Published November 2003
Carbon dioxide (CO2) is mostly generated as waste byproduct in processes involving fossil fuels combustion, chemicals production and synthetic fuels manufacturing. Natural gas (NG), itself, in some cases contains sizeable concentrations of CO2. Most of the CO2 produced in above processes is dumped into the air. However, due to greenhouse effect of gas, efforts are going on for sometime to develop new technologies-and upgrade the current ones-which would prevent or reduce CO2 generation at source, or capture the generated CO2 for subsequent disposal into safe sinks (oil & gas reservoirs, deep unmineable coal seems, saline aquifers, deep ocean, etc.). Another alternative under consideration is converting of the gas into a valuable chemical product. Synthesis gas (syngas), a mixture of hydrogen (H2) and carbon monoxide (CO) and a building block for several important chemicals, provides an excellent outlet for such waste CO2.
Syngas can be produced by reforming NG with CO2 or steam. Partial oxidation of NG and heavier hydrocarbon feedstocks is another means of producing syngas. Reforming of NG with CO2 can produce syngas with a H2/CO ratio of up to 1 at 1,652-1,832°F (900-1,000°C) and 1-20 atmosphere pressure. Group VIII metals are suitable as catalyst for NG-CO2 reforming with rhodium and ruthenium being the best. The high cost of the two metals is, however, a deterrent to commercial acceptance. Nickel, the least expensive, has a high tendency of coking under most reforming conditions, but is still a more preferred option. Palladium and platinum offer a compromise between costs and good functionality. Among the supports tested for above catalysts, -alumina, magnesia, silica, zirconia, and titania are note-worthy. This Review evaluates construction and operational costs of a grassroots syngas plant of 220,000 Nm3/hr-capacity, based on NG reforming with CO2. The estimates indicate that the developing NG-CO2 route has a fairly good potential of economic compatibility with the current NG-steam route (see cost details inside). The evaluation takes into consideration two scenarios:
1) Production of syngas with imported CO2: 2) Production of syngas with captured and recycled CO2. While the operating costs for two processes are identical, overall economies for second option are less favorable, but not glaringly away from economics of current commercial NG-Steam process. The biggest challenge, however, is development of an economical and stable catalyst suitable for use in commercial-scale plants. Another important issue is the cost-effective capturing of CO2 from some 24 million metric tons of gas emissions each year from fuels. If current CO2 capturing cost could be reduced by 35-40%, NG-CO2 reforming is potentially sustainable from CO2 recovered from flue gases.