Measurement and Testing
SMR Hydrogen yield improvement and CO2 emissions reduction using cryogenics
Mar 08 2023
Author: Stephen B. Harrison on behalf of sbh4 GmbH
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Much has been said about CCS – carbon capture and storage. The need to decarbonise is clear. Renewable power generation and green hydrogen may do much of the heavy lifting when they scale up in coming decades, but there are many steam methane reformers (SMRs) existing on refineries that must also be decarbonised.
Steam methane reforming of natural gas, refinery gas or naphtha feedstocks is the most common process to produce hydrogen is. When these fossil fuels are used to generate hydrogen without capturing the CO2 emissions, it is called ‘grey’ hydrogen. If most of the CO2 from the SMR is captured, the hydrogen is referred to as ‘blue’.
CO2 is released from the SMR in two locations, firstly as the feedstock is transformed to hydrogen, CO2 is produced within the process as a by-product. This is an unavoidable consequence of this chemical pathway. The second source of CO2 emissions are from the combustion of fossil fuels, generally the same natural gas feedstock, to create the heat that is required to drive the reforming chemical reactions that convert the feedstock to hydrogen.
CO2 capture from steam methane reformers (SMRs) is often regarded as a ‘quick-win’ in the decarbonisation of industrial processes. The CO2 concentration, pressure, and partial pressure in the SMR process gas is high. This leads to cost-effective CO2 capture. Furthermore, CO2 has been captured from SMRs for decades so that the CO2 can be used to make urea fertilizer, when reacted with ammonia that is produced from hydrogen made on the SMR. There is therefore a wealth of experience to leverage.
The use of cryogenics to capture and purify CO2 from SMRs is likely to be the next milestone in the development of CO2 capture from these units. The Cryocap™ H2 process from Air Liquide combines cryogenic separation of CO2 from the SMR process gas stream with membrane separation of hydrogen.
A demonstration project at an SMR in Port Jérôme, on the river Seine in France, showed that an additional 12% hydrogen yield from the SMR is achievable using the Cryocap™ H2 process. This can have a tremendous positive impact on operational economics and can help to fund the investment in the Cryocap™ H2 equipment.
With Cryocap™ H2 directing more hydrogen to the product stream, there is less hydrogen available for the SMR fired heater, so additional natural gas is required to compensate for the reduced heat energy available. However, the additional hydrogen production can more than offset the cost of the additional natural gas.
If liquid CO2 is required for food and beverage applications, additional CO2 purification is required. In the Cryocap™ H2 process, oxygen is added to react with hydrogen in the CO2 stream to produce water using catalytic oxidation. The water is then removed on regenerative dryer adsorption beds. Excess oxygen is separated from the liquid CO2 using cryogenic distillation. Mercury removal is a final polishing stage which is achieved on an activated carbon filter bed.
CO2 liquefaction is achieved using a heat exchanger to condense CO2 gas. The cold side of the heat exchanger is generally fed with a refrigerant gas from a typical mechanical refrigeration circuit. Electrical power is required to operate the refrigeration equipment, so the process can be decarbonised using renewable electricity.
The CO2 side of the liquefaction circuit is operated at a pressure of 15 to 25 bar. At elevated pressure, common refrigerant gases such as CO2, ammonia or F-Gases can be used to achieve the temperature required to liquefy the CO2.
As an alternative to mechanical refrigeration, ammonia absorption refrigeration can be used. This process avoids the mechanical compression of a refrigerant gas and derives the cold energy instead from the absorption and desorption of ammonia in water. To drive the ammonia out of the water, heat energy is required. If waste heat is available, this process can be more efficient than mechanical refrigeration.
After liquefaction, CO2 is stored and transported in tanks which are insulated to minimise boil off. Typically, liquid CO2 storage tanks are constructed of carbon steel and insulated with polyurethane foam. Often, a refrigeration unit is used to re-liquefy boiled off CO2. This avoids CO2 losses and over-pressurisation of the CO2 storage tank.
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Petro Industry News 24.2 - Apr/May 2023
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