Mar-2013

Driving down costs in hydrogen production

An optimised hydrogen plant design achieves the right balance of minimising both Capex and Opex costs, while meeting the specific objectives of the end user

Luigi Bressan and Chris Davis
Foster Wheeler

Article Summary

The global demand for refinery hydrogen has increased significantly over the past decade due to changes in available crude feedstocks and tighter environmental regulations, which have forced the refining industry to reduce sulphur, olefins and aromatics content in transportation fuels. This, coupled with the continued growth in diesel demand, means that refiners are investing heavily in both hydrotreating and hydrocracking facilities, and are constantly looking for access to low-cost, reliable sources of high-purity hydrogen.

Foster Wheeler pioneered steam methane reforming (SMR) technology and has delivered more than 100 hydrogen and synthesis gas plants around the world, with a total installed capacity of more than 3.5 million Nm3/h of hydrogen. The company’s patented and proprietary Terrace Wall reformer furnace was developed in conjunction with SMR technology in the early 1960s. Updates and improvements to plant efficiency, lower maintenance costs, simplified operations and enhanced plant safety have been documented in previously published articles.

These hydrogen-producing SMR plants process a wide range of feedstocks from natural gas to naphtha and range in size from 5000 to 200 000 Nm3/h. The range of hydrogen solutions provided include:
• Optimised plant design and operating parameters tailored to the operator’s requirements, integrating overall plant and reformer furnace design to reduce total lifecycle costs
• Full understanding of constructability issues and impact on total installed cost, with the ability to incorporate a high degree of modularisation of the Terrace Wall reformer, reducing construction costs
• One-stop shop, providing consistency through all design phases, ensuring single-point accountability for process and operational guarantees
• Safety in design that incorporates the latest state-of-the-art design principles as well as end user feedback, to enable safe and reliable plant operations.

Steam reformer-based technology
SMR continues to be the leading technology for hydrogen production and, although it is a mature technology, incremental economic improvements are being continuously developed, which improve overall plant efficiency, reduce the cost of hydrogen production and minimise the impact on the environment by reducing CO2 emissions.

The hydrogen production plant consists of five main sections:
•  reatment section Feedstock is hydrotreated and the resulting H2S is captured in a zinc oxide bed. Different schemes are available, with the most commonly used being a lead-lag reactor arrangement. Reaction temperatures are obtained by thermal exchange in the reformer’s heater convection section
• Pre-reforming section A pre- reforming section is generally installed to eliminate the long-chain hydrocarbons in heavier feedstocks before they enter the reforming section. When natural gas is used as a feedstock, the pre-reforming section helps to reduce the reforming duty, thereby lowering the initial investment cost of the reformer
• Reforming section This is the heart of the plant and will be discussed in detail in the next section. The Terrace Wall technology allows for steam reformer outlet temperatures of up to 930°C
• Syngas cooling and shift reaction section For the shift reaction, four options are available: high-temperature shift (HTS), HTS and low-temperature shift (HTS + LTS), medium-temperature shift (MTS) and isothermal shift (ITS). The syngas cooling section is normally optimised using pinch technology
• Pressure swing adsorption (PSA) section Final hydrogen purification is typically achieved using PSA, as this technology is both effective and well known. Process parameters need to be carefully defined to optimise both overall cost and operating expenses.
The centrepiece of Foster Wheeler’s hydrogen plant design is the Terrace Wall reformer. This design incorporates unique features that provide controlled heat transfer to the reformer catalyst tubes, which translates into longer tube life, longer catalyst life and better stability at turndown conditions.

General description
The Terrace Wall reformer, shown in Figure 1, features a radiant section consisting of a firebox, which contains a single row of catalyst tubes with burners on either side located at two terrace levels. Hot flue gases flow naturally upwards into the convection section very much like a conventional fired heater. The convection section, located on the top of the heater in between the radiant sections, has several coil sections, which recover much of the remaining heat from the flue gas for various process and steam duties. A close-coupled process gas boiler (PGB) with an internal bypass and associated steam generation system complete the reformer design.

Advantages
The advantages of the Terrace Wall reformer include the following:
• The sloped firing walls and terraces of the reformer are its trademark feature. Each terrace is capable of being independently fired to provide the particular heat flux desired in a given zone. Controlled delivery of heat to the reformer catalyst tubes is essential to control the reaction progressing along the tubes. The sloped walls are uniformly heated along the length of the furnace as a result of the special burner design, which provides for a continuous re-radiating plane with no marked discontinuity. The sloping walls also provide a uniform vertical flux profile, since the distance from the tube to the radiating wall decreases as the flue gas cools. The burners are selected to spread the flames both horizontally and vertically along the firing wall for a uniform planar heat flux pattern. Low-heating value, low-pressure PSA off-gas is stabilised against the brick firing wall, preventing the flame impingement or instability that is common in top-fired designs with free-standing burners.
• The burners are located at two levels in the radiant section, firing upwards adjacent to the brick firing walls. This configuration significantly reduces power requirements for the induced draft/forced draft (ID/FD) fans compared with competing technologies, which need to operate against the natural buoyancy of the flue gases, as well as to overcome the larger burner pressure drop required to shape the flames. Power consumption can be reduced by at least 40-50% compared with competing technologies.
• The burner flames adhere to the sloped firing walls, providing unmatched flame stability and virtually eliminating any possibility of flame impingement and catalyst tube failures compared with other technologies, which require constant observation and expensive instrumentation to confirm flames are not leaning into the catalyst tubes.