Small-Scale Hydrogen Production

5.9 Conclusions

Various stages of hydrogen production using a wide variety of feedstocks (hydrocarbons, ammonia, methanol, ethanol, etc.) in micro-reactor based systems have been successfully demonstrated. Significant advances in manufacturing and design of microengineered systems over the last two decades have generated and commercialized a plethora of micro-devices. This novel way to conduct chemical processing promotes enhanced mass and heat transport rates, due to reduced dimensions of microchannels leading to considerable process intensification. Several established micro-reactor suppliers, such as Heatric, Velocys, and IMM, make micro-processing a reality and enable the mass production of hardware tailored for hydrogen production for small-scale stationary and mobile applications. Despite significant progress though, small-scale hydrogen production at an effective cost remains difficult to achieve. The diseconomies associated with scaling down still continue to be a significant challenge.

2.5.1 Present situation

Although micro-scale hydrogen production systems are being developed for domestic use by some companies, it is likely that a distribution network of hydrogen would be required to supply hydrogen to end-users. Small-scale hydrogen production from electrolysis or biomass would be possible but large plants would otherwise be required for fossil fuel feedstocks so that the CO₂ by-product could be captured and stored. In the short-term, small-scale SMR could also be deployed at refuelling stations to facilitate the transition to a hydrogen economy but these would not be compatible with decarbonization in the long-term (Usher & Strachan, 2012).

Hydrogen consumption is expected to grow rapidly because of the development of fuel cells, and specifically for the transport sector. As stated previously, SMR, coal and residues gasification are mature technologies for H₂ production. However, as long as natural gas remains at low or even moderate cost, SMR will continue to be the technology of choice for massive H₂ production. This trend is expected to continue because of the rapidly growing interest in fuel cells in stationary and mobile applications. Accordingly, distributed hydrogen production via small-scale reforming at refueling stations could be an attractive near to mid-term option for supplying hydrogen to vehicles. A brief account of the present status and/or commercialization of H₂ production technologies based in fossil precursors have been examined in some detail (Navarro et al., 2007). Hydrogen is relatively difficult to store and transport in comparison with petroleum fuels. Hydrogen gas has two principle drawbacks: (1) the unusually low volumetric energy density of gaseous hydrogen means that the gas must be compressed to extremely high pressure to be used as a transport fuel and (2) the tiny molecules have a higher propensity to leak than other gases and require particularly complex storage materials. One method of avoiding these two difficulties is to compress the hydrogen into a liquid, but this is energetically expensive and difficult to handle because liquid hydrogen boils at around 20 K. Other forms of hydrogen, for example metal hydrides, are being researched but are at an early stage of development. Three methods are commonly used to deliver hydrogen to refuelling stations (Yang & Ogden, 2007). Pipelines to transport H₂ gas require large up-front capital investments but can transport large amounts of hydrogen very cheaply over short and sometimes long distances. The most cost-effective method of delivering small amounts of H₂ gas over short distances is using gaseous tube trailers, but for longer distances, liquid H₂ delivery by road tanker becomes optimal.

Refineries, chemicals and petrochemicals industries consume large amounts of hydrogen with a large-scale production and supply infrastructure. Regarding transportation of hydrogen, along with conventional means such as high-pressure or cryogenic tankers as well as cylinders, transportation via pipelines has been employed to cater to a specific range of mass-consuming users. The main difference in the delivery and supply aspects for hydrogen as a source of energy and hydrogen as an industrial gas lies in the fact that the former element has to deal with supplying an unspecified range of general consumers. For this reason, successful dissemination of the new technology necessarily requires not only development of technologies for producing and supplying hydrogen as energy but also examination into methods of transportation for its supply. From these points of view, after the turn of the century, various projects have been launched in Europe and the USA for technical as well as economic examinations regarding a method of transporting hydrogen by mixing it into the existing natural gas pipeline system wherein hydrogen is separated at the end users.

2.5 Hydrogen infrastructures and distribution

5.8 Future trends

The key issues to yielding success in the field of small-scale hydrogen production are related to the need to achieve simultaneously high-efficiency, high reliability, and high-durability for the alternative FPS that can be mass produced (like an appliance) at a very low cost. Regardless of the raw material used as feed, hydrogen production remains a complex process. Stages for feed purification, conversion of the hydrogen containing feed (hydrocarbon, ammonia, methanol), and product purification must be integrated in a compact structure. Even if significant advances have been made for fuel processing to generate syngas using innovative micro-reactor technologies, solution for efficient additional purification stages are still sought. Natural gas and hydrocarbons remain the most accessible source of hydrogen generation. In this case feedstock handling, and especially sulfur removal, is a significant cost increase contributor. At the same time, the quality of hydrogen fuel for fuel cell applications is quite stringent; usually, a low CO content is needed. While inert constituents, such as N₂ and CO₂, do not adversely affect fuel cell performance, substantial concentrations of inert diluent is undesirable, especially when hydrogen storage by compression and liquefaction is the ultimate goal. On-site hydrogen generation remains controversial. The argument for decentralized hydrogen production, advertised as zero emission fuel, implies that the CO₂ generated as a by-product when hydrocarbon feed is used must be captured and stored efficiently. Unfortunately, CO₂ capture (Kuramochi, 2012) remains expensive and cost prohibitive at any scale. From the operational point of view the argument of generating hydrogen ‘when it is needed’ implies a process design with high degree of flexibility with quick start-up capabilities and throughput flexibility to adjust to demand variability. This, in turn, requires advanced control systems and instrumentation, which only adds to the unit costs. To make decentralized hydrogen production competitive, the future focus should target cost reduction (O’Connell et al., 2012) for both fabrication techniques, and for catalyst selection (reduction in precious metal content is desirable). The necessity for market penetration at a faster pace remains critical to promote an increase of production volumes with the eventual goal of mass production, which will drive the fabrication costs down.

Despite significant investment, and developments in automotive industry for FCVs, hydrogen utilization for this purpose is still modest, mainly due to lack of either an efficient and cost-effective hydrogen distribution network, or decentralized hydrogen production. Still, other market niches are emerging (e.g. APU for recreational vehicles; hydrogen power fuel cells for forklifts driven by environmental and health regulations). Recently, the development of shale gas production may drive the need for novel natural gas processing at small scale using innovative solutions that go beyond conventional practice.

Small-scale hydrogen production

Delivery pathway 19

The delivery pathway 19 in Fig. 11 considers only the direct use of hydrogen from a small-scale hydrogen production plant through a small capacity G.G refuelling station for vehicles [103–106,115].

Fig. 11. Delivery pathway 17, delivery pathway 18 and delivery pathway 19.

Hydrogen production

The high temperature H₂/CO₂ separation membranes are also suitable for hydrogen production. Realizing the vision of ‘hydrogen economy’ and utilizing hydrogen as an ‘energy carrier’ will require increasing hydrogen production by more than an order of magnitude over the current production levels. About 42 Mt of hydrogen is currently produced per year worldwide. Most of the hydrogen is currently used captively on-site in ‘non-energy’ uses for the production of ammonia, methanol, and other chemicals and for hydro-processing in petroleum refineries primarily to remove sulfur and to upgrade the heavier fractions into more valuable products (ORNL, 2003). For utilization in hydrogen-powered cars, almost 40 Mt per year of hydrogen will need to be produced to be able to support 100 million cars in the US alone (US DOE, 2002). The demand for hydrogen is expected to escalate even more as hydrogen is used as an energy carrier for power generation applications for example, portable and/or distributed power. Both central as well as distributed hydrogen generation plants will be necessary to meet the projected hydrogen demand.

The potential demand for hydrogen may be determined from the projections for sales of hydrogen powered vehicles as well as for stationary and portable power systems using hydrogen PEM fuel cells. Hydrogen powered vehicles are currently expensive and they need a hydrogen infrastructure in place for supporting sales creating a ‘chicken and egg’ dilemma. Yet, Germany, the US (California), and Japan have recently announced plans for more than 200 hydrogen fueling stations between them by 2016 in anticipation of the commercial release of fuel cell electric vehicles (Fuel Cell Today, 2012). In the near future, gasoline hybrid vehicles are expected to replace current gasoline vehicles. However, ultimately, hydrogen fuel cell electric vehicles are projected to dominate as seen in one of the projections for new car sales in Fig. 1.2 (Thomas, 2011). The projections indicate that about 50 000 fuel cell electric vehicles will be on the road in the world in 10 years reaching a total of 1 million fuel cell electric vehicles on the road by year 2030. To support 1 million hydrogen fuel cell electric vehicles about 0.5 Mt of hydrogen supply would be needed per year in 2030 or about 1000 fueling stations providing 1500 kg/d hydrogen output. Again, assuming an average hydrogen flux of 100 scfh/ft² (30.5 std. m³/m²–h) approximately 250 ft. (23.2 m²) membrane area would be needed for an on-site distributed hydrogen production plant for each fueling station. With the US DOE office of Energy Efficiency and Renewable Energy year 2015 cost target for fully assembled membrane modules for distributed hydrogen fueling stations of $500/ft² ($5382/m²) (US DOE EERE, 2011), the market size for this opportunity is approximately $125 million in the year 2030. However, as seen in Fig. 1.2, fuel cell electric vehicles sales are projected to increase exponentially beyond 2030 in later years increasing the market size for membranes for hydrogen production also exponentially beyond 2030.

1.2. Projected distribution of new car sales. Gasoline internal combustion engine vehicles (ICVs), gasoline powered hybrid electric vehicles (HEVs), (cellulosic) ethanol-powered plug-in hybrid electric vehicles (PHEVs), and hydrogen powered fuel cell electric vehicles (FCEVs).

(Source: Thomas, C., ‘Making the case for hydrogen and fuel cell electric vehicles’, Proceedings of the 2011 Fuel Cell Seminar, Orlando, Florida, 3 November 2011.)

The commercial SMR process with PSA-based hydrogen purification has become a well established industry standard process for hydrogen production. Utilization of Pd-based membranes for hydrogen production from natural gas or coal derived syngas is still being demonstrated at a small scale. The technical feasibility of these membranes in regard to long-term durability and thermal stability in synthesis gas environments must be demonstrated for their commercial utilization. These membranes also need to exhibit sufficient economic advantage over PSA-based hydrogen production process. Membranes provide advantages of simplicity of operation and lower operating costs compared to a PSA-based system. However, one drawback of the Pd-based hydrogen selective membranes is that the product hydrogen is produced at a lower pressure. Thus, these membranes will have advantage in situations requiring CO₂ capture at high pressure while having the ability to utilize hydrogen at a lower pressure for power generation for example, in fuel cells.

Another application of H₂/CO₂ separation membranes is for hydrogen purification in small hydrocarbon (typically methanol) reformer-based hydrogen generators coupled with PEM fuel cells for small stationary power systems. A PSA-based system may prove cumbersome for such small scale hydrogen production applications, whereas, a membrane system may scale down quite well. Annual shipments of fuel cell systems for stationary power installations grew substantially from 2010 to 2011, over all categories. This application includes large stationary systems (generally over 100 kW) for prime power applications, as well as smaller units (below 50 kW and usually below 10 kW) for micro-CHP and uninterruptible power supply (UPS). In 2011 the number of stationary systems shipped during the full year increased to over 16 000 and the number of megawatts at over 81 MW, up from 8300 and 35 MW in 2010, increases of 94% and 133%, respectively (Adamson, 2009; Fuel Cell Today report, 2012). Pd-alloy foil and tube-based hydrogen purifiers are commercially used in reformer-based hydrogen generator systems.

2 Hydrogen production processes

Hydrogen production processes include all approaches and technologies designed to produce hydrogen [12]. At present, the vast majority of hydrogen, approximately 99%, is generated using fossil fuel methods, with 76% originating from natural gas and 23% from coal, while the remaining portion is generated through biomass and water electrolysis [11]. This section is devoted to presenting the primary features, pros, cons and challenges associated with the main mature methods currently used for producing hydrogen, including SMR, biomass gasification, coal gasification and water electrolysis processes.

2.1 Steam methane reforming

SMR is a widely utilized thermochemical procedure for producing gray hydrogen [30,31]. It consists of converting natural gas, which is the leading feedstock of methane, and steam (i.e., H₂O and heat) into synthesis gas. The latter is a combination of hydrogen and carbon monoxide (CO) that can serve as a vital feedstock for several chemical and petrochemical operations. Indeed, using a catalyst based on nickel, methane reacts with steam through the endothermic reaction, as outlined in Eq. (1) [32]. The amount of methane conversion can be increased by elevating the temperature and steam-to-methane ratio in the feed, whereas it diminishes with increasing pressure [33].

(1)$C{H}_4+H_{2}O\rightleftharpoons CO+3H_2$

After the reforming step, the released syngas contains an insufficient quantity of hydrogen compared to CO. To augment the produced amount of hydrogen, the syngas is passed via an exothermic water-gas shift reaction to transform CO into hydrogen and CO₂ (Eq. (2)). In this reaction, the combination of CO and steam results in the generation of more hydrogen and CO₂ [32].

(2)$CO+H_{2}O\rightleftharpoons C{O}_2+H_2$

Fig. 3 exhibits a simplified flow diagram of the hydrogen production process using SMR technique with CCUS. Although this process produces CO₂ as a byproduct, it is highly efficient for producing enormous quantities of hydrogen and syngas, making it suitable for large-scale hydrogen production. Nonetheless, it is vital to incorporate CCUS methods within the SMR process in order to capture CO₂ emissions instead of releasing them into the atmosphere, thereby performing an essential role in enhancing the environmental performance of this process [34]. As reported by the international energy agency (IEA), implementing CCUS techniques in the SMR process can alleviate carbon emissions by around 90% [11]. In addition, efforts are being made to develop more sustainable and environmentally friendly alternatives for hydrogen generation, such as the utilization of biomass gasification and water electrolysis techniques powered by renewable energy sources (RESs).

Fig. 3. Flow diagram of hydrogen production via SMR method with CCUS.

2.2 Biomass gasification

Biomass gasification is a thermochemical reaction that converts biomass materials into a versatile gas mixture known as syngas. The process involves exposing the biomass to high temperatures within a controlled environment where oxygen or air access is restricted. First, the biomass passes through a drying process to reduce moisture content, and then pyrolyzed at high temperatures, resulting in the formation of volatile compounds, coal and bio-oil. These products are then introduced into the gasifier, where partial oxidation reactions take place in an oxygen-deficient environment, leading to the creation of syngas. The latter is principally composed of CO, hydrogen (often referred as biohydrogen), CO₂, methane and other gas impurities. This syngas can be harnessed in various applications like power generation, heating, biofuels, and chemicals and materials production. The global reaction and a simplified scheme of hydrogen production using biomass gasification are highlighted in Eq. (3) and Fig. 4, respectively.

Fig. 4. Flow diagram of hydrogen production via biomass gasification with CCUS.

(3)$Biomass+H_{2}O+O_2\rightleftharpoons CO+C{O}_2+C{H}_4+H_2+Othergases$

The efficiency of the biomass gasification process depends on several elements such as the type and size of biomass, operating temperature, type of carrier gas and pretreatment conditions [35]. For example, minimizing the particle size results in increased contact surface area between the biomass and the gasification agents, which speeds up the chemical reactions and rises the overall rate of the gasification reaction [36]. Alleviating the tar content resulted from the biomass gasification process is also a key solution to augment the productivity of this process [37]. In this regard, implementing catalysts, such as dolomite, olivine, alkali and alkaline earth metallic species, is a promising approach to mitigate tar formation and rise the gasification process efficiency [38,39]. Nevertheless, their relatively high cost and non-recyclability are the major obstacles to the widespread integration of these catalysts. Therefore, it is imperative to stimulate further research and collaborative efforts in this area to effectively reduce the costs associated with the basic materials required for gasification operations. Additionally, exploring innovative approaches to recycling materials should be a key focus of these efforts.

3.2.2 Progress in hydrogen production technology

Under anaerobic conditions, cyanobacteria and green algae decompose water through photosynthesis to produce hydrogen and oxygen, which is a way for photosynthetic organisms to produce hydrogen. In this photosynthetic system, there are two independent but coordinated photosynthetic centers: photosystem II (PS II), which receives solar energy to decompose water to generate H⁺, electrons, and O₂, and photosystem I (PS I), which generates a reductant to fix CO₂.The electrons produced by PS II are carried by ferriredox protein through PS II and PS I to hydrogenase, and H⁺ forms H₂ under certain conditions under the catalysis of hydrogenase (Ramachandran and Menon, 1998). Hydrogenase is the key factor of hydrogen production in all organisms. Green plants cannot produce hydrogen because they do not have hydrogenase, which is an important difference between algae and green plants in the process of photosynthesis. Therefore, in addition to the formation of hydrogen, the photosynthetic law and research conclusions regarding green plants can be used to analyze the algae metabolism process. Benemann studied the mixed hydrogen production pathway of green algae. Green algae were cultured in an open pond to store carbohydrates (biomass of green algae) in CO₂, and then the cultured green algae were transferred into a dark and airtight anaerobic fermentation vessel for hydrogen production (Benemann et al., 1973). Belkin et al. isolated Chromatium sp. Miami pbs1071 and found that it is the fastest marine photosynthetic microalgae they had ever seen, with a doubling time of only 1.75 h. The study found that it could not use carbohydrates, but it could use a variety of other carbon and nitrogen sources for growth and reproduction (Belkin and Padan, 1978). Sasikala et al. studied the growth stage of Rhodobacter sphaeroides O.U.001, the pH value of hydrogen production matrix, and the relationship between glutamic acid content and the hydrogen production rate. The results showed that the static stage of bacterial growth was favorable for hydrogen production, and the pH value and glutamic acid content had a great influence on the hydrogen production rate and hydrogen production (Sasikala et al., 1995). At the same time, the researchers studied the relationships among light intensity, cell growth rate, and hydrogen production. The results showed that the growth and hydrogen production of cells were not inhibited by high light intensity, which was different from that of green algae. Many studies showed that the main obstacle to continuous hydrogen production is the simultaneous production of H₂ and O₂ by algae. Hydrogen-producing enzymes are extremely sensitive to oxygen, while the activity of hydrogen absorbing enzymes is not affected by O₂. Gaffron et al. found that green algae may have higher hydrogen production efficiency than cyanobacteria, because the nitrogen enzymes of cyanobacteria need the participation of energy carrier adenosine triphosphate to work (Gaffron and Rubin, 1942). There are many advantages in producing hydrogen from photodegradation water: only water is the raw material, the solar energy conversion efficiency is about 10 times higher than trees and crops, there are two photosynthetic systems, and so on, but there are also many disadvantages, such as the inability to use organic matter, the inability to use organic waste, the need for light, the need to overcome the inhibition effect of oxygen, the low efficiency of light conversion, the maximum theoretical conversion efficiency of 10%, and the complex photosynthetic system. The free energy needed to be overcome for hydrogen production is higher, which affects the development of photolysis water biohydrogen production technology.

The technology of combined light and dark fermentation hydrogen production has many advantages over one method alone. This technology includes the combined production of hydrogen by photosynthetic organisms and dark fermentation organisms, the two-stage combined production of hydrogen by dark and light fermentation, and the multistage combined production of hydrogen. The combination of the two fermentation methods can increase hydrogen production. Hydrogen production by photosynthetic and dark fermenting organisms is a technology that combines photosynthetic organisms such as algae, cyanobacteria, photosynthetic bacteria, and dark fermenting bacteria. Its chemical equation is: C₆H₁₂O₆ + 6H₂O → 12H₂ + 6CO₂. This technology can improve the conversion efficiency of light energy and the use efficiency of substrate as well as reduce the toxicity of volatile fatty acids to bacteria, so as to increase hydrogen production, and it is possible to achieve the complete degradation of organic matter and sustained and efficient hydrogen production. However, growth, the optimal pH value of hydrogen production, and the demand for light of the two kinds of bacteria in the combined hydrogen production technology are different, which limits the development and application of the technology to a certain extent.

The two-stage combined biohydrogen production technology of dark and light fermentation is a biohydrogen production technology that couples dark and light fermentation. The end products of dark fermentation are mostly small organic acids and alcohols such as acetic acid, ethanol, and butyric acid, which can be used by photosynthetic bacteria. The combination of the two fermentation methods can greatly improve the use efficiency of the substrate, increase hydrogen production, and realize the efficient degradation of organic matter. However, it is a difficult problem to select a light fermentation strain that can use the end products of the dark fermentation liquid phase, and it is also an important factor to restrict the cumulative hydrogen production of combined hydrogen production technology.

Multistage combined hydrogen production technology is an attempt to realize large-scale industrial production; it is based on two-stage combined hydrogen production technology, adding the enzyme hydrolysis process to improve the application scope and use efficiency of substrate.

The development of light and dark fermentation combined hydrogen production technology is a contentious and difficult point at home and abroad. It is a necessary stage of large-scale and continuous production and a key factor in promoting the development of biohydrogen production technology.

Hydrogen options

The studies reviewed are inconclusive with regards to the most appropriate pathway or steps for the UK to undertake in its journey to achieving its 2050 transport decarbonising targets. It is more likely that localised small-scale hydrogen production will suffice initially, perhaps from chemical processing plants or on-site production until the demand increases. Furthermore, the technique used to produce hydrogen will play a promising role in determining the timescale of when the hydrogen infrastructure will be installed. However, it is clear that no single hydrogen production method will be sufficient to produce enough hydrogen to fulfil the expected demand on its own.

From the hydrogen production options available, SMR is the cheapest while producing the least $C{O}_2$ emissions compared to the rest. The second most attractive option is coal gasification, which when utilised with CCS is both economically and environmentally attractive. For remote locations, water electrolysis is the most attractive option. It is inevitable that hydrogen will be produced on-site initially and localised production technologies such as electrolysis will play a crucial role in introducing hydrogen for early market adoption and low populations. However, introducing hydrogen into high populous areas will reduce the infrastructure costs with on-site production reducing distribution costs in contrast to centralised production. As hydrogen demand increases, incremental capacity can be added to increase the capacity as required. This will help keep costs low with the uptake of HFCVs increasing.

Liquefied hydrogen is most economic using large power plants and with dispersed hydrogen demand [64]. Hydrogen produced from renewable energy will be a crucial role in reducing global warming impacts and fuel consumptions [63,92], alongside transportation of hydrogen to it [44,45]. Availability of biomass for hydrogen production alongside CCS are also critical in achieving low costs and emissions [49]. But hydrogen from non-renewables is expected to dominate initially because of cost and infrastructure being in place already. Policies and $C{O}_2$ targets will help the transition from fossil-based hydrogen to green hydrogen. Furthermore, cost reductions in renewable technology achieved by mass production, feedstock costs and availability will strongly influence the cost of green hydrogen.

Large power plants are not required initially due to high costs in transporting the fuel to the required locations [93], however centralised production plants are an ideal route to the hydrogen economy [32] and perhaps ideal at low market penetration [61]. Industrialised hydrogen will also play a role in initiating the transition to a hydrogen economy with onsite SMR supporting the demand before moving to more centralised production [27].

In terms of distribution, huge investments are required before a sizeable infrastructure is in place to maintain the demand expected. There are many options available to transport hydrogen depending on the volume of hydrogen, delivery distances etc. compressed gaseous and liquid hydrogen can be distributed by trucks and rail with gaseous hydrogen through a pipeline infrastructure. However, the cost of building a centralised infrastructure for distribution such as pipelines has high capital costs [70]. A localised distribution infrastructure will initiate and maintain the hydrogen local hydrogen until it is seen viable and profitable before governments and stakeholders will invest in a more centralised infrastructure. Investing in a pipeline distribution is ideal for low market penetration [61], while truck and rail delivery will offer a competitive option in the UK [21,32].

Having an adequate refuelling infrastructure in place alongside the deployment of HFCVs in the market will meet the expectations of potential customers. The hydrogen infrastructure required is expected to be similar to the current infrastructure [22] where customers can expect the deviation time to be similar to the current reported time in order to refuel a HFCV at one of the proposed 68 stations [69]. However, in a different study it was found that only 11–14% of the hydrogen refuelling stations can provide comparable accessibility to drivers compared to gasoline [68]. Introducing hydrogen in clusters is the most effective strategy for an efficient design [55,62], as this and longer vehicle ranges will reduce the need of having many refuelling stations [26].

From the literature, it can be determined that initially the infrastructure will have to be developed alongside HFCVs in the UK, and most likely with the involvement of the vehicles manufacturers themselves. Once the number of HFCVs on the road increases, then more investment into the infrastructure will become necessary exceeding the rate of HFCV growth. Both, renewable and non-renewable energy sources will be utilised until sufficient renewable plants are in place to accommodate the hydrogen demand. So, therefore, beyond the initial period, the growth rate of infrastructure will have to be significant to attain long-term sustainability in the UK's road transport network.

9.3.1 Hydrogen generation processes

Hydrogen is an effective medium for energy storage, the use of which provides many advantages in FCs for stationary, transportation, and portable power applications. Hydrogen can be obtained through various processes from a number of sources, both renewable and nonrenewable. Global hydrogen production has so far been dominated by fossil fuels, with steam reforming of hydrocarbons (e.g., natural gas) being the most important new technologies. Electrolysis of water, an energy-demanding process, also produces pure hydrogen. Another process of hydrogen production is from biomass. Fig. 9.11 provides a summary of the methods used for the production of hydrogen along with their primary energy and material sources.

Figure 9.11. Overview of hydrogen production methods.

9.3.1.1 Hydrogen generation from fossil fuel

Conventionally large scale hydrogen production in industries is performed from fossil fuels. Natural gas reforming, coal gasification, and partial oxidation (POX) of hydrocarbons (natural gas or heavy oil) are several industrial processes to generate hydrogen from fossil fuel. These three processes are briefly explained later.

9.3.1.1.1 Hydrogen from natural gas methane-steam reforming

Natural gas reforming is the most prominent method of hydrogen production all over the world. The technology is currently mature and industrially adopted in a mass scale, especially for power plant applications. The main component of natural gas is methane (CH₄), using thermal processes like steam reformation, hydrogen can be produced from it [18]. Fig. 9.12 shows a simplified diagram of hydrogen production process from methane-steam reforming [19].

Figure 9.12. Simplified flow diagram of methane-steam reforming for hydrogen production.

In methane-steam reforming process, there are three major steps: (1) syngas generation, (2) water–gas shift, and (3) hydrogen purification. Apart from these steps, there are several additional steps such as desulfurization and heat recovery, which are necessary to maintain purity, temperature, and pressure of the gas. First, the methane from natural gas is desulfurized and then fed to the reformer. In the syngas generation process, high temperature (700°C–1000°C) steam reacts with the methane under 3–25 bar (43.5–362.5 psig) of pressure. The output of the reaction is hydrogen and carbon monoxide (CO). This reaction is an endothermic reaction, which implies that to proceed the reaction to generate hydrogen heat must be supplied. The CO generated from the reforming reaction is then involved in another reaction called “water gas shift reaction,” where CO reacts with steam and generate carbon dioxide (CO₂) and additional hydrogen. Small amount of heat is generated in this stage of the reaction. The final stage of the process is named as “pressure swing adsorption.” In this stage, the impurities and byproduct gases are eliminated from the gas stream and pure hydrogen is collected at the end. This process is also applicable for hydrogen production from ethanol, propanol, or even gasoline. The chemical reactions that take place in different stages are given as follows:

Steam-methane reforming reaction:

(9.24)${\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}\left(+\mathrm{heat}\right)\to \mathrm{CO}+3{\mathrm{H}}_2$

Water–gas shift reaction:

(9.25)$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\left(+\mathrm{small}\mathrm{amount}\mathrm{of}\mathrm{heat}\right)$

9.3.1.1.2 Hydrogen from hydrocarbon partial oxidation

POX is another well-known method to produce hydrogen from fossil fuel. Hydrocarbon-based fossil fuels such as natural gas, coal, and heavy oil can be converted in to hydrogen from this method. There are two types of POX: catalytic that occurs at around 590°C and noncatalytic that occurs at around 150°C–1315°C. Noncatalytic POX is ideal for hydrogen generation from heavy hydrocarbons that cannot react over catalyst instantly. This process is also called as gasification when coal is the source. A simplified diagram of POX process to generate hydrogen is shown in Fig. 9.13 [19]. The fuel used in the diagram is heavy oil. Thus several additional steps are added to manage the byproducts.

Figure 9.13. Simplified flow diagram of partial oxidation of hydrocarbon.

There are three principle steps of the process: (1) syngas generation, (2) water–gas shift reaction, and (3) hydrogen purification. The basic idea of syngas generation step is to oxidize the hydrocarbon with limited supply of oxygen, so that the hydrocarbon does not get fully oxidized. The product of this reaction is hydrogen and carbon monoxide (CO). If the oxidization is performed with air instead of pure oxygen, as byproduct nitrogen, small amount of carbon dioxide, and other components can be generated. In the next step of the process named as “water gas shift”, the CO reacts with water and produces more hydrogen and carbon dioxide. The total process is exothermic, which means heat is generated as a byproduct of the reactions. In the final step, hydrogen gas is purified from CO₂ and other components before collection. The chemical reactions are given as follows:

POX reaction:

(9.26)${\mathrm{CH}}_4+1/2{\mathrm{O}}_2\to \mathrm{CO}+2{\mathrm{H}}_2\left(+\mathrm{heat}\right)$

Water–gas shift reaction:

(9.27)$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\left(+\mathrm{small}\mathrm{amount}\mathrm{of}\mathrm{heat}\right)$

9.3.1.1.3 Hydrogen from coal gasification

Coal gasification is another way of hydrogen production using fossil fuel. The conventional technology is environmentally harmful; however, with carbon capture, storage, and utilization technology coal gasification process can be improved and vastly used for hydrogen production [20]. Coal has a very complex chemical property and it can be converted into various type of fuels. It can be converted into a range of liquid or gaseous fuels by using coal gasification. A simplified diagram of industrially accepted process of coal gasification is shown in Fig. 9.14 [19]. The process is adopted by Kopper–Totzek, thus named as Kopper–Totzek (K–T) gasifier.

Figure 9.14. Simplified flow diagram of Kopper–Totzek coal gasification for hydrogen generation.

In this process, the crushed coal is partially oxidized by steam and oxygen at atmospheric pressure in the K–T gasifier. The oxygen is extracted by the air separation process. The raw gas produced in the gasifier is then cooled and quenched with water to remove the ash particles from the raw gas. The purified gas also called as syngas is then passed through the compression chamber, shift conversion chamber, and purification chamber. At the output hydrogen is collected at about 2.8 MPa (400 psig) pressure and of purity greater than 97.5%. The chemical reaction associated with the process is shown as follows:

Coal gasification reaction:

(9.28)${\mathrm{CH}}_4+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CO}+{\mathrm{CO}}_2+{\mathrm{H}}_2+\mathrm{other}\mathrm{species}$

The pressure of the produced hydrogen is not enough for general use. Thus it needs to compress further to store in hydrogen tanks. Furthermore, coal gasification process involves handling of rock-solid coals and removal of large amount of coal ash. These processes have a significant economic impact on the production cost. All these limitations make coal gasification less favorable than the other liquid or gaseous hydrocarbon-based hydrogen generation processes.