,000	0
TLV ≤ 10,000	1
TLV ≤ 1000	2
TLV ≤ 100	3
TLV ≤ 10	4
TLV ≤ 1	5
TLV ≤ 0.1	6

details the concentration ranges that define the score for this index, allowing us to identify how dangerous accidental contact with the chemicals present in the process would be.

The corrosion (I_COR) sub-index estimates the intrinsic hazard associated with the deterioration or damage to equipment materials due to the chemical nature of the substances being handled.

Table 12. Scores for corrosion sub-index.

Required Material of Construction	Score
Carbon steel	0
Stainless steel	1
Special materials	2

shows the hazard levels associated with this factor, making it possible to anticipate the need for specialized materials or protective coatings to prevent structural failures caused by chemical wear in the plant.

The assessment using the ISI is always conducted based on the worst-case scenario. To this end, subindices that depend on chemical substance data—such as toxicity or interactions—are obtained directly from the materials’ safety data sheets. In the original ISI framework, a total index value of 24 has been used as a reference boundary for interpreting process inherent safety. In the present biomass-gasification application, this value is used as a comparative benchmark rather than as an absolute safety-certification threshold, because the original boundary was not specifically validated for indirect biomass gasification systems.

Since air ingress inherently introduces nitrogen alongside oxygen, its presence was implicitly considered. However, following the worst-case scenario criterion of the ISI methodology, nitrogen was not individually scored, as it is inert, non-flammable, and non-toxic and therefore does not govern the I_FL, I_EX, I_TOX, or I_INT sub-indices. Its potential dilution effect on the flammability limits of the mixture was not explicitly quantified, as the ISI evaluates sub-index based on the safety-worst chemical rather than on mixture-specific recalculations.

3. Results

3.1. Chemical Sub-Index Analysis

The I_ch sub-index analysis shows that the heat of reaction scores are determined by the endothermic gasification reaction and the exothermic secondary combustion. As a shown in

Table 13. Reactions involved in the gasification process.

Process	Reaction	Heat of Reaction at 298 K	Source
Gasification	C (Char) + H2O (Steam) → CO + H2	+131.3 kJ/mol	[16]
Combustor	C (Char) + O2 → CO2	−393.5 kJ/mol	[17]
WGS	CO + H2O (Steam) ↔ CO2 + H2	−41.2 kJ/mol	[18]

, the primary gasification reaction (ΔH = +131.3 kJ/mol) is endothermic, requiring external heat and posing no thermal runaway risk; hence, it receives I_MR,max = 0. Conversely, the combustion of unreacted char in the combustor releases ΔH = −393.5 kJ/mol, corresponding to a heat release magnitude of 393.5 kJ/mol (32,764 J/g) for pure carbon. EFB char, however, contains ash and has a reported calorific value of 19–30 MJ/kg [15]. Even at the lower bound of 19,000 J/g, this heat release magnitude far exceeds the 3000 J/g threshold defining the extremely exothermic category in Table 7, justifying a score of I_SR,max = 4 for the secondary reaction sub-index.

The chemical hazard profile of the process, illustrated in Figure 4, is dominated by the combined flammability–explosiveness–toxicity term of Equation (3). Under the ISI methodology, this term is determined by identifying the single worst-case chemical species rather than by independently maximizing each sub-index across different compounds. Applying this criterion to the species present in the syngas (

Table 14. Determination of the Explosiveness and Toxicity subindex.

Chemical Component	Explosiveness (UEL–LEL) %v/v	Score	Toxicity (TLV, ppm)	Score
H2	71	4	Non toxic	0
CO	65.1	3	23	3
CO2	Not applicable	0	5000	1
CH4	10	1	Not applicable	0
N2	Not applicable	0	Not applicable	0
SO2	Not applicable	0	2	4
O2	Not applicable	0	Not applicable	0

and

Table 15. Determination of the Flammability subindex.

Chemical Substance	Flash Point °C	Boiling Point °C	Flammability Type	Score IFL
H2	There is no information	−252.8	Very flammable	4
CO	There is no information	−191.5	Very flammable	4
CO2	Not applicable	There is no information	Non flammable	0
CH4	−188	−161.5	Very flammable	4
N2	Not applicable	−195.6	Non flammable	0
SO2	Not applicable	−10	Non flammable	0
O2	Not applicable	−182.9	Non flammable	0

), CO yields the highest combined score (I_FL = 4, I_EX = 3, I_TOX = 3; total = 10), followed by H₂ (I_FL = 4, I_EX = 4, I_TOX = 0; total = 8). The SO₂ speciation is specific to the component set and thermodynamic representation adopted in the conceptual process model and should not be interpreted as a general prediction of the dominant sulfur species in all reducing biomass-gasification environments. The SO₂ contributes (I_FL = 0, I_EX = 0, I_TOX = 4; total = 4). Consequently, CO governs the combined flammability–explosiveness–toxicity term. Thus, although H₂ exhibits the highest individual explosiveness score and SO₂ the highest individual toxicity score [19], these individual maxima are not combined across different substances in the calculation of the Chemical Inherent Safety Index. Figure 5 therefore distinguishes the CO-specific scores that determine the controlling combined term from the individual property maxima of the other compounds, which are shown only for comparison [20].

The flash point concept is not applicable since these are permanent gases, meaning they exist entirely in the vapor phase under all realistic process and storage conditions.

Because H₂ and CO are permanent gases and do not have a conventional liquid flash point, the flash-point criterion of the original ISI table cannot be applied literally. For the present conceptual assessment, these flammable gases were conservatively assigned I_FL,max = 4 because a released gas can form a flammable mixture with air without first requiring liquid vaporization.

The syngas generated in the indirect gasification section contains mainly H₂, CO, and CH₄, all of which contribute to the intrinsic flammability of the process. The gasification stage reaches a maximum operating temperature of 900 °C, whereas lower temperatures may occur in downstream syngas streams after thermal conditioning. In addition, in indirect gasification the absence of nitrogen as a diluent—when steam and external combustion of char are used—increases the concentration of fuel components, which increases the fire potential [16].

The I_INT,max was rated with a value of 4 because the most severe credible interaction is the formation of flammable/explosive mixtures following air ingress into sections containing H₂, CO, and CH₄. Contact of these combustible gases with oxygen can create ignitable mixtures and substantial heat release [20,21,22]. This worst-case interaction is sufficient to assign the maximum I_INT score under the ISI methodology. In addition, tar deposits present in the syngas can undergo exothermic polymerization in hot zones, causing localized deposits, blockages, and overheating.

Regarding I_COR,max, the process involves high temperatures, water vapor, sulfur-containing species, and tar-related compounds that may promote material degradation under specific operating conditions. The recent literature indicates that these effects can be managed through appropriate material selection, including stainless steels and high-temperature corrosion-resistant alloys [23]. Accordingly, the corrosivity sub-index was assigned a score of 2, reflecting the requirement for special construction materials. Although this contribution is lower than those associated with chemical interactions, inventory, and severe operating conditions, corrosion remains an important consideration for subsequent equipment design and material selection.

3.2. Process Sub-Index Analysis

The inventory sub-index I_I received a score of 4. Following the original ISI methodology for conceptual process design, the ISBL inventory was estimated from the representative mass flow associated with each major process vessel using a nominal residence time of one hour. For each unit, the nominal inventory was calculated as $M_i={\dot{m}}_i{\tau }_{nom}$, with ${\tau }_{nom}=1$h. The representative streams used for each unit are reported explicitly in

Table 16. ISBL Equipment Inventory Results.

Process Unit	Representative Stream	Mass Flow (t/h)	Nominal Residence Time (h)	Nominal Inventory (t)
EFB feed (MILL/DRYER)	S3	41.07	1	41.07
Gasification (GASF1)	S4	41.07	1	41.07
Gasification (GASF2)	S6	45.17	1	45.17
Separator (SEP)	S8	23.31	1	23.31
Char combustion (COMBS)	S7	21.86	1	21.86
Scrubber (CLEANER)	S10	56.49	1	56.49
High-temperature WGS reactor (HTS)	S13	26.49	1	26.49
Low-temperature WGS reactor (LTS)	S15	26.49	1	26.49
Post-WGS separator (SEP3)	S16	26.49	1	26.49
PSA unit	S18	1.24	1	1.24
Total ISBL	-	309.68	-	309.68

to ensure full traceability of the calculation. In particular, GASF1 is represented by S4 = 41.07 t/h and GASF2 by S6 = 45.17 t/h. The previously reported S5 = 224.37 t/h value resulted from an incorrect stream assignment and has been removed from the calculation. The individual nominal equipment inventories range from 1.24 t for the PSA unit to 56.49 t for the scrubber. Their sum gives a total nominal ISBL inventory of 309.68 t, which falls within the 200–500 t interval and therefore corresponds to I_I = 4. This value represents the preliminary-design inventory defined by the ISI methodology and should not be interpreted as a detailed mechanical hold-up calculated from final vessel dimensions.

In contrast, as a shown in

Table 17. Available OSBL inventory information at the conceptual-design stage.

Process Unit	Stream	Observation
Biomass storage (EFB) prior to milling	Not specified in the design	Data not available at this design stage
Process water storage	Continuous stream (WATER)	No explicit storage tank reported
Hydrogen product storage	Not specified (HYDROGEN)	Not defined in the conceptual design
PSA byproduct storage (SUBP)	Not specified	Not defined in the conceptual design

, the OSBL index could not be accurately quantified at this conceptual design stage, since the storage volumes for feedstocks and products have not yet been defined in the simulated process flowsheet.The H₂ production process includes high-hazard equipment associated with the indirect gasification section. The gasification stage reaches a maximum verified operating temperature of 900 °C and a maximum pressure of 60 bar. These severe operating conditions, together with the reactive nature of the gasification system and the handling of combustible syngas, justify the assignment of I_EQ,max = 3 to the high-risk reactor category. The independent operating temperature of the char combustor is not specified in the conceptual simulation and is therefore not used as the basis for the temperature sub-index. The maximum verified process temperature is 900 °C in the indirect gasification section; since this value exceeds the >600 °C threshold of the ISI methodology, I_T,max = 4.

As for the I_P,max, the process simulation streams associated with the gasifier and combustor sections report a pressure of 60 bar, which constitutes the highest pressure recorded across the entire process. The PSA purification unit, by comparison, operates at 45 bar, representing a moderate hazard contribution on its own but not the governing condition for this sub-index. According to Table 4, the 60 bar value falls within the 50–200 bar bracket, and following the worst-case scenario criterion established in Section 2.4 for the overall ISI assessment, the I_P sub-index was scored at 3.

Finally, the I_ST,max sub-index was rated 2. Although no specific accidents are reported in biomass gasification plants with this exact configuration, the literature on high temperature chemical processes indicates the possibility of incidents such as steam cloud explosions or fires [24]. The contribution of each indicator to the overall Inherent Safety Index assessment is illustrated in Figure 6.

Based on the controlling values summarized in

Table 18. Summary of the ISI calculation for the indirect gasification–PSA hydrogen production process.

Index/Sub-Index	Abbreviation	Controlling Parameter	Numerical Input/Criterion	Score	Source/Basis
Main reaction heat	IMR	Main gasification reaction	Endothermic C + H2O → CO + H2 ΔH ≈ +131.3 kJ/mol	0	Table 7 [2,16]
Secondary reaction heat	ISR	Char combustion	Exothermic C + O2 → CO2 ΔH ≈ −393.5 kJ/mol Heat release > 3000 J/g	4	Table 7 [2,15,17]
Chemical interaction	IINT	Air ingress/Fuel gas + O2	Potential fire/explosion	4	Table 8 Figure 4 (compatibility analysis) [20,21,22]
Combined Flammability Explosivity Toxicity	(IFL + IEX + ITOX)MAX	CO	Very Flammable UEL-LEL: 65.1% v/v TLV: 23 ppm	10	Table 9, Table 10, Table 11, Table 14 and Table 15 [20]
Corrosivity	ICOR	Most demanding material requirement	Special high-temperature corrosion-resistant material requirement	2	Table 12 [23]
Chemical Inherent Safety Index	ICh	Chemical subtotal	-	20	Equation (3)
Inventory	II	ISBL nominal inventory	309.68 t using τnom = 1 h	4	Table 2 and Table 16 [14]
Temperature	IT	Indirect gasification section	900 °C	4	Table 3 [2]
Pressure	IP	Gasification section	60 bar	3	Table 4 [2]
Equipment	IEQ	High-risk reactor category	Indirect gasification reactor	3	Table 5 [25]
Process structure	IST	Process structure	No direct industrial record; neutral/analogy-based classification	2	[14]
Process Inherent Safety Index	IPs	Process subtotal	-	16	Equation (2)
TOTAL				36

, the Chemical Inherent Safety Index is I_Ch = 20 and the Process Inherent Safety Index is I_Ps = 16, resulting in a total ISI of 36, as a shown in Figure 7.

To evaluate the ISI under fluctuations in operating conditions, a sensitivity analysis was performed by independently varying the reactor temperature and process pressure. Figure 8 illustrates the behavior of the total ISI when these variables deviate from the baseline case.

A limited threshold-sensitivity analysis was performed to examine how the discrete temperature and pressure scoring intervals of the ISI methodology affect the calculated overall index. This analysis is exclusively an ISI threshold-sensitivity exercise and does not represent technically feasible alternative operating scenarios. Maximum process temperature and pressure were varied independently across the scoring intervals defined by Heikkilä while all other sub-indices were held constant. The resulting points therefore represent the mathematical response of the ISI scoring system rather than re-simulated process configurations. Under the corrected baseline process, the maximum verified temperature is 900 °C, corresponding to (I_T,max = 4), whereas the maximum pressure is 60 bar, corresponding to (I_P,max = 3).

These variations are theoretical threshold-response cases and are not proposed as operating modifications. In particular, lowering the maximum temperature or pressure would require modification of the gasification operating conditions and could affect biomass conversion, syngas composition, reaction equilibria, downstream separation requirements, and the overall mass and energy balances. Quantification of these effects would require re-simulation and optimization of alternative process configurations and therefore constitutes a separate process-redesign study rather than part of the present ISI assessment.

Consequently, the analysis is used only to determine the sensitivity of the calculated ISI to the governing temperature and pressure categories. Other inherently safer design strategies identified from the ISI results, including inventory minimization, sulfur-species reduction, prevention of air ingress, and modifications to the hydrogen-purification section, are discussed qualitatively as opportunities for subsequent design development and are not presented as simulated scenarios in this study.

4. Discussion

The total Inherent Safety Index obtained for the indirect gasification–PSA hydrogen production route is 36, comprising a Chemical Inherent Safety Index of 20 and a Process Inherent Safety Index of 16. This value exceeds the reference value of 24 reported in the original ISI framework and therefore indicates an unfavorable inherent-safety profile for the evaluated conceptual configuration. However, the value of 24 should not be interpreted as an absolute safety-certification or pass/fail threshold for the present process because it was not specifically developed or validated for indirect biomass gasification systems. In this study, it is therefore used as a comparative reference for identifying the dominant intrinsic hazard contributors and establishing priorities for inherently safer process development.

The chemical hazard profile is governed by the single substance presenting the highest combined flammability, explosiveness, and toxicity score rather than by independently combining the maximum scores of different compounds. In the present process, CO controls this term with a combined score of 10. Hydrogen exhibits the highest individual explosiveness score, whereas SO₂ exhibits the highest individual toxicity score; however, neither produces a combined score as high as that of CO. This distinction is important because it prevents the Chemical Inherent Safety Index from being artificially increased by combining unrelated maxima from different substances. It also shows that the controlling chemical hazard cannot necessarily be identified from a single-property ranking and instead requires a compound-specific evaluation of the three hazard dimensions considered by the ISI methodology.

The high chemical interaction score (I_INT,max = 4) reflects the susceptibility of the system to form explosive mixtures upon air ingress, given the coexistence of combustible gases (H₂, CH₄, CO) with oxygen. This finding highlights the need for preventive strategies centered on leakage prevention, inert atmosphere control, and airtight equipment design.

From a thermal standpoint, the governing condition is the indirect gasification stage, which reaches a maximum verified operating temperature of 900 °C and consequently determines I_T,max = 4. Although gasification is globally endothermic, its coupling with char combustion involves substantial thermal-energy transfer and highly exothermic oxidation reactions. The detailed operating temperature of the char combustor and the thermal balance of the circulating heat-transfer medium are not independently resolved in the conceptual simulation and are therefore not used to define the temperature sub-index. The previously reported high-temperature steam condition associated with the HTS reactor is no longer used as a governing temperature for the ISI.

Among the process sub-indices, inventory (I_I,max= 4) and temperature (I_T,max = 4) are the largest contributors. The nominal ISBL inventory is 309.68 t according to the one-hour residence-time convention of the ISI methodology. The equipment sub-index (I_EQ,max = 3) reflects the presence of high-hazard reactor equipment in the indirect gasification section.

The pressure sub-index (I_P,max = 3), governed by the 60 bar streams from the gasifier and combustor sections, is moderate but relevant.

The safe process structure sub-index (I_IST,max = 2) was estimated by analogy with coal gasification and petrochemical reforming, as no direct operational history exists for this specific configuration. While this analogy-based approach is standard for emerging technologies, it introduces uncertainty that should be addressed as pilot-scale experience becomes available.

The parametric analysis shows that the ISI responds discretely to the temperature and pressure intervals established by the methodology. A theoretical reduction in the maximum process pressure from 60 bar to below 50 bar would decrease I_P,max from 3 to 2 and reduce the total ISI by one point, provided that all other sub-indices remain unchanged. Likewise, moving the maximum process temperature from the >600 °C interval to 600 °C or below would reduce I_T,max from 4 to 3 and decrease the total ISI by one point. These results represent the mathematical response of the ISI to its scoring thresholds and should not be interpreted as technically validated operating alternatives. Implementing either modification would require re-simulation of the gasification process to determine its effects on biomass conversion, syngas composition, energy requirements, and downstream performance.

The ISI results also identify several potential directions for future inherently safer design development. These include reducing material inventory through process intensification or equipment-volume minimization, decreasing the presence of hazardous sulfur-containing species where technically feasible, preventing air ingress into hydrogen- and CO-containing sections, and evaluating alternative purification configurations. These alternatives are identified from the hazard drivers revealed by the ISI and should be regarded as design opportunities rather than quantitatively demonstrated improvements in the present study. Their implementation would alter process flows, compositions, equipment requirements, and operating conditions and would therefore require dedicated process re-simulation and subsequent recalculation of the ISI. Such process-redesign scenarios are outside the scope of the present work and constitute a logical direction for future research.

The present assessment is limited to the inherent-safety characteristics of the conceptual hydrogen production process itself. It does not constitute a detailed quantitative risk assessment and does not estimate accident frequencies, consequence distances, individual risk, or societal risk. Likewise, the study does not include downstream CO₂ capture, transport, or storage, nor does it provide a techno-economic or life-cycle sustainability assessment of the proposed design modifications. The results should therefore be interpreted as an early-stage screening of intrinsic process hazards and as a basis for identifying priorities for subsequent process redesign, detailed safety analysis, and integrated sustainability assessment.

5. Conclusions

The inherent safety of hydrogen production from oil palm empty fruit bunches through indirect gasification followed by PSA purification was evaluated using the Inherent Safety Index methodology at the conceptual-design stage. The assessment resulted in a Chemical Inherent Safety Index of I_Ch = 20, a Process Inherent Safety Index of I_Ps = 16, and a total ISI of 36. This value exceeds the reference value of 24 used in the original ISI framework and indicates an unfavorable inherent-safety profile for the evaluated configuration, while the reference value is interpreted here as a comparative benchmark rather than an absolute safety-certification threshold.

The chemical contribution is governed by CO, which presents the highest combined flammability–explosiveness–toxicity score I_FL + I_EX + I_TOX = 10. Hydrogen exhibits the highest individual explosiveness score and SO₂ the highest individual toxicity score, but these individual maxima are not combined across different substances in the ISI calculation. The potential formation of flammable or explosive mixtures following air ingress also leads to the maximum chemical-interaction score.

From the process perspective, inventory and temperature are the largest contributors. The corrected equipment-by-equipment inventory calculation gives a nominal ISBL inventory of 309.68 t, corresponding to I_I = 4. The maximum verified temperature of 900 °C in the indirect gasification section gives I_T,max = 4, while the maximum process pressure of 60 bar gives I_P,max = 3. These results identify inventory minimization, prevention of air ingress, appropriate equipment and material selection, and evaluation of less severe operating conditions where technically possible as relevant priorities for subsequent inherently safer design.

The temperature and pressure cases explored in the sensitivity analysis represent only the mathematical response of the ISI to its discrete scoring thresholds. They are not technically validated operating alternatives. Any modification of the gasification temperature, pressure, purification configuration, or process inventory would require re-simulation of the process, verification of mass and energy balances, evaluation of hydrogen production performance, and subsequent recalculation of the ISI.

Accordingly, the present study does not establish the technical feasibility of the proposed process modifications and should not be interpreted as a detailed quantitative risk assessment. Rather, it provides an early-stage inherent-safety diagnosis that identifies the dominant hazard drivers and establishes a technical basis for future process redesign and more detailed safety assessment of biomass-based hydrogen production via indirect gasification and PSA purification.