Clean Energy Technology

This thermochemical heat storage is based on gas-gas or gas-solid reactions, by using thermal energy to dissociate compounds (“AB”) into two reaction products (“A” and “B”). Upon subsequent recombination of the reactants, an exothermic reverse reaction occurs and the previously-stored heat of reaction is released. This allows for the theoretically lossless storage of thermal energy. 95% of the installed systems are in R&D and have reached a TRL of 3-4.

Several biotechnologies can produce green H2 from organic biomass:. 1.Dark fermentation (DF) uses organic matter as the substrate. It relies on the biological conversion of organic compounds (waste/effluents/biomass) into H2 and other valuable green biobased molecules. Contrary to other biological processes, DF is light-independent. The DF process presents the simplest reactor design and the easiest way of operation which reduce drastically the costs of production. DF is carried out by strict or facultative anaerobic bacteria, that release H2 during the fermentation of organic substrates. Although DF can be applied on most of the waste or effluents, carbohydrate-rich waste presents the highest performances and can be more easily used as feedstocks. Nonetheless, H2 yields in dark fermentation are limited by thermodynamics. It is now well known that microbial interactions and dynamics in dark fermentation can strongly impact process performances and better understanding of these interactions is a key factor to improve and stabilise fermentative hydrogen production. 2.Water-splitting photosynthetic processes: under certain conditions, green algae and cyanobacteria can be used to generate molecular hydrogen. Two type of processes exists: direct biophotolysis, when light is irradiated during hydrogen evolution, or indirect biophotolysis when light is not irradiated during hydrogen evolution. Direct biophotolysis is an attractive process since solar energy is used to convert a readily available substrate, water, to oxygen and hydrogen. Microalgae, such as green algae and Cyanobacteria (blue-green algae), containing hydrogenases, have the ability to produce hydrogen. The main drawback of direct biophotolysis is that the process is limited because of the strong inhibition of hydrogenase by the oxygen produced (it is necessary to maintain the oxygen content below 0.1%, which is very difficult without additional energy demand and cost). In indirect biophotolysis, cyanobacteria and microalgae produce hydrogen through photosynthesis, with oxygen and hydrogen production occurring at different times. This avoids enzyme deactivation and explosive gas mixtures, but also makes hydrogen purification easier. Biophotolysis, is an immature technology, applied only at laboratory scale. 3.Bioelectrochemical systems for hydrogen production. At lab-scale, microbial electrolysis cells can be divided into single and double chamber depending on the presence of an ion exchange membrane (IEM) that separates the anodic and cathodic compartments. Electroactive microorganisms grow as a biofilm on the anode whereas abiotic hydrogen evolution takes place at the cathode. These microorganisms can degrade organic matter and transfer electrons extracellularly, which travel from the anode to the cathode through the electric circuit and reduce protons on the cathode, thereby forming hydrogen.

Pyrolysis is a thermochemical process in which a solid feedstock (in this case biomass or waste) is heated at high temperatures in the absence of any oxidant to be transformed into a gas mixture of H2, CO, CO2 and other light hydrocarbons (called syngas), along with other byproducts (char and tars). The gaseous fraction is treated to maximise H2 and CO proportions. Following treatment, the gas is passed through a water gas shift reactor in which steam reacts with CO in the presence of a catalyst to generate H2 and CO2. Then, the CO2 is captured and a high-purity H2 stream is obtained (99.9 vol% if Pressure Swing Adsorption is used). This technology has the potential to produce low carbon H2 and even to generate negative emissions if the captured CO2 is stored and the char (whose production is significantly larger than in the case of gasification) is used in applications that prevent its carbon content being released in the form of CO2. Biomass has an inherent low (

Coal gasification is a very mature technology that has been in operation in China for many years for the generation of H2 to be used as a feedstock. It consists of a thermochemical process that transforms coal into syngas (H2, CO, CO2 and other light hydrocarbons) which is then upgraded to maximise H2 and CO proportions. The upgraded syngas is used in a water gas shift process to increase H2 production while transforming CO into CO2, which separates more easily than CO from H2. Finally, CO2 is separated to produce high purity H2. The CO2 separated from H2 can be captured and stored to minimise the carbon footprint of the process. Partial capture is widely used in ammonia production plants where co-production of urea takes place. In addition, two demonstration plants in China are storing CO2 underground for EOR.

Sorption processes can be used to absorb and release heat through adsorption (physical bonding) and absorption (uptake/dissolution of a material). In adsorption, the reactants (e.g. zeolite and water) are separated during charging and the heat of reaction is released after recombination. The sorption principle can be applied for thermal energy storage as well as for chemical heat pumps. Whereas sorption heat pumps are commercially available, sorption-based thermal energy storage with discharging cycles of more than 1 hour are still in research and development. Sorption storage systems are at a TRL 5-7, with the exception of sorption heat pumps which have been fully commercialised (TRL 9).

Gasification is a thermochemical process in which a feedstock (in this case biomass or waste) is heated to high temperatures in the presence of an oxidant (oxygen, air and/or steam) under non-stoichiometric conditions to avoid complete combustion. Depending on the operating conditions, such as temperature, catalysts and oxidant, a flue gas is produced consisting of varying percentages of H2, CO, CO2, CH4, CxHx and tars. The flue gas is upgraded and conditioned in a water gas shift (WGS) reactor in which steam reacts with CO in the presence of a catalyst to generate H2 and CO2. The gas leaving the WGS typically contains 65-70 vol% hydrogen, which can be purified by pressure swing adsorption (PSA), resulting in a high purity (99.9%) hydrogen stream. The CO2 is separated into a concentrated waste stream as an integral part of the process, thus providing a favourable source for CO2 capture and storage and, consequently, negative CO2 emissions. Biomass has an inherent low (

Chemical looping combustion (CLC) is a process for generating energy from fossil fuels while capturing carbon dioxide (CO2) emissions. Unlike traditional combustion, which burns fossil fuels in the presence of air, CLC uses metal oxides as oxygen carriers to provide oxygen for combustion while also capturing and separating CO2 from the flue gas. Fluidised bed chemical looping is a process in which two reactors work in parallel to generate hydrogen and a high-purity CO2 stream. In the first reactor, an oxygen carrier (metal oxide) is oxidised with steam, thus producing H2. The oxidised oxygen carrier is sent to the second reactor, where it is brought into contact with a fuel. The fuel is oxidised generating CO2 while reducing the oxygen carrier, which is sent back to the first reactor, working in a loop. In the fixed-bed chemical looping hydrogen process, the oxygen carrier is not physically circulated. Instead, the oxidation and reduction steps take place alternately in the same fixed-bed reactor, which allows for easy operation without moving parts and high hydrogen purity. The asynchronous use of multiple reactors enables continuous hydrogen production and recovery of heat from exothermic oxidation for endothermic reduction, which increases the overall efficiency of the process.

Electrolysis uses electricity to split water into its basic components (H2 and O2). Alkaline electrolysers are a type of electrolyser that uses nickel-based electrodes and a concentrated alkaline solution (25-35 wt% KOH) to enable the splitting of water. Alkaline electrolysers have been applied on 10+ MW scale for over hundred years, where they were traditionally used to produce hydrogen for ammonia plants based on hydropower until gas reforming became the cheapest technology. These electrolysers were traditionally operated in a baseload manner. Both atmospheric and pressurized versions (15-30 bara) do exist. The electrolysis products from anodes via water splitting may not only be O2. For example, the product is Cl2 from the chlor-alkali process, which is today the largest market for alkaline electrolysers. The selectivity of the products is dependent on the electrolytes and catalysts.

Hydrogen can be produced by water/steam aluminum oxidation. The reaction can be carried out in different ways depending on pressure and temperature operating conditions: 2Al+6H2O --> →2Al(OH)3+3H2 -886 kJ/mol (1) 2Al+4H2O --> →2AlO(OH)+3H2 -862 kJ/mol (2) 2Al+3H2O --> →Al2O3+3H2 -817 kJ/mol (3). Reactions (1) and (2) are typical of oxidation with water and are carried under pressure (up to 40 bar and over for recent supercritical water applications) and low temperatures, i.e., up to 300°C (requiring catalysts). They have notable drawbacks tied to the need of using alkaline compound (to promote the reaction inibited by the oxide layer formation). Other methods include mechanical activation and metal alloying. Good results in terms of H2 yield and reaction time are obtained in batch operating conditions, but not suitable for continuous operation required by H2 use in fuel cells and turbine.

Gasification is a thermochemical process in which a solid feedstock (in this case biomass or waste) is heated at high temperatures in the presence of an oxidant (oxygen, air and/or steam, under stoichiometric conditions to avoid complete combustion) to be transformed into a gas mixture of H2, CO, CO2 and other light hydrocarbons (called syngas), along with other by-products (char and tars). The gaseous fraction is treated to maximise H2 and CO proportions. Following treatment, the tar is removed and then the gas is passed through a water gas shift reactor in which steam reacts with CO in the presence of a catalyst to generate H2 and CO2. Then, the CO2 and other inpurities (sulphur, alkaline metals, fine particles) are captured before a high-purity H2 stream is obtained (99.9 vol% if Pressure Swing Adsorption or Membrane separation technique is used).

Coal gasification is a very mature technology that has been in operation in China for many years for the generation of H2 to be used as a feedstock. It consists of a thermochemical process that transforms coal into syngas (H2, CO, CO2 and other light hydrocarbons) which is then upgraded to maximise H2 and CO proportions. The upgraded syngas is used in a water gas shift process to increase H2 production while transforming CO into CO2, which separates more easily than CO from H2. Finally, CO2 is separated to produce high purity H2. The CO2 separated from H2 can be captured and stored to minimise the carbon footprint of the process. There are no demonstration or commercial plants operating with high capture rates (>90%).

Electrolysis uses electricity to split water into its basic components (H2 and O2). Anion exchange membrane electrolysers are a type of electrolyser that uses an anion-exchange membrane in combination with nickel-based electrodes. In contrast to alkaline electrolysis the use of the anion-exchange membrane enables the use of a dilute alkaline electrolyte or even pure water. In contrast to PEM, AEM enables the use of non-noble metal electrodes. This makes AEM a potentially highly competitive technology. The potential benefit of AEM electrolysers is low cost. Capital cost is reduced because of alkaline conditions which are milder than the acid conditions of PEM. This means platinum group metal free catalysts like Ni alloys and Ni, Fe, Co oxides may be used. In addition, and more importantly when it comes to cost, low-cost stainless steel current collectors and Ni based porous transport layers may be used instead of the expensive Pt coated Ti required for PEM. Operational cost is reduced by the membrane which reduces resistance in the cell making AEM electrolysis more electrically efficient than traditional alkaline electrolysis. It is important to note that AEM systems may still use an alkaline feed solution of up to 1 M KOH. The low ionic conductivity of current membranes and ionomers means operation with a pure water feed results in low performance. At the same time, AEM still requires significant development in terms of performance (efficient operation at high current densities) and scaling up cell area.