Aug-2021

CO2 capture and natural gas savings in SMR process

Study of the efficiency optimisation of an H2 production plant through natural gas reforming and its contribution to preserving the environment.

Marcelo Tagliabue
Air Liquide Argentina S.A.

Article Summary

Sixty-six per cent of the world’s hydrogen production, estimated at 70 million tonnes, is used as an input in refining oil, the production of ammonia, methanol and, in recent years, in the development of engines that run on hydrogen (H2).

At the start of 2021, 70% of the world’s economy has made ambitious commitments on carbon neutrality, and H2 will play a fundamental role in reducing emissions. Currently, the best-known and most developed production methods are:

- Electrolysis of water: currently limited to 4% of H2 production, it is achieved by dissociating the water molecule into its components (hydrogen and oxygen) using electricity. It is known as green H2.

- Steam reformed from natural gas (SMR): it accounts for 96% of current production worldwide. It is a thermochemical process that requires high temperatures and a subsequent purification of the final stream, obtaining the so-called grey H2. If you capture the CO2 produced in this process, you are in the presence of so-called blue H2.

To achieve the highest possible conversion of crude oil into gasoline, gas oil or middle distillates, the availability of H2 has become a critical point in modern refineries. This article looks at the efficiency optimisation of an H2 production plant through natural gas reforming and its contribution to preserving the environment. One of our business units consists of an SMR, mainly to satisfy the H2 needs of a refinery. Additionally, part of the CO2 generated is captured, purified and sold to the local market with food grade.

In a modern, natural gas-fired steam reforming H2 plant, up to approximately 60% of the total CO2 produced is contained in the syngas produced (and then in the tail gas pressure swing absorption, PSA). The remaining 40% is the product of the combustion of the additional fuel gas required by the steam reformer. In our case, to recover CO2, we use point 1 shown in Figure 1.

Experimental, theory and/or calculations
The plant used to develop this study is a side-fire type SMR fed with natural gas. It consists of a reactor downstream of the main reformer — to eliminate CO from the syngas as much as possible (water-gas shift reaction, HTS) — a plant for capturing CO2 using an absorption process with activated amines, and a liquefaction plant for CO2 (for later sale). Previous literature mentioned that in addition to maintaining the production of H2, it was possible to substantially reduce the amount of CO2, CO and CH4 vented to the atmosphere from the non-condensable gases of the CO2 liquefaction unit, using them as feed to the reformer and thus decreasing the consumption of natural gas.

Secondly, to increase the profits of the business unit and to continue reducing CO2 emissions, an unused asset — a CO2 production plant that burned natural gas to produce it — was used to capture CO2 from the SMR flue gases vented to the atmosphere (see Figure 1, point 2). Thus, about 25 t/d of CO2 is recovered and sold in the local market, reducing emissions to the atmosphere by the same amount.

This technique is very favourable for the industry since the stream rich in CO2 is a gaseous effluent sent directly to the atmosphere. However, the practical use of this technique has some drawbacks, which must be known in advance so as not to make costly mistakes due to problems of amine solution degradation or internal corrosion, mainly in pumps and heat exchangers. Therefore, it is crucial to choose the right amine for each process, with the correct concentration, pressure and operating temperature. You also have to be careful about how the SMR operates in terms of excess O2 in the combustion gases. If it is above 2-3%, amine degradation processes are aggravated, as are any corrosion problems.

As clean fuel standards become more and more demanding, a refinery’s consumption of H2 continues to increase. Considering this, our company installed a new SMR very close to the first. In this case, the technology is top fired, which does not have units to recover CO2.

In this last stage, taking advantage of the design of the second SMR, which includes an adiabatic pre-reformer for greater operational flexibility (to process different types of feed), the use of a stream of refinery off-gas (ROG)1,2 as feed to the SMR was investigated. The expectation was to reduce fossil fuel, such as natural gas, consumption and use the ROG that would otherwise be burned and sent to the atmosphere.

Additionally, the composition of ROG can fluctuate significantly in a refinery as rates for different units change and, in particular, if a unit goes offline. Typical conditions for different ROG streams in a refinery can be seen in Table 1.

When it comes to incorporating ROG as feed to the SMR, keep in mind a few things for smoother operation:

- High level of heavy hydrocarbons
- High H2 content
- High sulphur levels

The H2 content in ROG varies between 5 and 10%, up to values of 90%. Therefore, its use in the production of H2 as a feed to an SMR should be a commitment by case. If the H2 content is as high as 80%, care must be taken to mix this stream with, for example, natural gas so that the heat required by the SMR is not less than that supplied by the tail gas from the PSA.

In our case, H2 content in the ROG varies from 70-77%, so to have a safe operation and achieve the H2 flow required by our client, it was mixed with natural gas in different proportions. The relationship between the H2 produced as a function of the ROG/natural gas ratio can be seen in Figure 2.

Figure 3 shows how the fuel supplied to the furnace varies, depending on the ROG/natural gas ratio. Here, when the ROG/natural gas ratio increases, there is more H2 in the feed and fewer hydrocarbons to reform, so the energy required diminishes.

As mentioned above, an adiabatic catalytic pre-reformer was installed upstream of the main reformer. The adiabatic pre-reforming process is based on a set of reactions: hydrocarbon steam reforming followed by water-gas exchange reactions and methanisation.