Apr-2013

Minimising the cost of hydrogen production

Feed and fuel costs or the demand for export steam drive the choice of plant architecture and operating parameters in SMR hydrogen production

Lorena Mosca, Palma Contaldo, Menica Antonelli and Paola Volpi
Tecnimont KT

Article Summary

The oil refining and petrochemical industry regards hydrogen more and more as a utility. From this perspective, the selection of a hydrogen manufacturing scheme is simply judged on the cost of production; in today’s plants, variable operating costs (mainly feed, fuel, power and steam credit) amount to two-thirds of such production costs. It is therefore evident that minimising operating costs and not simply energy efficiency is the key to the selection of an appropriate hydrogen plant architecture.

Although steam methane reforming (SMR) is a quite mature technology, at the practical level there are a large number of variables affecting the cost of producing hydrogen. It is essential, then, that the designer selects plant configurations suited to the specific site or project, rather than offering the same standard process scheme in all cases.

This article summarises how the cost of feed or fuel, or the need to produce a certain amount of export steam, affect the choice of plant architecture and operating parameters, which need to be driven economically.

Site requirements and cost backgrounds
The availability at any site of different feeds and fuels, their cost and the other requirements that must be fulfilled in parallel with hydrogen production may greatly affect the architecture of a hydrogen plant, which should provide the best economic balance of project requirements and fulfilments.

One of the most common requirements for a new hydrogen plant is to produce a stream of high- pressure export steam. The reason for this is that hydrogen production based on steam methane reforming generates a heat surplus in the steam reforming furnace that may be conveniently used for producing extra-high-pressure steam in addition to the amount needed for the steam reforming process itself. In order for it to be produced economically, the amount of export steam should be limited to what can be obtained as a real “byproduct” after optimisation of the hydrogen process, and should never be produced at the expense of a valuable feedstock.

Another factor to evaluate related to the steam export requirement is whether it may be produced economically at the cost/price conditions fixed by the plant owner. The amount of export steam that can be produced economically is greater in a site where low-cost fuel is used as additional fuel.

The relative cost of feed and fuel is also important for the selection of an economic configuration of the plant architecture. For instance, at a site where the same stream, say natural gas, is used as feed and fuel, their sum use needs to be minimised, while at a site where a high-value feedstock is used the minimum feed flow rate will be sought.

Process optimisation that targets minimum feed plus fuel consumption¹ may not always produce the lowest hydrogen production cost, at least in those sites where a high-cost feed is coupled with a low-cost fuel and export steam is valorised. Criteria for minimising feed should prevail over minimum feed plus fuel consumption in this case, especially if there is no export steam requirement.

Combining export steam requirements with feed/fuel cost requirements leads to four site scenarios:
•  Zero or minimum steam export with a feed cost > fuel cost
•  Zero or minimum steam export with a feed cost = fuel cost
•  Maximum export steam with a feed cost > fuel cost
•  Limited export steam with feed cost = fuel cost.

These scenarios are the basis for our analysis, which is intended to show how to select plant architecture and design parameters in order to realise minimum hydrogen production cost.

Design parameters and plant architecture
Selecting the optimal hydrogen plant architecture and setting the values of the design parameters is a real challenge for the process engineer, who has to consider many variables together and foresee the impact of all of them on plant performance, taking into account subsequent variation in operating costs.

Preliminary identification of the site scenario is therefore essential and can assist in making basic choices and the selection of process design parameters with the aim of minimum production cost.

The main contributions to hydrogen production costs are derived directly from plant performance in terms of feed consumption, fuel consumption and steam credit, when export steam is a requirement. Feed and fuel consumption and export steam credit are typical indicators of a hydrogen plant’s performance, and their values are a direct result of the plant architecture selected and its assigned design parameters.

Plant architecture also depends on the type of feedstock available at the site, but some general design options are applicable to all cases.
In the present analysis, we consider those design options that have a considerable impact on plant performance, the most important of which are:
•  SMR process parameters Steam reforming temperature and steam/carbon (S/C) ratio have an influence on feed consumption, fuel consumption and steam credit
•  Introduction of a pre-reformer This reduces SMR duty and affects fuel consumption and steam credit
•  Shift reactor temperature Selection of high-, medium- or low-temperature steam has a direct influence on feed consumption
•  Air preheating level Has an influence on fuel consumption and on export steam credit.