Other Carbon-Based Fuel Gases

Natural gas is not the only carbon-based gas which can be used as a fuel. One of the simplest is derived from coal. The reaction of coal, coke, or charcoal with insufficient air for complete combustion forms producer gas which is a mixture of CO and N2 in a 1 2 ratio  

Any moisture in the air results in the formation of hydrogen by the water gas reaction 

When coal is heated to about 500°C, the CH4 trapped in the pores is released. At higher temperature (1,000°C), the organic matrix in the coal is decomposed, forming mostly H2 with some COx. This coal gas is also formed during the coking process. 

The gasification of coal with oxygen and steam under pressure is called the Lurgi process and the gas is called Lurgi gas. As the pressure is increased from 5 to 20 atm, the CH4 and CO2 increases, while the H2 and CO decrease. This gas, once popular as a town gas, is seldom used today. 

The approximate composition and heating values of the various fuel gases are given in Table 6.3. The flame speeds relative to that of H2 are also shown and account for the need to change burner nozzles when fuel gas composition is significantly changed. 

---

The primary product is fuel-grade, coal-derived gas which is similar to natural gas. The basic gasification process can also be applied to other carbon-based feedstocks such as biomass or municipal waste. 

Combustion is an oxidation-reduction reaction between a nonmetallic material and molecular oxygen. Combustion reactions are characteristically exothermic (energy releasing). A violent combustion reaction is the formation of water from hydrogen and oxygen. As discussed in Section 9.5, the energy from this reaction is used to power rockets into space. More common examples of combustion include the burning of wood and fossil fuels. The combustion of these and other carbon-based chemicals forms carbon dioxide and water. Consider, for example, the combustion of methane, the major component of natural gas ... 

SASOL. SASOL, South Africa, has constmcted a plant to recover 50,000 tons each of 1-pentene and 1-hexene by extractive distillation from Fischer-Tropsch hydrocarbons produced from coal-based synthesis gas. The company is marketing both products primarily as comonomers for LLDPE and HDPE (see Olefin polymers). Although there is still no developed market for 1-pentene in the mid-1990s, the 1-hexene market is well estabhshed. The Fischer-Tropsch technology produces a geometric carbon-number distribution of various odd and even, linear, branched, and alpha and internal olefins however, with additional investment, other odd and even carbon numbers can also be recovered. The Fischer-Tropsch plants were originally constmcted to produce gasoline and other hydrocarbon fuels to fill the lack of petroleum resources in South Africa. 

The production of coke involves the heating of coal in the absence of air, called the carbonization or destructive distillation of coal. Carbonization, besides its main purpose of production of coke, also results in a coproduct called coke oven gas from which various liquid products such as tar, benzol, naphthalene, phenol, and anthracene are separated. There are two main types of carbonization based on the temperature to which the coal is heated in the absence of air. One type is low-temperature carbonization (LTC) the other is high-temperature carbonisation (HTC). Some features of LTC and HTC are listed in Table 1.28. The LTC Process is mainly carried out to manufacture domestic smokeless fuel. This presentation, however, concentrates on the HTC process by which metallurgical coke is produced. 

This system includes several mixing and heat exchange units. A concept for an integrated, microtechnology-based fuel processor was proposed by PNNF [8]. As examples for unit operations which may be included in future integrated systems the same publication mentions reactors for steam reforming and/or partial oxidation, water-gas shift reactors and preferential oxidation reactors for carbon monoxide conversions, heat exchangers, membranes or other separation components. 

Derivation (1) From natural gas by absorption or adsorption. (2) From coal mines for use as fuel gas. (3) From a mixture of carbon monoxide and hydrogen (synthesis gas) obtained by reaction of hot coal with steam the mixed gas is passed over a nickel-based catalyst at high temperature. See methanation. Methane can also be obtained by a nickel-catalyzed reaction of carbon dioxide and hydrogen. (4) Anaerobic decomposition of manures and other agricultural wastes. (5) By horizontal drilling of coal seams. 

In polymer electrolyte membrane fuel cells, like in many other kinds of fuel cells, gas-diffusion electrodes are used. They consist of a porous, hydrophobic gas-diffusion layer (GDL) and of a catalytically active layer. The diffusion layers (often called backing layers) usually consist of a mixture of carbon black and about 35% by mass of polytetrafluoroethylene (PTFE) applied to a conducting base (most often a thin graphitized cloth). The GDLs yield a uniform supply of reactant gas... 

In an ideal combustion of fuel purely based on stoichiometric conversion, fuel is burnt to CO2 and H2O 100% with 0% excess air so that there is no oxygen left in the combustion flue gas. However, in reahty, industrial fired heaters require excess air. To achieve complete combustion, a minimum of 10-15% excess air (2—3% O2 in flue gas) is required for fuel gas. Otherwise, carbon monoxide and unbumed hydrocarbon could appear in the flue gas leaving stack. Fuel oil usually requires 5-10% higher excess air than fuel gas. In other words, a minimum of 15-25% excess air (3-5% O2) is required for fuel oil for complete combustion. 

---