The Fischer-Tropsch synthesis (FTS) process collectively refers to the process of converting syngas to liquid hydrocarbons (chemicals composed of hydrogen and carbon) over a transition metal catalyst. The FTS process can be used to produce liquid transportation fuels (gasoline and diesel) and other chemicals (e.g., waxes, lubricants, phenol and cresol, kerosene, alcohols, ammonia, etc.).
Syngas is composed mainly of carbon monoxide (CO) and hydrogen (H2). It is produced from the gasification of feedstocks at temperatures in excess of 1100°F under conditions where the amount of oxygen (from air, pure oxygen, or steam) is less than what is needed for complete combustion. Syngas can be produced from many feedstocks, including natural gas, petroleum products, coal, and biomass.
FTS process. FTS process. Fischer-Tropsch (FT) products are produced using four main steps--syngas generation, gas purification, FT synthesis, and product upgrading (figure 1).
The Fischer-Tropsch synthesis (FTS) process collectively refers to the process of converting syngas to liquid hydrocarbons using a metal catalyst. The FTS process can be used to produce liquid transportation fuels (gasoline and diesel) and other chemicals.
Syngas is composed mainly of carbon monoxide and hydrogen. It is produced by gasifying feedstocks at high temperatures under conditions where the amount of oxygen is less than what is needed for complete combustion. Syngas can be produced from many feedstocks, including natural gas, petroleum, coal, and biomass.
In the FTS process, autothermal reforming alone, or in combination with steam reforming, are genereally recommended for the production of syngas from natural gas. For coal, the syngas is produce via high temperature gasification in the presence of oxygen and steam. FT synthesis temperatures are usually kept below 400°C (~780°F) to minimize methane (CH4) production and reactors are operated at pressures ranging from 145–580 psi. About 20% of the carbon contained in natural gas is converted to CO2 during reforming, while about 50% of the carbon in coal is converted due to the lower hydrogen content of coal. The theoretical maximum conversion rate of CO to FT products is 33% with coal as the feedstock and 78% with natural gas (low heat value basis).
The mix of products depends on reactor temperature and pressure, the feed gas composition (H2 to CO ratio), and the types of catalysts and promoters used. Depending on the types and quantities of FT products desired, either low (200–240°C; ~420-490°F) or high temperature (300–350°C; ~600-690°F) synthesis is used with either a cobalt or iron catalyst respectively. Low temperature synthesis yields high molecular weight waxes while high temperatures produce gasoline and low molecular weight olefins (e.g., ethylene and propylene). Production of gasoline products is highest under conditions of high temperatures using an iron catalyst in a fixed fluid bed reactor. The theoretical maximum is 48% conversion of the syngas. Production of diesel fractions is maximized in a slurry reactor using low temperatures and a cobalt catalyst with maximum production of about 40%.
A single pass of the syngas through the reactor always produces a wide range of olefins (alkenes, CnH2n), paraffins (alkanes, CnH2n+2), and oxygenated products (i.e., alcohols, aldehydes, acids and ketones) with water as a byproduct. Product selectivity can be improved using multiple step processes to upgrade the FTS products. Upgrading involves use of a combination of hydrotreating, hydrocracking, and hydroisomerization in addition to product separation. Hydrotreating involves adding hydrogen and a catalyst to remove impurities like nitrogen, sulfur, and aromatic hydrocarbons. Hydrocracking is a catalytic cracking process assisted by an elevated partial pressure of hydrogen gas. Hydroisomerization involves the addition of hydrogen and a catalyst to drive isomerization processes.
FTS catalysts. Catalysts play a pivotal role in converting syngas to products. The basic concept of a catalytic reaction is that compounds adsorb onto the catalyst surface, rearrange, and combine into products that desorb from the surface. One of the fundamental functional differences between syngas synthesis catalysts is whether or not the adsorbed CO molecule dissociates on the catalyst surface. For FTS and higher alcohol synthesis, CO dissociation is a necessary reaction condition.
Iron (Fe) and cobalt (Co) catalysts are most commonly used in FT synthesis.
Fe is very active; readily forms compounds which show FTS activity; and undergoes the water gas shift (WGS) reaction. The use of promoters and supports are essential for Fe catalytic activity. Co catalysts tend to have longer lifetimes than Fe catalysts and do not have water gas shift activity, which leads to improved carbon conversion to products because CO2 is not formed. In FTS processes, Co catalysts yield mainly straight chain hydrocarbons rather than oxygenated hydrocarbons that occur with Fe catalysts. Co catalysts are 230 times more expensive than Fe but are still used because they demonstrate activity at lower synthesis pressures so the higher catalyst cost can be offset by lower operating costs. Co catalysts are not sensitive to the addition of promoters. Other catalysts can also be used (e.g., Ni, Ru) but are more expensive.
Carbon (coke) deposits can deactive the catalyst. Coke deposition is affected by the addition of promoters, and reaction temperature and pressure. It is generally unavoidable during FT synthesis, and thus the process must be operated in a way that balances the tradeoffs between lower output due to coke deposition and catalyst regeneration and replacement costs. Because of its high activity, coke deposition rates are typically higher for Fe catalysts than Co catalysts, and consequently, Co catalysts have longer lifetimes.
Catalyts can be deactivated by impurities in the syngas. Sulfur is the major catalyst poison which is present in both natural gas and coal. Sulfur compounds rapidly deactivate both Fe and Co catalysts but Co catalysts are more sensitive to sulfur poisoning. Other syngas impurities that can poison FTS catalysts include halides, nitrogen compounds (NH3, NOx and HCN), and water. Commercial processes are available to clean syngas to meet impurity tolerances. The level of gas cleaning required must balance the costs of cleaning with the reduced lifetime of the catalyst. Gas cleaning costs are a function of the size of the operation. Given the relative cost of Co versus Fe, more efficient sulfur removal should be expected for FTS with Co catalysts.
The focus of catalyst development is on improved catalyst lifetimes, activity, and selectivity which are affected by the use of chemical and structural promoters, catalyst preparation and formulation, pretreatment and reduction, selective poisoning, and shape selectivity.
FTS reactors. FTS reactions generate a large amount of heat and reactor design and process development have focused on heat removal and temperature control. Insufficient heat removal leads to localized overheating which causes high carbon deposition on the catalysts and subsequent deactivation of the catalyst. Methane formation also dominates at higher temperatures at the expense of desired FTS products. Commercial FTS reactors consist of 4 basic designs (figure 2).
The ARGE high capacity FT reactor (fixed bed tubular design) has been used for many years and contains 2,000 tubes filled with Fe catalyst immersed in boiling water for heat removal. The water bath temperature is maintained in the reactor by controlling the pressure. Syngas is introduced into the top of the reactor, flows through the tubes, and the products exit at the bottom of the reactor. Wax accounts for 50% of the products. Conversion efficiencies are on the order of 70%. The reactor is operated at 20-30 bar at an operating temperature of 220-260ºC (~455-525ºF). Additional temperature control is obtained by using high gas velocities and gas recycling. Catalyst lifetimes are around 70-100 days and removal can be difficult.
High temperature circulating fluidized bed reactors (Synthol reactors) have been developed for gasoline and light olefin production and operate at 350°C (~690°F) and 25 bar. The combined fresh and recycled gas feed enters at the bottom of the reactor and entrains (suspends) the catalyst which is carried into the reaction zone where heat is removed through heat exchangers. Product gases and the catalyst are then transported into a hopper where the catalyst settles out and the product gases exit through a cyclone. These reactors have been successfully used for many years, but are constrained by the physical complexity of the reactors and their limited capacity.
In the fixed fluidized bed Sasol Advanced Synthol reactor, the syngas is introduced through a distributor and bubbles up through the catalyst bed which contains a heat exchanger to remove heat. Process conditions are similar to those in the Synthol reactors. Compared to the Synthol reactor, these new reactors have the same capacity, better thermal efficiency, and maintain more consistent pressure within the reactor, but are half the cost and size. Operating costs are also lower, there is more flexibility in the types of products that can be produced, and they can be scaled up to 20,000 barrel per day of production.
The low temperature slurry reactor is a 3-phase reactor consisting of a solid catalyst suspended and dispersed in a high thermal capacity liquid (often the FTS wax product). Syngas is bubbled through the liquid phase achieving contact with the catalyst while also keeping the catalyst particles dispersed. Slurry reactors are optimized at low temperatures for FTS wax production with low methane production. Compared to the fluidized bed reactors, liquid slurry bed reactors have better temperature control, lower catalyst loading, and significantly lower catalyst attrition rates. The improved isothermal conditions in slurry bed reactors allows for higher average reactor temperatures leading to greater conversion of syngas to products. Compared with multitubular fixed bed reactors, slurry reactors have lower pressure differences across the reactor resulting in lower costs. However, any poisons in the syngas will affect all of the catalyst in the reactor, whereas in a fixed tube design, they will primarily affect only the catalyst near the gas inlet. These reactors are beginning to be used in commercial applications.
Commercial production of FTS compounds. The first FT plants began operation in Germany in 1938, but were closed following World War II. Numerous FT plants of varying reactor designs have been established in South Africa and produce more than 200 fuel and chemical products using coal as the feedstock. Plans are underway to fully or partially replace coal with natural gas in many of these facilities. Shell produces 500,000 tonnes (1.1 billion pounds) of FT products/yr using natural gas and a cobalt catalyst at its facility in Malaysia. Syntroleum operates a 10,000 barrel per day (BPD) plant in Australia using natural gas to produce liquid fuels and specialty products.
Most of the capital cost for FTS plants is for the generation of the syngas.
The capital cost of the Shell plant was about $660 million. The capital cost of replacing 16 South African circulating fluidized bed reactors with 8 fixed fluidized bed reactors was $225 million. It is estimated that a two-train gas-to-liquid (GTL) plant composed of slurry phase reactors can be constructed at a capital cost of about $25,000/BPD capacity for a 30,000 BPD and $30,000/BDP capacity for a 10,000 BDP facility.
The feedstock cost is a major component of the operating costs. The cost of producing syngas from methane is 30% lower than that from coal and is a more efficient process. At a natural gas price of $0.50/MMBtu (inexpensive), the feedstock accounts for $5/BBL of the product price. Other operating costs (fixed and variable) are estimated to contribute an additional $5/BBL to the cost of production.
For a 50,000 BPD plant and a natural gas cost of $0.50/MMBtu (inexpensive), the estimated cost of producing FT liquid fuels is $24 to $26/BBL ($0.76 to $0.83/gallon) and $52/BBL ($1.70/gallon) if the price of the natural gas is $4/MMBtu. The cost using Illionois #6 coal as the feedstock was estimated to be $46/BBL ($1.50/gallon). The estimated cost of production using biomass feedstocks ranged from $1.10/gallon to $4.10/gallon depending on assumed feedstock price, conversion rates, and technology used among other factors.
Tests performed on heavy duty trucks showed decreased vehicle emissions of HC, CO, NOx, and PM (particulate matter) when using a FT fuel. Overall, FT diesel showed a reduction in regulated as well as some unregulated emissions compared to conventional diesel. Life cycle assessments (LCA) indicate that FT liquid fuels produced from fossil fuel feedstocks offer little energy and greenhouse gas emission benefits compared to traditional transportation fuels. Use of biomass feedstocks has the potential to alter this situation.