Efforts are underway to develop hydrogen as a future transportation fuel, but today, hydrogen is mostly used as a building block compound to produce other chemicals. World consumption of hydrogen (1999) was 15,864 billion ft3, 20% of which was consumed in the U.S. About 60% of world consumption is used to produce ammonia, followed by capture and use in oil refineries (23%) and methanol production (9%). Only 8% was produced as merchant hydrogen.
Hydrogen is produced from syngas. In its simplest form, syngas is composed of carbon monoxide (CO) and hydrogen (H2). Syngas is produced from the gasification of feedstocks at temperatures in excess of 1100°F and under conditions where the amount of oxygen (from air, pure oxygen, or steam) is less than what is needed for complete combustion.
In principle, syngas can be produced from any hydrocarbon feedstock, including natural gas, naphtha, residual oil, petroleum coke, coal, ethanol, methanol, and biomass. Under today’s conditions, the least expensive feedstock is natural gas and presently, 77% of the world hydrogen production comes from natural gas (methane), 18 % from coal, 4 % from water electrolysis, and 1% from other sources.
Steam methane reforming is the dominant technology used to produce hydrogen, and involves four steps--feed pretreatment, steam reforming, CO shift conversion, and hydrogen purification. For natural gas, pretreatment involves removing the sulfur. The natural gas is then fed into a reformer reactor, where it reacts with steam to produce carbon oxides (CO, CO2) and H2. The reformer reactor consists of tubes filled with a catalyst and surrounded by a firebox that provides the heat needed for the reactions. The gas that exits the reformer is cooled to about 650°F and then undergoes further chemical reactions (i.e., the water gas shift (WGS) reaction) in a reactor. Current commercial operations typically use a high temperature shift (HTS) converter. The gas is then purified to greater than 99.99% purity. High rates of hydrogen recovery (85-90%) can be achieved under proper conditions.
Catalysts play a pivotal role in converting the syngas to other chemicals. The basic concept of a catalytic reaction is that chemical compounds adsorb onto the catalyst surface, rearrange, and combine into products that then desorb from the catalyst surface. High temperature shift catalysts are typically a mixture of iron oxide and chromium oxide. Typical lifetimes for catalysts are 3-5 years. Sulfur compounds are the main poison of catalysts and must be removed from the syngas prior to its undergoing additional reactions. This typically involves physical or chemical scrubbing of the gas. Alternatively other compounds (called promoters) can be used with some catalysts to provide a higher tolerance to sulfur, and efforts are underway to develop more sulfer tolerant catalysts. Commercial catalyst suppliers include BASF, Dycat International, Haldor Topsoe, ICI Katalco, and United Catalysts.
Numerous hydrogen reformer (converter) designs exist and can be used in a number of process configurations. The main components of the reformer are an air/fuel combustion system, a radiant heat transfer section, and a convection section. The combustion and radiant sections combust the air/fuel mixture and transfer the heat to the catalyst tubes. The convection section recovers heat by cooling down the gases that exit the reactor (flue gas). Reformers are not very efficient and only about half of the heat in the radiant section is absorbed by the furnace tubes.
In most reformers the feed gas flows upward through the catalyst tubes, but reformer furnaces can also be side-, terrace-, top-, or bottom-fired. Top-fired reformers are generally suited for larger production units. For small units (<24 tubes), side-fired units are more economical. The terrace-fired reformer is a variation of the side-wall design and bottom-fired reformers not commonly used. The reformers are typically combined with fixed bed gasifiers. Today, numerous firms license components of the steam reforming reactors and supply gas clean-up units.
Information regarding delivered hydrogen prices is not readily available, and prices vary substantially depending on the type of delivery, the quantity required, and the delivery distance. Pipeline delivery is the most economical form of transport followed by bulk liquid hydrogen delivery. The list price for liquid hydrogen (2001) was $45/GJ with considerably lower average purchase prices ($18-$24/GJ) for large-volume, bulk deliveries.
The cost of producing hydrogen from natural gas in large-scale, central production facilities is estimated to be about $5-$8/GJ. Processing difficulties, and thus capital costs, increase progressively as the feedstock changes to light hydrocarbons, heavy hydrocarbons, and to solid feedstocks. The estimated cost of producing hydrogen from biomass ranges from $7/GJ to $21/GJ with costs differing depending on the year in which the analysis was conducted, assumed feedstock costs, assumed conversion rates, and assumed value of co-products among other factors.
Hydrogen itself is a clean burning fuel. However, depending upon the feedstock used, its production can generate a considerable amount of CO2. Additionally, steam reformers produce NOx from fuel combustion. Controlling emissions becomes increasingly difficult as the feedstock becomes less hydrogen-rich (from heavy fuel oil, coke, or coal). These feedstocks also contain other impurities such as sulfur and heavy metals.