The Influence of Hydrogen Concentration on the Hazards Associated with the Use of Coke Oven Gas

Abstract

Coke oven gas (COG), as a by-product of the coking process and a mixture with a high hydrogen content, is an important potential component of the sustainable economy of the coking industry. Ongoing studies and analyses are looking at many opportunities for the utilization of coke oven gas, including for the production of hydrogen, methanol or other chemicals. However, it is important not to forget that all processes for the utilization of this gas may pose a potential hazard to humans and the environment. This is due to the physicochemical properties of COG and the content of flammable gases such as hydrogen, methane or carbon monoxide in its composition. Potential hazardous events are also related to the content of toxic substances in the composition of coke oven gas. The publication focuses on the occurrence of a fire or explosion as a result of the uncontrolled release of purified coke oven gas from the installation. The potential hazard zones associated with the occurrence of these phenomena are presented concerning different levels of hydrogen concentration in coke oven gas and the influence of selected factors on the range of these zones. Zones related to human deaths due to fire of coke oven gas reached a maximum range of about 130 m from the site of the failure, depending on the gas composition, level of damage and parameters of the installation. Zones related to human deaths due to the explosion of the coke oven gas did not occur. The zone related to the injury of humans as a result of the COG explosion reached a maximum range of about 12 m.

1. Introduction

The exhaustion of fossil fuels and the continuous increase in energy demand associated with the rapid development of the economy and population growth lead to investigations and research to find alternative energy sources, but also to more efficient and effective use of the existing sources. These activities include, among others, the use of hydrogen or the modernization of existing technologies to obtain clean and emission-free fuels. For many years, fossil fuels have made a significant contribution to the development of industry, transport and domestic electricity consumption. Coal, with a consumption level of over 8000 million tonnes in 2023, is the largest source of power generation and steel and cement production [1,2]. It is also the largest contributor to greenhouse gas emissions from the use of fossil fuels. The introduction of relevant regulations, legislation and the modernization of existing technologies is aimed at reversing this trend, both in energy generation and in many other industries.

The coking industry also contributes to greenhouse gas emissions. The process of producing coke and recovering by-products is not neutral to the environment. This is because of the specificity of those processes. Therefore, the coking industry is also subject to various types of regulation and pressure related to limiting greenhouse gas emissions into the atmosphere. The activities include technological solutions leading to solving the problem of air emissions associated with the traditional production of coke. The activities carried out include among others changes in the process of operation and maintenance of existing coking plants in order to increase their profitability and extend their service life, the expansion of the range of materials used for coking in order to reduce costs without loss of coke quality or stability of the process as well as the introduction of new technologies that decrease emissions [3].

The coking industry is also looking for technological solutions that will allow full use of the potential of coke oven gas (COG) while reducing emissions.

Currently, coke oven gas is mostly used as a gaseous fuel and combusted. Analyses and research conducted concern, among others, the reduction of process emissions or the improvement of combustion conditions. Experiments and numerical calculations to investigate the combustion properties of coke oven gas are presented in [4]. A chemical kinetic model and mixing rule for the estimation of the COG burning velocity are also proposed in this publication. The measurement of the laminar burning velocity of CH₄/H₂/O₂/CO₂ mixtures generated from oxy-fuel combustion of coke-oven gas to understand the flame propagation process is presented in [5]. Investigations of the temperature profile of the combustion chamber and the effect of this profile on fuel consumption, i.e., coke-oven gas, are presented in [6]. Combustion modelling in the context of emission reduction including NOx is presented in [7,8,9]. A proposal to optimize the design of a coke oven heating flue to provide a significant reduction in NOx emissions is presented in [10]. In turn, an analysis of coke oven gas combustion using the MILD technique is presented in [11]. MILD combustion is a high-efficiency technique used in industrial furnaces to obtain low emissions and significant energy savings by high air heating. The main objective of the publication was to study the behaviour of two hydrogen-rich fuels burning in the MILD regime at different furnace temperature levels. Both fuels had a good performance, but the one with higher H₂ content showed a greater capability to keep a well-placed reaction zone at a higher heat transfer to load and consequently a wider range of flue gas temperatures. As an example of an alternative application of coke oven gas, an analysis of its combustion in a spark-ignition engine using CFD modelling was conducted [12]. Analysis and simulation of an innovative trigeneration process (coke oven gas-fed combined power, methanol and oxygen production system) based on a solid oxide fuel cell and methanol synthesis unit was studied in [13].

There is also a constant search for other alternative and efficient methods of using coke oven gas. Current analyses are largely focused on the possibility of using the gas to extract hydrogen. It is assumed that such a process should lead to potential economic and environmental benefits for the coking industry [14]. A comparative analysis of the environmental and economic effects of different ways of using coke oven gas is presented in [15]. The authors studied the processes of using coke oven gas to produce methanol, hydrogen and synthetic natural gas. The obtained results showed that the method of using coke oven gas to produce hydrogen by pressure swing adsorption (PSA) has the lowest environmental and economic effects. A similar analysis related to the analysis of energy consumption, carbon dioxide emissions and financial costs in the life cycle of the production process of hydrogen from coke oven gas and synthesis gas from coal gasification is presented in [16]. The authors showed that hydrogen production from coke oven gas is characterized by better technical, environmental and economic parameters.

Important analyses in the context of hydrogen extraction from coke oven gas are also subjects related not only to economic and environmental aspects but also to the development of technology. Studies on the production of hydrogen in a cobalt-free catalytic membrane reactor by water splitting combined with the oxidation of coke oven gas to syngas are presented in [17]. A study on the optimization of a high-purity hydrogen production process using artificial neural networks is presented in [18]. The analysis of the operational parameters of the steel slag enhanced reforming process for the production of blue hydrogen from coke oven gas is presented in [19]. The technological process proposed by the authors requires higher total capital investment while requiring a lower total production cost than the most commonly used variable pressure adsorption technology (PSA). In [20], the separation of hydrogen from coke oven gas using ZIF-8/ethylene glycol-water slurry was investigated.

Despite the trend of development of research related to the coke industry, it should be remembered that coke oven gas is a hazardous gas due to its composition. This gas mainly contains hydrogen and methane, i.e., gases that pose a fire and/or explosion hazard. Each installation, both existing and built in the future, e.g., for the transport or storage of this gas, should meet safety requirements to limit the negative effects of potential failures. Previous publications related to the safety of the coke industry have largely focused on hazards resulting, for example, from the release of hazardous volatile organic compounds (HVOCs). This paper presents the hazard zones associated with an uncontrolled release of coke oven gas from installation. The presented analyses were performed based on different purified COG compositions, considering the influence of hydrogen content, the proportion of which in coke oven gas can vary from 55–60%. The analyses also investigated the influence of selected factors, e.g., the operating parameters of the installation, on the hazard zones.

2. Coking Industry and Coke Oven Gas (COG)

The coking industry is an important part of the energy and raw materials industry. It has an important role in the economy by influencing selected sectors of the industry. In 2020, the global production of coking coal reached 1016 Mt. Currently, the world’s largest producer and consumer of coking coal is China, which is responsible for more than 50% of global production—Figure 1 [1].

Coal coking is a process of heating a mixture of coal without access to air. The process in the coking chamber is based on a series of complex physicochemical processes, the culmination of which is the formation of coke. The coking process includes preparation of coal, loading of coal, pushing of coke, coking, quenching of coke, screening of coke storage, recovery of by-products, water treatment and other processes [21]. The finished coke is sorted into appropriate sorts depending on customers’ needs. Coke has a wide range of applications; the most common application of coke is the blast furnace process in steel production. Other uses of coke included: foundry processes and the production of iron alloys and nonferrous metals. For example, coke is also used to produce quicklime and glass wool (Figure 2).

During the thermal decomposition of coal, the volatile compounds in the coal are vaporized and removed. They leave the coke oven chamber in the form of hot, raw coke oven gas. Due to its high content of hydrogen and methane, this product has a high calorific value, constituting a good fuel for use in energy processes. However, purifying it in the appropriate installations is necessary due to the content of compounds such as tar, BTX, ammonia and hydrogen sulfide. The raw coke oven gas is cooled. As a result of cooling, a liquid condensate stream and a gas stream are created. Both streams are then processed to recover coal chemicals and to treat the raw coke oven gas. Further gas processing involves the removal of undesirable components such as ammonia and hydrogen sulfide, for example, and the separation of BTX. The purpose of coke oven gas purification is to enable its use as a gaseous fuel. Purified coke oven gas is called pure coke oven gas or coke oven gas (COG) [20,22,23,24].

During the coking process, a large amount of coke oven gas is generated, which can be considered a by-product. Typical COG is composed mainly of hydrogen and methane, so it has a significant useable value. However, not all COG resources are fully utilized. In China, for example, more than half of produced coke oven gas is released into the air. This causes a huge waste of resources and huge environmental pollution [20].

Coke oven gas is a mixture of different substances, and its composition and properties can vary depending on the type of coal used and the applied technology used. The low-temperature carbonization process (temperature of 700 °C) results in COG with lower hydrogen content. The high-temperature carbonization process results in COG with a high content of hydrogen [22]. Usually, there is 300–350 m³ of coke oven gas per ton of coke [23].

After purification, pure COG usually consists of 54–59% hydrogen and 24–28% methane, as well as CO (5–8%), CO₂ (1.5–3%) and hydrocarbons (2–4%). It can also contain a certain amount of impurities including hydrogen sulfide and naphthalene. However, the use of coke oven gas as an energy source requires that their levels be adjusted to European Directives on environmental protection. For purified coke oven gas, the concentration of residual hydrogen sulfide must be <300–1000 mg/Nm³ or <10 mg/Nm³, depending on the technology used. In addition, during the combustion of coke oven gas, emission levels must be <200–500 mg/Nm³ for SO_x and <350–650 mg/Nm³ for NO_x [20].

Most COG is currently used as a gaseous fuel and combusted. About half of COG production is used for the technological needs of coking plants related to firing coke oven batteries or powering technological installations in the production of by-products. COG is also used as a gaseous fuel to produce heat and electricity in power units. Part of the COG is still available for development, but the possibilities in this case are limited (Figure 2).

Currently, six technological routes for the use of coke oven gas are assumed. These include COG for electricity, COG for methanol, COG for hydrogen, COG for synthetic natural gas, COG for synthetic ammonia and COG for olefins. It is expected that they will compete with conventional production of heat energy, methanol, hydrogen, natural gas and ammonia. Among these technologies, the highest indicator of energy consumption and carbon dioxide emissions is the production of methanol and electricity from COG. In turn, the production of olefins is considered to be the most promising and high-quality route for the utilization of coke oven gas. This is due to the competitiveness of the market and the significant demand for such products.

Compared to the high water and energy consumption of all pathways, the best technical process parameters of the process concern the ammonia production path. In turn, the best environmental parameters are shown by the production of hydrogen, ammonia and natural gas. On the other hand, the economic indicators of the processes depend, among others, on the market prices of raw materials and final products and are therefore relatively difficult to estimate. The influence of political factors such as state financial subsidies or environmental taxes is also significant here. Appropriate policies and standards aimed at promoting the high value of the use of various COG routes also depend on the economy of the individual countries and are related to the specific location. The economics of the selected process also depend, for example, on the stage of consumption of COG-based products or the need to build appropriate infrastructure and facilities [15,23,25].

Other methods of using coke oven gas also include the production of aromatics and ethanol. The fuelling of COG into the engine and the gas-steam combined cycle are also considered to be advanced technologies for the use of coke oven gas. Coke oven gas (COG) heat utilization technologies are also being developed [23,25].

The development of technologies and processes for using COG is related to the limited supply of fossil fuels and concerns about climate change. All activities are aimed at improving the energy efficiency of the coking industry and environmental protection. The development of technologies and processes for using COG is related to the limited supply of fossil fuels and concerns about climate change. The conducted analyses and research have shown, for example, that the path of using coke oven gas for hydrogen production compared to the use of natural gas is better from the point of view of reducing carbon dioxide emissions in the life cycle. The development of COG waste heat recovery technology also contributes to reducing carbon dioxide emissions and reducing energy consumption [15,23,25,26].

3. Characteristics of Coke Oven Gas

As mentioned above, coke oven gas is a by-product produced during coke production. It is a mixture of flammable and non-flammable gases. In the past, coke oven gas was used as urban gas to supply the energy needs of towns and cities. Over time, it was replaced by natural gas. Its composition depends on the coal mixture used and the conditions of the coking process. The initially obtained raw coke oven gas has a yellowish-brown colour and an organic odour. It contains mainly hydrogen and methane. Additionally, raw coke oven gas contains other valuable components and impurities. Coal tar and light oil (benzene, toluene, xylene and other aromatic hydrocarbons) can be disposed of after separation, while impurities such as ammonia or hydrogen cyanide, etc. must be removed due to emission requirements [20]. After purification, pure coke oven gas of a similar composition is obtained. However, the proportions of the main components of this gas, i.e., hydrogen and methane, are changed. Several compounds (impurities) are removed (e.g., tars) and the levels of others are reduced to the level required by the European Directives on environmental protection.

Pure COG is a colourless gas with an odour characteristic of hydrocarbons and hydrogen sulfide. Examples of coke oven gas compositions are shown in

Table 1. Composition of coke oven gas.

References	H2	CH4	CO	CO2	N2	CxHx	O2
[19]	59%	29%	7%	2%	3%
[27]	60.5%	26.3%	7.2%	1.5%	4.7%
[28]	57.8%	31.6%	7.6%	3%
[29]	54–59%	24–28%	5–8%	1.5–3%		2–4%
[30]	60.9%	20.7%	6.9%	1.8%	7.8%	2%
[31]	54.8%	24%	6.6%	3.2%	9.2%	1.8%	0.4%
[24]	55–60%	23–27%	5–8%	1–2%	3–6%	1.5–2.3%
[32]	55–60%	22–28%	6.5–10%	1–3%	3–5%		0.3–0.8%

.

The wide range of variability in the components of coke oven gas also influences the variability of its heating value (

Table 2. Heating value of selected fuels [22,24].

Fuel	Heating Value
Natural gas	56.6 MJ/kg
Coke oven gas	33–41.8 MJ/kg
Water gas	21.9 MJ/kg
Blast furnace gas	2.7 MJ/kg
Producer gas	5.2 MJ/kg

) [22,24].

The development of technology related to, among others, the reduction of greenhouse gas emissions and the increase in the price of fossil fuels leads to the search for alternative fuels. There is huge potential to use the COG to produce high-value products, achieving significant socio-economic benefits. This is because coke oven gas is distinguished from other fuels by its high content of valuable compounds. The purification and conversion of COG makes it possible to obtain these valuable products. Therefore, it is assumed that the promotion of COG energy recovery pathways is a step forward towards sustainable development in the industry. In this context, the most important valuable compound is hydrogen, which gives COG the position of a promising source of clean energy. Hydrogen is a resource for the production of chemicals or used, for example, in refinery processes. It is also used in the production of food or pharmaceuticals. Most importantly, however, it is a potential zero-emission fuel [24].

Current technologies for the utilization of COG include [26,27,28]:

• The combustion of coke oven gas. This is one of the basic methods of COG utilization. Raw COG can be combusted on-site in blast furnaces and coke oven batteries in the coking process. It can also be combusted in small combustion units, such as process heaters and boilers. A surplus of gas can be used to generate power and heat.
• The use of coke oven gas as a feedstock for hydrogen separation. Hydrogen is considered as a future clean energy source. Coke oven gas containing 50–60% hydrogen is a high-potential source of H₂. The processes used to obtain hydrogen from COG are the process of pressure swing adsorption (PSA), membrane separation and cryogenic distillation. The PSA process and the membrane separation process are considered to be highly energy-intensive. They are commercially available. The membrane separation process, in turn, is less industrially developed.
• The use of coke oven gas to produce synthesis gas. Currently, synthesis gas is produced using steam reforming of natural gas and oil. Using coke-oven gas as an alternative to syngas production makes it possible to produce syngas in a less energy-intensive and cleaner way. Synthesis gas, in turn, is a valuable source of hydrogen and a raw material for the production of chemicals and fuels.
• The use of coke oven gas in methanol synthesis. Methanol is used in many industries, e.g., for the production of chemicals. The high hydrogen content in coke oven gas means that COG is considered a good element for sustainable methanol production. This is because COG is used to produce syngas, which is in turn useful in methanol synthesis. However, the process is not free of disadvantages. The problems associated with the use of COG to produce syngas are related to the reduction of hydrogen content in the finished syngas, which in turn results in a low H₂:CO ratio, which is unsuitable for the synthesis of methanol.
• The methanation of coke oven gas. The catalytic co-methanation of CO and CO₂ (CO_x) in COG to CH₄ enrichment is considered a simple and efficient way to produce a gas with a high heating value and a wide range of industrial and commercial uses. Methanation of COG can occur without the addition of other reagents, while CH₄ can be separated as a valuable fuel. An important factor in the process is the selection of a catalyst, especially for low-temperature methanation. This is to provide long-term thermal stability and minimize operating costs for large-scale applications.
• Others.

4. Hazards Related to the Use of Coke Oven Gas

Each activity associated with the use of fuels poses a potential hazard to humans and the environment. This is due, among other things, to their physical and chemical properties and concerns both harmful emissions occurring during their combustion, but also their conversion processes. The situation is similar to the coking industry and technologies for the use of coke oven gas.

Coke production is problematic from an environmental and health point of view [33]. Emissions from coke ovens are a mixture of hazardous volatile substances, creosote, coal tar, polycyclic aromatic hydrocarbons (PAHs), etc. Exposure to coke oven emissions can occur to workers in various industries (e.g., steel), but also to ordinary citizens [34]. Long-term exposure to this type of emissions can cause conjunctivitis, severe dermatitis and lesions of the respiratory system and digestive systems. A study of coke plant workers has also shown an increase in cancers of the lungs, trachea, bronchi, kidneys, etc. [35]. Coke production can also be associated with water pollution and landscape degradation as a result of coal mining. In addition, the potential fire and explosion hazards of furnace interference should not be forgotten [36].

The hazard associated with the use of coke oven gas depends largely on its composition, i.e., whether it is raw coke oven gas containing, for example, hazardous ammonia, hydrogen sulfide, etc., or purified gas. Coke oven gas is generally a mixture of flammable gases, such as H₂, CH₄ and CO, and non-flammable gases, including CO₂ and N₂ [22].

The main hazards associated with the presence of various gases in the composition of raw and purified coke oven gas include hazards associated with the possibility of fire and/or explosion and toxicity due to the presence in their composition of, among others:

• Hydrogen (H₂)—is considered to be the future energy carrier. It is a raw material in the production of chemicals, refining processes, etc. [24]. The potential of hydrogen is also used in the power generation and transport sectors. Under normal conditions, hydrogen is a gas with a very low density. The product of its combustion is water and a significant amount of released energy. A characteristic property of hydrogen that has a major impact on the safety of its use is its wide flammability range of 4 to 75%. Another property that affects the potential hazard of accidental ignition of hydrogen is its low ignition energy, i.e., the minimum amount of external energy that, if supplied to hydrogen, can ignite it. This value is only 0.02 mJ, while for other fuels these energies are more than ten times higher. Such a low value of hydrogen ignition energy means that any uncontrolled leakage of hydrogen can result in a fire, and an igniting spark may be generated as a result of the friction of the flowing hydrogen stream itself or may come from electrostatic interactions [37].
• Methane (CH₄)—is a non-toxic, colourless and odourless gas. Methane is flammable and burns with a blue and yellow flame. The minimum ignition energy of methane is 0.28 mJ, and its flammability limits range from 5 to 15%. In the right proportions, a mixture of air and methane has flammable and explosive properties [38].
• Carbon monoxide (CO)—is a flammable gas. It burns in the air with a small, brightening blue flame. Carbon monoxide is also toxic. It quickly combines with haemoglobin, causing a decrease in cellular respiration, which is particularly harmful to the central nervous system. Initial symptoms of poisoning include headache, nausea, vomiting and blurred vision. Then chest pain, shortness of breath, weakness and fainting. In case of severe toxicity, it causes cardiac arrhythmia, myocardial ischemia, cardiac arrest, pulmonary oedema and coma [39,40].
• Carbon dioxide (CO₂)—is a colourless and odourless gas lighter than air. Because it is odourless, it is difficult to detect. The effect of carbon dioxide on humans and the environment depends on the concentration and time of exposure. Lower concentrations of CO₂ cause higher breathing rates. Headaches and ear buzzing may also occur. At higher concentrations, blood pressure increases, and hallucinations, loss of consciousness and convulsions may occur. Spending a long time in high concentrations of carbon dioxide can cause death. Exposure to concentrations exceeding 30% causes immediate human death [38].
• Ammonia (NH₃)—is considered a toxic and flammable substance. Ammonia is difficult to ignite in air, but it creates a flammable/explosive mixture in closed spaces. Ammonia is toxic, irritating and caustic. In the form of gas and vapour, it causes eye pain, conjunctival redness, cough, sore throat, nausea, vomiting and shortness of breath. Laryngeal oedema with a feeling of shortness of breath, bronchospasm, respiratory arrest and pulmonary oedema may also occur. Immediately after ammonia poisoning, acute bronchitis, pneumonia and fibrosis of the lung tissue with severe respiratory failure may occur. In contact with the skin, ammonia, its mist and solutions cause chemical burns with deep ulcerations. Liquid ammonia causes frostbite of the skin. The negative effects of ammonia on the human body depend on the concentration of ammonia vapour and the exposure time [38].
• Hydrogen sulfide (H₂S)—is a colourless and extremely flammable gas. It has a characteristic rotten egg smell and is detectable even at very low concentrations. It is absorbed into the body through the lungs and to a small extent through the skin. Hydrogen sulfide is highly toxic, irritating and chemically asphyxiating. Lower concentrations of hydrogen sulfide cause irritation and inflammation of the eyes and the respiratory tract. Higher concentrations cause cough, headache, eye pain and swelling of the eyelids. In very high concentrations, hydrogen sulfide causes severe irritation of the respiratory system. Respiratory and heart problems, loss of unconsciousness and death can occur [38].
• Others—depending on the final composition of the raw and purified coke oven gas.

This publication focuses on the hazards associated with the uncontrolled release of purified coke oven gas and the occurrence of fire and explosion phenomena. To analyze the hazardous consequences of these phenomena, PHAST v6.7 software was used [41]. This software is a solution for modelling discharge, dispersion, fire, explosion and other hazardous scenarios. A limitation of modelling effects compared to computational fluid dynamics (CFD) software is that it does not take into account obstacles in the hazard zone that could affect dispersal, fires or explosions. Where it is important to consider obstacles or undulating terrain, computational fluid dynamics (CFD) is the preferred solution. The computational models implemented in PHAST software are mainly developed in C++ or Fortran [41]. Verification and validation of these models are presented, among others, in [42,43].

5. Methodology

As already mentioned, any operation involving the treatment of fuels (including coke oven gas) is associated with hazards. These are due to, among other things, the physicochemical properties of the fuels and the parameters of the installation’s operation. In the case of raw and purified coke oven gas, hazardous accidental phenomena initiated by its release include fires, explosions and the movement of a toxic cloud. An analysis of the consequences of selected accident scenarios was carried out using the PHAST v6.7 software package [41].

In the case of a leakage of a pressurized coke oven gas through a hole (‘failure’), a jet fire is involved. A long, stable flame with intense radiation is then created. Experimental semi-empirical models are used to estimate the consequences of this type of phenomenon. These are divided into three groups: point source models, multi-point source models and surface source models [41,44]. In the analysis presented here, the quantities associated with this type of fire were calculated using PHAST v6.7 software [41] with an implemented surface model. This model is based on the assumption that the flame is a spherical solid in the form of a truncated cone (cone frustum) with dimensions depending, for example, on the wind speed: W₁—diameter of the lower base of the frustum, W₂—diameter of the upper base of the frustum, R_l—height of the frustum (Figure 3) [41,44,45,46].

Another type of fire that can occur as a result of a leak in a coke oven gas installation is a fireball, which can be caused by the BLEVE (Boiling Expanding Liquid Vapour Explosion) phenomenon. This phenomenon is the result of a sudden and complete rupture of, for example, a tank. The size of the resulting fireball and its duration are determined using empirical formulas. The estimation of the heat flux generated from this type of fire is based on the determination of the flame emissivity, which depends, among other things, on the amount and properties of the fuel taking part in the phenomenon and the duration of the phenomenon [37,41].

The analysis of the consequences of a coke oven gas explosion in the form of an overpressure wave was based on the trinitrotoluene model (TNT model) implemented in the PHAST software. Its main assumption is to convert the explosion energy created by the pressure-wave explosion into an equivalent amount of trinitrotoluene (TNT) that would be exploded. The model is mainly based on observations of real explosions and experiments of explosions. The assumptions of this model assume that the effects of an explosion due to an overpressure wave are the same at some scaled distance z, depending, among other things, on the amount of equivalent mass of trinitrotoluene. In turn, each normalized distance z corresponds to an overpressure value Δp read from graphs developed from experimental studies [41,47].

6. Analysis of Hazards Related to the Fire and Explosion of Coke Oven Gas

The technology of using coke oven gas is an element of many studies and analyses. This is because currently a significant amount of this gas is not used. When introducing installations e.g., coke oven gas processing, transport or storage into the industry, it should not be forgotten that coke oven gas is a hazardous gas. The high content of hydrogen in its composition makes it a flammable gas with a relatively wide flammability range. In a mixture with air, coke oven gas creates an explosive mixture. In the case of COG, as with all other flammable gases, safety rules must therefore be followed during its transport, storage, processing and use.

Failure of the coke oven gas installation and its uncontrolled leakage can result in a number of different hazardous events for humans and the environment. These include, for example, fire and explosion. The hazardous consequences of these events are the direct impact of the flame, the heat flux generated in the case of a fire, the flying debris of ruptured installations and the overpressure wave in the case of an explosion.

Analyses of the level of these effects depending on the adopted parameters are shown in the figures below. The analyses presented, carried out using PHAST v6.7 software, were conducted for purified coke oven gas with different concentrations of hydrogen. The adopted analysis assumed different operating parameters of the installation and different levels of damage (puncture and complete rupture). The assumed weather conditions were a wind speed of 2 m/s and an ambient temperature of 20 °C.

The compositions of purified coke oven gas with different hydrogen concentrations assumed in the analysis are:

• COG₁—60% H₂, 29% CH₄, 6% CO, 2% CO₂, 3% N₂
• COG₂—55% H₂, 29% CH₄, 8% CO, 2% CO₂, 6% N₂

(a)

Fire of coke oven gas

Depending on the course of the failure, a fire of coke oven gas released from the installation may take the form of a jet fire or a fire in the shape of a fireball, respectively for partial and complete damage to the installation.

The presented hazard zones refer to heat flux generated from the fire at the level of 12.5 kW/m² and 37.5 kW/m² which causes burns and death of humans in the vicinity of the failure site, respectively (Figure 4). Such heat flux values with sufficiently long exposure times, i.e., over 30 min, also cause the melting of plastics and damage to other installations and tanks, respectively [48]. The analysis assumed a failure of the installation of the coke oven gas resulting in a puncture rupture of 100 mm in diameter. The gas pressure in installation is 40 bar. In such a case of ignition of the gas jet flowing out under pressure, a jet fire occurs. The results presented were obtained by using the PHAST v6.7 software [41].

The results of the analysis assuming a complete rupture of the installation containing 1 m³ of coke oven gas are shown in Figure 5. In such a case, the ignition of the coke oven gas results in a fire in the shape of a fireball.

Analyzing the above figures, it can be seen that a change in the hydrogen concentration in coke oven gas at the level of 5% will not cause significant changes in the range of the zones associated with burns and human deaths in the event of partial damage of the installation and gas leakage through the hole. The difference in the range of zones is about 0.5 m. In the case of a complete rupture of the coke oven gas installation, a zone with a range of 6 m associated with human deaths will appear for gas with a higher hydrogen concentration. For coke oven gas with a lower hydrogen concentration, this zone will not occur.

The pressure of the gas in the installation, the amount of gas in the installation or the size of the damage are important factors affecting the level of hazard associated with a release and a coke oven gas fire. The influence of these parameters on the hazard zones for humans and the environment is shown in Figure 6, Figure 7, Figure 8 and Figure 9.

The ranges of the hazard zones associated with human deaths in the case of a coke oven gas jet fire for the different diameters of the holes caused by the damage to the installation are shown in Figure 6. The gas pressure in installation is 40 bar.

The ranges of the hazard zones for different pressures of coke oven gas in the installation are, in turn, shown in Figure 7 and Figure 8. These refer to the zones associated with human deaths from a jet fire of coke oven gas leaking through a 100 mm diameter hole (damage) and a fireball of coke oven gas.

The impact of varying the amount of coke oven gas in the installation on the range of hazard zones associated with fireball and human deaths is shown in Figure 9.

Analyzing the charts above, it can be seen that the range of hazard zones associated with partial damage to the installation, a jet fire of coke oven gas and human death varies from about 60 to 130 m, depending on the diameter of the hole (damage). In the event of a 4-fold increase in gas pressure in the installation, the range of hazard zones associated with fire and human death increases by about 25 m in the event of partial damage to the installation and COG₁ and COG₂ and by about 6 m in the event of complete damage and COG₁. In the event of complete damage to the installation and COG₂, the hazard zones associated with human death will not occur.

(b)

Explosion of coke oven gas

The other type of hazardous incident that may occur in the event of damage to a coke oven gas installation is an explosion.

The hazard zones for the explosion of coke oven gas are shown in Figure 10. They refer to the overpressure wave generated from the explosion at the levels of 13.8 kPa and 82.7 kPa, causing eardrum damage and human lung damage, respectively [48]. In the case of partial and complete damage to an installation, the resulting hazard zones are the same. In the case of an explosion of 1 m³ of coke oven gas with different hydrogen concentrations, the hazard zones associated with human death will not occur. The results presented were obtained by using the PHAST v6.7 software [41].

The influence of selected factors on the level of hazard associated with an explosion of coke oven gas is shown in Figure 11 and Figure 12. The range of hazard zones is almost the same for both COG1 and COG2 and the case of partial and complete damage to the installation

The ranges of zones associated with the explosion of different amounts of COG and human lung damage are shown in Figure 11. The pressure of the gas in the installation is assumed to be 40 bar.

The ranges of the hazard zones for different pressures of coke oven gas in the installation are shown in Figure 12.

In the case of changes in the amount and pressure of coke oven gas in the installation, it can be seen that in the case of zones with explosion and human lung damage, a difference of 5% in the concentration of hydrogen in the coke oven gas composition results in no significant change in the range of zones.

7. Summary and Conclusions

The safety of conducting various processes in industry is one of the basic conditions that allow the development of an economy concerning the protection of humans, the environment and the climate. The improvement of the safety of all processes associated with the use of different fuels should be based, among others, on experience and on an analysis of past failures and accidents in the industry. This is no different in the case of the coking industry and the use of coke and coke oven gas.

To efficiently use coke-oven gas from a technical, economic and environmental point of view, all technologies for its current and future processing should take into account the safety aspects of its use. This is because there are many installations and factors in the industry that can lead to damage and the uncontrolled release of this gas. This situation, in turn, is a potential source of hazard to humans and the environment because of the properties of this gas. The COG contains flammable and toxic gases.

This paper focuses on the hazardous events of fire and explosion of coke oven gas with varying concentrations of hydrogen. The presented hazard zones associated with human deaths and injuries varied from 0 to 130 m. This is due both to the fact of the different hydrogen content in the COG composition and also to the different operating parameters of the installation. For the assumed conditions, the zones associated with a COG fire have reached greater ranges than in the case of an explosion. Important factors that increase the level of hazard to human health and life are also the pressure, the amount of coke oven gas in the installation and the level of damage. For example, a fourfold increase in gas pressure increased the range of the zone associated with fire and human death by about 25 m for partial damage to the installation and about 6 m for complete damage to the installation. In the case of the explosion of coke oven gas, the zones associated with human deaths did not occur. A change in the level of damage to the installation (diameter of the damage hole) from 100 mm to 400 mm increased the range of hazard zone associated with human death by about 60%, for both analyzed coke oven gas compositions.

It should be noted that the obtained coke oven gas may have a different composition. The operating parameters of the installations using COG will also be different. The ambient conditions in the vicinity of potential failure and uncontrolled release of coke oven gas may also be different. All these factors will affect the results obtained in the analyses. An important factor influencing the consequences of potential failures involving coke oven gas is safety installations, such as shut-off valves. Correct operation of safety systems can significantly affect the amount of released coke oven gas and reduce the hazardous effects on people and the environment.