PEP Review 2001-7
Single-Step Hydrogen Gas Synthesis
Published: September 2003
Hydrogen has long been an important raw material for chemical and petroleum industries. Its primary use occurs in ammonia, fertilizer, methanol and petroleum products treatment. Hydrogen can be produced through thermal, electrolytic, photolytic, and enzymatic processes from fossil fuels, water and biomass. Other sources for it include oxygenated materials such as methanol or ethanol and petroleum products like liquefied petroleum gas, naphtha and gasoline. At present, only thermal processes and, to a little extent electrolytic processes are used for commercial production of hydrogen. Among thermal processes, steam reforming of natural gas is the most dominant process both for large-scale as well as small-scale (<5,000 Nm3/hr) production.
Hydrogen is expensive to produce. Major portion of cost on production incurs on steam reforming of natural gas to produce syngas, a hydrogen precursor composed of a mixture of carbon monoxide and hydrogen. Steam-natural gas reforming is done generally at 1,562 - 1,652°F (850 - 900°C) and 15 - 25 atmosphere. Such conditions require special materials for construction of reformer and high mechanical strength as well as resistance against carbon deposition of catalyst materials. For hydrogen as the final product, reforming is characteristically done in two separate stages - steam-methane reforming and water-gas shift reaction.
Single-step reforming, a recently applied commercial approach to hydrogen production, on the other hand can be conducted at much milder conditions of temperature and pressure, usually in the range of 842 - 1,112°F (450 - 600°C) and 1 - 3 atmosphere. At these conditions, hydrogen equilibrium concentration in syngas is appreciably higher than that of CO and the process can be completed in just one step. Shifting of CO with H2O to H2 (and CO2) in separate step is not needed. Process conditions can be adjusted so that minimal amount of CO is produced. Conversion of methane is, however, somewhat lower. Mild process conditions and single-reactor process configuration are considered to result in lower capital investment and reduced production costs of hydrogen than two-step reforming technologies. The technology is aimed for small-sized, onsite hydrogen plants, and may be regarded as an improved version of two-staged, onsite hydrogen production technologies commercialized recently.
Our estimates per PEP's standard costing methodology show that a 500 Nm3/hr grassroots hydrogen plant based on steam and natural gas single-step reforming may be slightly more capital intensive in terms of investment and on operating cost basis than the two-step hydrogen plant (see cost details inside).