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Feb-2020

SMR integration with increased CO2 production

Optimising the efficiency of a plant producing hydrogen by natural gas reforming reduces CO2 emissions and lowers fossil fuel consumption.

MARCELO TAGLIABUE
Air Liquide Argentina

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Article Summary

In recent years, hydrogen has been attracting great interest as a clean fuel for combustion engines and fuel cells. It can be produced by several routes. The main route is the catalytic reforming of methane, which includes, among others, steam methane reforming (SMR)1,2 and dry-methane reforming (DMR).3

Carbon dioxide (CO2) has been identified as the most significant greenhouse gas arising from anthropogenic activities. It is important to reduce anthropogenic CO2 emissions in order to counteract global warming. One such method, which is presently being investigated extensively, is the sequestration of CO2 produced by concentrated sources (such as industrial plants and power stations). However, no one can be sure of the potential influence of CO2 buried in the ecosystem in the long term. Conversion of CO2 instead of sequestration is presently being explored as a potential alternative solution. Production of useful value-added products (chemicals products, fuels) by dry reforming of methane appears to be an interesting method.

In recent decades, numerous processes have been developed to produce synthesis gas (also referred to as syngas) as one of the most important feedstocks in the chemical industry. Syngas is a gaseous mixture containing hydrogen and carbon monoxide (CO), and may additionally contain other gaseous components such as carbon dioxide (CO2), steam, methane, and nitrogen. Natural gas and light hydrocarbons are the predominant starting materials for the production of synthesis gas. Syngas is used successfully as a synthetic fuel and also in numerous chemical processes such as methanol or ammonia synthesis, Fischer-Tropsch type and other olefin synthesis, hydroformylation or carbonylation reactions (oxo processes), reduction of iron oxides in steel production, hydrogen production, and so on.
SMR reactions are:

CH4 + H2O ⇔ CO + 3H2  ∆H°298 = 206 KJ/mol 2                                   (1)

CO + H2O ⇔ CO2 + H2   ∆H°298 = -41 KJ/
mol 5  (WSR)                    (2)

The DMR reaction is:

CH4 + CO2 ⇔ 2CO + 2H2 ∆H°298 = 247 KJ/
mol 3, 6                   (3)

These reforming reactions1,2,3 are strongly endothermic, while the concomitant reaction of water displacement is moderately exothermic.2 These processes therefore require a reactor in which heat management is extremely important. For the steam reforming process, several types of reactors are possible, such as conventional reformers with top ignition or side ignition, which are widely used. In practice, an SMR unit can contain from 40 to 1000 tubes, each of which is typically 6-12 m long, 70-160 mm in diameter, and 10-20 mm in wall thickness. These pipes are arranged vertically in a housing or rectangular combustion chamber, the so-called radiant section. The reactor tubes contain a nickel based catalyst, usually in the form of small cylinders or rings. The reactor tubes are heated by burners which can be located at the bottom, on the side wall, or at the top. Fuel combustion takes place in the radiant section. After the chimney gas has supplied its heat to all the reactor tubes, it passes to the convection section where it is cooled further by heating other currents such as process feed, combustion air and boiler feed water, as well as for steam production. The produced gas, which typically leaves the reformer at a temperature of 850-950°C, is cooled in a heat recovery boiler to produce process steam for the reformer.

If our interest is not only in the production of hydrogen, but also to reduce CO2 emissions and produce it for sale to the industries that use it, there are several points in the scheme of an SMR for its recovery.

In a modern steam reforming hydrogen plant fed by natural gas, up to approximately 60% of the total CO2 produced is contained in the shift gas (and then in the PSA tail gas), while the remaining 40% is a product of combustion of the additional fuel gas required by the steam reformer.
In our case, to recover CO2 we use points 1 and 3 shown in Figure 1.

Experiment, theory and calculations
The plant used to develop this study is a side fire type, with a feed of natural gas. This SMR plant consists of a high temperature shift plant (HTS), a CO2 capture plant through an absorption process with activated amines, and a CO2 liquefaction plant (for later sale).

This work was a theoretical and practical study with the objective of seeing how a stream of non-condensables, usually vented to the atmosphere (rich in CO2 and H2) and coming from the stripper of the CO2 liquefaction plant, could be added to the feed of the SMR plant, in order to reduce emissions of CO2 and other compounds responsible for the greenhouse effect, and to reduce consumption of natural gas while maintaining hydrogen production.

In the second stage, following a theoretical study of the impact on the SMR when this feed is added, it was decided to proceed to experimental work to learn how the major variables of the process were affected.4

In the field work, a line was built to link the non-condensables pipe from CO2 liquefaction (at a pressure of 17.0 barg) with the suction line to the natural gas compressor (at a pressure of 7 barg). Additionally, use of hydrogen from the non-condensables was considered to supplant part of the recycled hydrogen (extracted from the flow of produced hydrogen) which is usually employed to eliminate sulphur from natural gas in hydrotreatment.

The practical results of the test were encouraging. As dry reforming is more endothermic than conventional reforming, natural gas consumption increases initially because of the need for a higher input of combustibles and a lower flow of off-gas from the pressure swing adsorption unit (PSA) due to higher conversion of CH4. When the tube walls cool down, the excess air can be reduced. In addition to providing O2 for complete combustion, excess air is used to cool the tubes. Thus the consumption of natural gas can be reduced.

Stage three
In a third stage, in order to increase the profits of the business unit and mainly to continue reducing emissions of CO2 to the environment, an unused but available unit (a CO2 production plant burning natural gas to produce it) was used to capture CO2 from the SMR flue gases vented to the atmosphere (see Figure 2). Thus, about 25 t/d of CO2 are recovered for the local CO2 market, reducing emissions to the atmosphere by the same amount.

For this stage, the stack of the combustion gases of the SMR was linked to the absorber of the CO2 production plant by means of a duct and two blowers, one hot blower next to the stack and one cold blower to reduce the temperature of the flow of flue gas after heat exchange with cooling water.

The absorber and stripper were operated at low pressure, and an 18 vol% MEA solution was used. Liquefaction of CO2 was carried out at 17 barg using a refrigeration circuit.

This technique is attractive for the industry since the stream rich in CO2 is normally waste. However, the practical use of this technique has some disadvantages, which must be identified in advance to avoid costly errors.

As is well known, when SMR already incorporates sequestration of CO2 in the syngas stream (MDEA), the flue gases are poor in CO2 (4-8 vol%) and separation is more difficult. On the other hand, the flue gases contain a high concentration of NOx, even when low NOx burners are used. This calls for the inclusion of a purification unit.


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