Converting low quality gas master

Converting low quality gas into a valuable power source

The Wärtsilä GasReformer utilizes low quality gases that contain large amounts of heavier hydrocarbons, or that vary in their composition, to produce a valuable energy resource.

Text: REETTA KAILA & PEIK JANSSON Photo: -

Associated gas, or volatile organic compounds (VOCs), cannot normally be utilized as a source of energy due to the low quality and unreliability of the gas. For the purposes of this article, gas quality is defined as being its uniformity and ability to withstand auto-ignition of the yet unburned fuel-air mixture part during the combustion process (methane number, MN). Due to the impulsive nature of this auto-ignition, destructive forces will lead to damage of the engine. The phenomenon of such auto-ignition is generally referred to as knocking, because of the typical sound associated with it. All combustion engines based on the Otto cycle, including Wärtsilä’s dual-fuel engines, need a high and stable fuel gas MN (>80) to operate at full performance.

Higher hydrocarbons (C2+), such as ethane, propane or butane, notably decrease the MN in gaseous fuels. When the MN is too low, instantaneous combustion of the yet unburned mixture will occur, unless the engine output is reduced. A reduced power output also results in lower fuel efficiency. In the offshore environment, the gases that are released during the oil separation process, or from crude cargo handling, are typically either flared or, even worse, directly vented to the atmosphere.

The Wärtsilä GasReformer provides an alternative option. Its technology is  based on steam reforming (SR), a catalytic process where non-methane hydrocarbons (NMHCs) are converted into methane, thereby improving the MN to 100±5. Regardless of the initial gas quality or variability, high and stable MN’s in the product gas are achieved. 

The GasReformer is not only a solution for the recovery of VOCs or associated gas. It also improves the performance and fuel flexibility of Wärtsilä dual-fuel engines. Together with a Wärtsilä dual-fuel engine, the system attains an overall efficiency of up to 44% in producing electricity (Figure 2).

The Wärtsilä GasReformer

Wärtsilä’s GasReformer technology has been actively developed over the past 5-6 years. After successful testing of the prototype “GasPac”, it was decided to industrialize this technology as a customer offering. Design work for the Wärtsilä GasReformer began in December 2010, and marketing to select customers has been taking place since 2011. Wärtsilä owns the patent for the application.

The Wärtsilä GasReformer product has gone through two validation tests. These were (1.) the Proof of Concept test in 2007-2010, whereby the 3 MW prototype was successfully tested together with a 6-cylinder Wärtsilä 34SG engine in the Vaasa Test Laboratory, and (2.) a full scale, 8 MW Wärtsilä GasReformer Factory Acceptance Test (FAT) in December 2012 relative to the Pilot Delivery Project. The FAT was witnessed by the customer and approved by a classification society. The Wärtsilä GasReformer Pilot meets all the requirements of the Offshore Industry and fulfils the Det Norske Veritas (DNV) standard requirements. The on-board commissioning of the system will take place in late 2013. These validation tests show that the technology is reliable, safe, and efficient for use in combination with Wärtsilä’s dual-fuel engine technology.

The 8 MW Wärtsilä GasReformer is designed to supply fuel gas for an engine having a shaft power of 8 MW, for instance the 16-cylinder Wärtsilä 34DF engine. The dimensions of this Wärtsilä GasReformer Pilot are 9000(L) x 3500(H) x 2500(W) in mm, while the overall weight is 22 tons. The second generation 8 MW GasReformer is already under design so as to optimize the weight and size (Figure 1). The offering will also be enlarged by increasing the power range with 4 MW and 18 MW GasReformer units.

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Fig. 1 – Design of the second generation 8 MW Wärtsilä GasReformer.
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Fig. 2 – Sankey diagram for a Wärtsilä GasReformer and dual-fuel engine with generating-set configuration for an example case where the feed gas MN is 46.

The process and chemistry

The Wärtsilä GasReformer process is based on known steam reforming technology, whereby hydrocarbons react with steam in the presence of a catalyst. This catalyst is sensitive to sulfuric compounds, which are removed from the gas through a desulfurization absorbent before entering the catalytic reformer. Excess steam is condensed and separated from the product gas. The condensed water is recycled back to generate steam, thus minimizing the need for fresh water. The dry product gas as such is ready for use in Wärtsilä dual-fuel engines. 
The schematic of the Wärtsilä GasReformer process, and its interface to a Wärtsilä dual-fuel engine, can be seen in Figure 3.

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Table 1 – Technical Specification of the 8 MW Wärtsilä GasReformer.
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Fig. 3 – The Wärtsilä GasReformer process and its interface to a Wärtsilä dual-fuel engine.

Steam Reforming

Steam Reforming (SR) is a technology that has been used in the petrochemical industry and in refineries since the beginning of the 20th century. Conventionally, SR is used to produce synthesis gas (H2 + CO) from natural gas (NG) or even naphtha, in locations where NG is not available. This continuous process is operated on a nickel-based catalyst at temperatures as high as 700-900 °C, and with an excess of steam to achieve high amounts of hydrogen. The steam or oxygen to carbon (O/C) molar ratio is usually 2.5–4.5, whereas the reaction stoichiometry requires only O/C =1:

  • CH4 + H2O  3 H2 + CO (1)

Hydrogen production from hydrocarbons, generally:

  • CnHm + n H2O  (n + m/2) H2 + n CO  (endothermic) (2)

In addition to these primary reactions, there are always equilibrium reactions present: a water gas shift (WGS, Eq. 3) and methanation (Eq. 4) that affect the final product composition:

  • H2O + CO  H2 + CO2 (3)
  • CO + 3 H2  CH4 + H2O (exothermic) (4)

The intensity and equilibrium of these reactions are controlled with operational parameters, such as the temperature, pressure, O/C feed ratio, and catalyst.

In the Wärtsilä GasReformer, all non-methane hydrocarbons are first converted into synthesis gas (Eq. 2), followed by methanation (Eq. 4). High methane loadings are obtained by operating the catalytic process under milder conditions (350–420 °C), and with a lower O/C ratio (1 mol/mol) than usual. At these temperatures, the Water Gas Shift reaction (Eq. 3) equilibrium is shifted to H2 + CO2, which is seen in the product composition. The technical specification of the Wärtsilä GasReformer is presented in Table 1, and the GasReformer Inlet Gas and Dry Product Gas compositions for an example case in Figures 4 and 5, respectively.

The product (Figure 5) contains a fairly high level of hydrogen (H2 < 11%) that tends to decrease the compressibility and MN of the fuel gas. However, CO2 is formed in the same ratio (Eq. 3), which compensates for this effect. One should not forget that the presence of CO2, or other inerts (N2), degrades the LHV (Lower Heating Value (MJ/kg) of the fuel gas, which imposes certain limitations on the Wärtsilä GasReformer’s operational window (Figure 6). Nevertheless, the Wärtsilä GasReformer is able to improve the MN of the feed gas from 25 – 65 up to a product gas MN of 100±5.

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Fig. 4 – GasReformer Inlet Gas Composition (MN 46).
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Fig. 5 – GasReformer Dry Product Gas Composition (MN 104).
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Fig. 6 – Operational window for the Wärtsilä GasReformer.

Carbon deposition

One of the major challenges encountered with the nickel catalyst placed in the reformer is its deactivation due to carbon deposition on the catalyst’s surface. Carbon deposition is promoted in the presence of unsaturated hydrocarbons (alkenes, or olefins) or aromatics. Carbon formation, however, is minimized with the right operational conditions and can partly be removed with excess steam. Catalyst deactivation decreases the hydrocarbon conversion rate and, therefore, the product quality. In the worst case, carbon deposition may block the catalyst bed.

Desulfurization

Nickel catalysts are highly sensitive to sulfuric compounds. Sulfur occupies the active nickel sites and causes an immediate decrease in activity. This sulfur poisoning prevents both the conversion of hydrocarbons to synthesis gas as well as the resultant reactions to follow. Unreacted hydrocarbons will degrade the quality of the product and lead to carbon deposition in the reformer.

The most common sulfur compound present in natural or associated gas is hydrogen sulfide (H2S). Desulfurization vessels upstream of the reformer ensure that all H2S is removed from the inlet gas by absorbing the H2S into ZnO:

  • ZnO + H2S = ZnS + H2O (5)

When the absorbent from the first vessel becomes saturated, the vessel needs to be replaced. The second desulfurization vessel acts as a guard bed to prevent sulfur slip into the reformer.

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Fig. 7 – Desulfurization and Reformer Vessels of the 8 MW GasReformer unit.

Applications

The main application area for the GasReformer is in offshore oil and gas production, where self-sustaining electricity production is a necessity. In the offshore sector, the traditional way to get rid of associated gas, and even recovered volatile organic compounds (VOCs), is either by 
flaring, venting, or by burning it in boilers or gas turbines. This involves high operational costs and low efficiency. The Wärtsilä GasReformer has been developed and designed to meet these industry standards, and is the first of its kind in the world.

Downstream of the Wärtsilä GasReformer, the Wärtsilä dual-fuel engine can exploit a wide range of fuel gases, and is able to refine not only natural or associated gas and VOCs, but also condensates and LPG into high quality fuel gas with increased knocking resistance (MN). Thus, much potential is seen also for land based power plant applications and, for example, in the shale oil and gas industry, which has been very much in the news lately, especially in Europe and the USA.

Ecological and Economic Excellence

The flaring of offshore gas is increasingly recognized as a major environmental issue, contributing more than 1% of the global emissions of CO2.

In crude oil pumping and handling, for instance, the crude oil is heated to improve its viscosity. During the heating process, the lighter hydrocarbon fractions of the crude are evaporated. These VOCs are traditionally vented to the atmosphere. Similarly, the associated gas separated from the crude oil is traditionally flared and not utilized for the reason that it varies in composition, and contains a lot of heavier hydrocarbons. In general, associated gas is considered as waste or as an unreliable source of energy.

With the Wärtsilä GasReformer and Wärtsilä dual-fuel engine combination, this insight is changed. These undesired, volatile hydrocarbons can be recovered (using the VOC Recovery System by Wärtsilä-Hamworthy) and utilized to produce a gas with high efficiency, reliability, and flexibility. Not only can the operator run the Wärtsilä dual-fuel engine at full load and efficiency, but the gas source itself can be changed during operation. 

Since the flare gas can be utilized for power production, the operator has no need for flaring. Indeed, the introduction of the Wärtsilä GasReformer for offshore applications reduces greenhouse gas emissions (CO2 and CH4), and together with the proven Wärtsilä dual-fuel engines, lowers NOX emission levels. The carbon footprint of the entire offshore operation can, therefore, be substantially reduced. 

When utilizing the associated gas or recovered VOCs for energy production, the operator can achieve self-sufficiency in terms of the energy supply, thereby decreasing the need for bunkered fuel oil. For example, an 8 MW dual-fuel generator set equipped with a Wärtsilä GasReformer, operating on associated gas or recovered VOC, can save up to 20 tons of bunkered fuel per day.

CONCLUSIONS

Flared gas and vented VOCs are known problems in the offshore and onshore oil and gas industries, since both emit unnecessary greenhouse gases into the atmosphere. To this day, one of the main challenges in utilizing flare gas has been its constantly changing quality. The GasReformer turns these waste flows into a valuable source of energy and enables self-sustaining power generation for the offshore operation, and in so doing, reduces the need for additional energy, normally seen as MDO bunkering. By combining a Wärtsilä-Hamworthy VOC recovery system and a Wärtsilä GasReformer with Wärtsilä dual-fuel engine technology, the utilization of this energy source is efficient, reliable, safe, and flexible. The Wärtsilä GasReformer not only provides cost savings for the operator, but has the even larger benefit of environmental sustainability, since no flaring or venting of the VOCs or associated gas is needed.

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