The global hydrogen production is currently around 50 million tons per years, with an annual growth rate of about 10%.
Hydrogen can be produced from a variety of feedstocks and processes. Main hydrogen sources include hydrocarbons, water and biomass. Steam reforming, partial oxidation (POX), autothermal reforming (ATR) and electrolysis are the most common and well-developed hydrogen production technologies.
Currently, more than 95% of the world’s hydrogen is produced by hydrocarbons reforming. Although water electrolysis is a mature and well-known technology, which has been industrially used for over 100 years and which has the advantage to produce extremely pure hydrogen, it only contributes to 4% of the worldwide hydrogen production. More specifically, hydrogen from water electrolysis is used in unique situations, when hydrogen from large scale production plants is not available (space crafts, rockets, marine) or is too expensive (medical applications, electronic industry, food industry).
For large-scale hydrogen production, conventional reforming technologies are the preferred solutions. Hydrogen produced with these processes is used for hydrotreating in refineries and for the production of ammonia and methanol in chemical industries.
Steam reforming is the most cost-effective and worldwide method for industrial hydrogen production. It only uses light hydrocarbons, such as methane and naphtha, and does not require oxygen. When methane is used as feedstock, the process is referred to as steam methane reforming (SMR).
In the steam reforming process, the fuel reacts with steam in presence of a catalyst at around 800°C to produce a mixture of hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2) called syngas (from synthesis gas). As the process is usually endothermic, heat generally needs to be added to the reformer unit. The process efficiency is normally about 75% and goes up to 80-85% in large-scale production plants. When the feedstock includes sulfur or other impurities, a cleanup pretreatment is required.
Partial oxidation is an exothermic process in which light and heavy hydrocarbons are converted to hydrogen by a partial combustion of the feedstock with oxygen. For this aim, the oxygen present in the system is in a sub-stoichiometric amount.
Partial oxidation can be operated with or without a catalyst. In the first case, the operating temperature is about 800-900°C and the process is referred to as catalytic partial oxidation (CPO). In the second case, the operating temperature is around 1300-1500°C and the process is referred to as non-catalytic partial oxidation or thermal partial oxidation (TPO). However, both CPO and TPO are less efficient than steam reforming.
Coal gasification, which is the second most important and widespread hydrogen production method, entails a controlled partial oxidation of coal. This process includes a series of reaction steps that convert coal into a syngas composed of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), oxides of sulphur (SOx), hydrogen sulfide (H2S) and nitrogen (N2). The products of primary coal gasification also include small amounts of liquids and solids.
The conversion of coal into syngas is obtained by introducing steam and oxygen (or air) into a reactor vessel containing coal feedstock in the presence of a catalyst at high temperatures. Generally, the following reactions occur simultaneously along with partial oxidation: steam gasification, carbon dioxide gasification, hydrogasification and water gas shift (WGS) reaction. Partial oxidation, which is exothermic, provides the heat required for the gasification reactions.
The composition of the product gases depends on the type and composition of the coal, the relative amounts of steam and oxygen introduced into the gasifier and the process parameters (temperature and pressure). At the end of the main process, downstream processes are performed according to the final application of the syngas.
Besides as a hydrogen feedstock, coal gasification syngas can be used as a substitute of natural gas for pipeline applications, as a fuel for electricity generation via integrated gasification combined cycles (IGCC) or as a chemical feedstock after conversion to liquid hydrocarbon via the syngas/Fischer-Tropsch route.
Autothermal reforming is a process in which a light hydrocarbon reacts with both steam and oxidant. It can be considered a combination of steam reforming and POX. The reaction with steam is endothermic whereas the partial oxidation reaction is exothermic. The total reaction is exothermic, so that no heat needs to be supplied to the system.
Water electrolysis is a process in which water is split into hydrogen and oxygen through the application of electrical energy. In operational terms, a direct-current (DC) electrical power source is connected to two inert metal electrodes, called anode and cathode, immersed in an electrolyte. When the power source is on, electrochemical reactions take place on the surface of the electrodes. More precisely, a reduction reaction occurs at cathode where hydrogen is produced, and an oxidation reaction occurs at anode, where oxygen is obtained as a useful byproduct.
The two metal electrodes, together with the electrolyte, form an electrochemical cell. An assembly of multiple electrochemical cells form an electrolyzer stack. One or more stacks form an electrolyzer module. Finally, a single or multiple modules form an electrolysis unit.
According to the type of connection between the cells, electrolyzers are classified as monopolar or bipolar. In the monopolar arrangement, the cells are connected in parallel so that the total module voltage is equal to the individual cell voltage. In the bipolar configuration, the cells are connected in series and the total voltage is the sum of the cell voltages.
Electrolyzers can be also classified according to electrolyte type used, operating temperature and operating pressure. Typical efficiencies of commercial electrolyzer units are around 70%. This figure includes the energy required to compress the product gas.
When water is split by electrolysis, electrical energy is converted into chemical energy in the form of hydrogen. Like electricity, hydrogen energy can be easily stored and transported over long distances.
Besides electrolysis, other hydrogen production methods involve water-splitting such as photo-electrolysis, thermochemical cycles and biophotolysis.
In the photo-electrolysis process, sunlight is used to split directly water into hydrogen and oxygen within a single device named photo-electrolyser. Basically, a photo-electrolyser is composed by a set of photoelectrochemical cells (PECs) having a semiconductor photoelectrode and a counter-electrode. When exposed to sunlight, the photoelectrode operates as a photovoltaic cell, generating enough electrical energy to promote hydrogen evolution at the cathode and oxygen evolution at the anode.
Thermochemical water splitting cycles use high temperature heat (up to 2000°C) to drive multiple chemical reactions that have the net effect of decomposing water into hydrogen and oxygen. The reactions produce a closed loop in which the chemical compounds are continuously reused. A sustainable way to provide the process heat is through a field of mirror heliostats that concentrate sunlight on a receiver/reactor placed at the top of a central tower.
In the biophotolysis process, light energy is directly or indirectly converted to hydrogen through photosynthesis of blue-green algae, a group of unicellular nitrogen fixing bacteria containing hydrogenase or nitrogenase, two enzymes capable to catalyze hydrogen formation. The main advantage of this method is that it only utilizes sunlight and water, which are no/low-cost and available almost everywhere. Unfortunately, in addition to hydrogen, oxygen is also produced, which inhibits the hydrogenase activity leading to a reduction in hydrogen production rate. This problem is attenuated when hydrogen evolution and oxygen evolution are separated spatially or temporally (indirect biophotolysis).
Biophotolysis along with photofermentation and dark fermentation is one of the most promising metabolic biological processes for hydrogen production.
Photofermentation is a process in which photosynthetic bacteria, such as purple non-sulfur (PNS) bacteria and green sulfur (GS) bacteria, use light energy and the nitrogenase enzyme to convert organic acids to hydrogen. A major advantage of this method is that organic wastes can be used as substrate. The main disadvantage is that it has a very low light conversion efficiency, which does not exceed 5%.
Dark fermentation is a metabolic process in which some species of anaerobic bacteria produce hydrogen together with low molecular weight organic acids by digestion of organic matter, in the absence of light. Unlike other methods, such as electrolysis or biophotolysis, dark fermentation processes do not yield pure hydrogen, but a mixed biogas containing hydrogen (H2), carbon dioxide (CO2) and small amounts of carbon monoxide (CO) and methane (CH4).
At present, metabolic hydrogen production processes are yet under research and their technology needs further development.
Hydrogen from biomass/biofuel
Besides metabolic processes, biological processes also include the generation of hydrogen from biomass used as a fuel (biofuel). The term biomass refers to organic material associated with any living organism, including vegetable matter (such as wood from varies sources and agricultural residue), animal tissue, manure and human waste. In the near term, biomass represents the most likely renewable organic substitute of the fossil fuels.
Biomass-to-hydrogen conversion can be obtained by several technologies: thermochemical biomass conversion to syngas via gasification or via pyrolysis, biomass conversion to biogas (via anaerobic digestion) to hydrogen-rich gas (via steam reforming) and direct biomass conversion to bioliquids (via fermentation or other processes) to hydrogen-rich gas (via steam reforming).
Gasification and pyrolysis are the most promising technologies for production and commercialization of hydrogen from biomass in the medium term.
Biomass gasification is a very mature technology, which is very similar to coal gasification. The core of the process is the partial oxidation of the feedstock material, which is converted into a syngas composed of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and nitrogen (N2). Net greenhouse gas emissions are rather low because the carbon dioxide released during the gasification process is largely offset by the carbon dioxide that was absorbed during the original growth of the biomass.
As for coal, biomass gasification takes place at high temperatures in the presence of a gasification medium (steam, air, oxygen or a mixture of these). The process can be performed with or without a catalyst.
The properties of the product syngas depend on the feedstock characteristics, process parameters and the composition of the gasification medium. Contaminants such as char, tar and ash are produced along with syngas. Specifically, tar formation is one of the major issues in biomass gasification as it reduces hydrogen yield. As for coal gasification, downstream processes are performed according to the final application of the syngas.
Pyrolysis is a mature technology in which the feedstock (biomass with low water content) is heated at high temperatures in absence of oxygen. At the end of the process, the organic material is converted into liquid oils, solid charcoal and gaseous compounds. Since no oxygen is present, no carbon oxides (CO and CO2) are formed unless unexpected air and/or water from too moist feedstock materials are/is present in the reactor.
Pyrolysis represents the precursor of all gasification processes and can be classified into slow pyrolysis and fast pyrolysis.
Slow pyrolysis occurs at temperatures less than 450°C and takes several hours to complete. As the main resulting product is charcoal, this process is not much considered for hydrogen production.
Fast pyrolysis is currently the most used pyrolysis system. It occurs in a time of few seconds at temperatures of about 700-800°C. The resulting products can be found in all gas, liquid and solid phases. Gaseous products include hydrogen (H2), methane (CH4) and other gases depending on the characteristics of the feedstock. Liquids products are composed of tar and oils (bio-oil). Finally, solid products include charcoal and other inert materials.
Temperature, residence time, heating rate, particle size and type of catalyst used are key control parameters in pyrolysis process. Specifically, high temperature, high heating rate and long residence time of the volatile compounds result in an increase of the gaseous products, among which hydrogen.
Bio-oil is a dark brown liquid composed of water, water-soluble compounds (acids, esters, etc.) and water-insoluble compounds (phenolic oligomers, usually called pyrolytic lignin).
The high oxygen content and the presence of water, acids, aldehydes and large molecular oligomers makes bio-oil viscose, corrosive, thermally unstable and ultimately not suitable as a direct substitute for fossil fuels. To overcome this issues, it needs to be chemically or physically upgraded. In particular, the water-soluble fraction of the bio-oil can be steam reformed over a catalyst to generate hydrogen.
Anaerobic digestion is a process in which microorganisms convert biomass into biogas in the absence of oxygen. Basically, all organic materials can be used as feedstock, including food waste, agricultural waste, animal by-products, manure, grass, waste paper, sewage sludge, etc. The process takes place in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The first two stages correspond to the dark fermentation process. In the last stage, methane-forming bacteria (methanogens) convert hydrogen and acetic acid into methane and carbon dioxide. The final biogas is a mixture of methane (CH4), carbon dioxide (CO2), nitrogen (N2) and small amounts of ammonia (NH3), hydrogen sulphide (H2S) and various other gases.
Biomass can be converted directly into liquid fuels, called bioliquids or (liquid) biofuels. Bioliquids already in wide use include bio-ethanol and biodiesel.
Bio-ethanol is produced by fermentation of sugars and starch derived from crops such as sugarcane, sugar beet, sorghum, wheat and maize. Biodiesel is obtained from vegetable oils such as soybean, rapeseed and palm oils as well as fats from the food industry through the transesterification process.
In the transesterification process, an alcohol (usually methanol) is reacted with a vegetable oil in the presence of an alkali catalyst to produce fatty acid alkyl esters (biodiesel) and glycerol as by-product.
Both bio-ethanol and biodiesel can be used in pure form, blended with conventional fuels or vaporized and reformed to produce a hydrogen-rich gas.