Synthesis Gas. James G. Speight
the particle size, the more contact area between the coal and the reaction gases, leading to a more rapid reaction. For medium-rank coal and low-rank coal, reactivity increases with an increase in pore volume and surface area, but for coal having a carbon content greater than 85% w/w these factors have no effect on reactivity. In fact, in high-rank coal, pore sizes are so small that the reaction is diffusion controlled. Other feedstocks (such as crude oil residua and biomass) are so variable that gasification behavior and products vary over a wide range. The volatile matter produced during the thermal reactions varies widely and the ease with which tar products are formed as part of the gaseous products makes gas cleanup more difficult.
The mineral matter content of the feedstock also has an impact on the composition of the produced synthesis gas. Gasifiers may be designed to remove the produced ash in solid or liquid (slag) form. In fluidized or fixed-bed gasifiers, the ash is typically removed as a solid, which limits operational temperatures in the gasifier to well below the ash melting point. In other designs, particularly slagging gasifiers, the operational temperatures are designed to be above the ash melting temperature. The selection of the most appropriate gasifier is often dependent on the melting temperature and/or the softening temperature of the ash and the feedstock which is to be used at the facility.
Gasification reactors are very susceptible to ash production and properties. Ash can cause a variety of problems particularly in up or downdraught gasifiers. Slagging or clinker formation in the reactor, caused by melting and agglomeration of ashes, at the best will greatly add to the difficulty of gasifier operation. If no special measures are taken, slagging can lead to excessive tar formation and/or complete blocking of the reactor. A worst case is the possibility of air-channeling which can lead to a risk of explosion, especially in updraft gasifiers. Whether or not slagging occurs depends on the ash content of the fuel, the melting characteristics of the ash, and the temperature pattern allowed by gasifier design. Local high temperatures in voids in the fuel bed in the oxidation zone, caused by bridging in the bed, may cause slagging even using fuels with a high ash melting temperature.
Generally, slagging is not observed with fuels having mineral matter ash contents less than below 5 to 6% w/w. Severe slagging can be expected for fuels having mineral matter contents in excess of 12% w/w/. For fuels with mineral matter contents between 6 and 12% w/w, the slagging behavior depends to a large extent on the mineral matter composition – reflected in the ash melting temperature – which is influenced by the presence of trace elements giving rise to the formation of low melting point eutectic mixtures.
Updraft and downdraught gasification reactors are able to operate with slagging fuels if specially modified (continuously moving grates and/or external pyrolysis gas combustion). Cross draught gasification reactors, which work at temperatures on the order of 1500°C (2700oF) and higher, need special safeguards with respect to the mineral matter content of the fuel. Fluidized bed reactors, because of their inherent capacity to control the operating temperature, suffer less from ash melting and fusion problems.
High moisture content of the feedstock lowers internal gasifier temperatures through evaporation and the endothermic reaction of steam and char. Usually, a limit is set on the moisture content of feedstock supplied to the gasifier, which can be met by drying operations if necessary. For a typical fixed bed gasifier and moderate carbon content and mineral matter content of the feedstock, the moisture limit may be on the order of 35% w/w. Fluidized-bed and entrained-bed gasifiers have a lower tolerance for moisture, limiting the moisture content to approximately 5 to 10% w/w of the feedstock. Oxygen supplied to the gasifiers must be increased with an increase in mineral matter content (ash production) or moisture content in the feedstock.
Depending on the type of feedstock being processed and the analysis of the gas product desired, pressure also plays a role in product definition (Speight, 2011a, 2013). In fact, some (or all) of the following processing steps will be required: (i) pretreatment of the feedstock, (ii) primary gasification, (iii) secondary gasification of the carbonaceous residue – char – from the primary gasifier, (iv) removal of carbon dioxide, hydrogen sulfide, and other acid gases; (v) shift conversion for adjustment of the carbon monoxide/hydrogen mole ratio to the desired ratio, and (vi) catalytic methanation of the carbon monoxide/hydrogen mixture to form methane. If high heat-content (high-Btu) gas is desired, all of these processing steps are required since gasifiers do not yield methane in the concentrations required (Speight, 2008, 2011a, 2013).
Thus, the reactivity of the feedstock is an important factor in determining the design of the reactor because feedstock reactivity, which determines the rate of reduction of carbon dioxide to carbon monoxide in the reactor, influences reactor design insofar as it dictates the height needed in the reduction zone. In addition, certain operational design characteristics of the reactor system (load following response, restarting after temporary shutdown) are affected by the reactivity of the char produced in the reactor. There is also a relationship between feedstock reactivity and the number of active places on the char surface, these being influenced by the morphological characteristics as well as the geological age of the fuel. The grain size and the porosity of the char produced in the reduction zone influence the surface available for reduction and, therefore, the rate of the reduction reactions which are facilitated by reactor design.
In terms of feedstock quality, mixed feedstock must be given careful attention by virtue of the composition – related to the amount of each feedstock component in the mixture – as well as the potential interaction of the components with each other during the initial heating stage just prior to the gasification stage in the gasifier.
Both fixed-bed and fluidized-bed gasifiers have been used in co-gasification of coal and biomass – these include a downdraft fixed-bed gasifier (Kumabe et al., 2007; Speight, 2011a). However, operational problems when a fluidized-bed gasifier was employed that included (i) defluidization of the fluidized-bed gasifier caused due to agglomeration of low melting point ash present in the biomass, and (ii) clogging of the downstream pipes due to excessive tar accumulation (Pan et al., 2000; Vélez et al., 2009). In addition, cogasification and co-pyrolysis of birch wood and coal in an updraft fixed-bed gasifier as well as in a fluidized-bed gasifier has yielded overhead products with to 6.0% w/w tar content while the fixed-bed reactor gave tar yields on the order of 25 to 26% w/w for cogasification of coal and silver birch wood mixtures (1 :1 w/w ratio) at 1000°C (1830oF) (Collot et al., 1999).
From the perspective of the efficient operation of the reactor, the presence of mineral matter has a deleterious effect on fluidized-bed reactors. The low melting point of ash formed from the mineral matter present in woody biomass can lead to agglomeration which influences the efficiency of the fluidization – the ash can cause sintering, deposition, and corrosion of the gasifier construction metal. In addition, biomass containing alkali oxides and salts can cause clinkering/slagging problems (McKendry, 2002).
2.5 Gasification Processes
All of the main types of gasifiers (Chapter 3) can be adapted to be used with various waste types as feedstock, but plasma gasification is becoming a technology of the near future, especially in regard to the treatment of municipal solid waste. In the plasma gasification process, the plasma (ionized gas at high temperature which conducts a strong electrical current) allows extremely high gasification temperatures of 4000oC (7200oF) to over 7000oC (12600oF). These high temperatures completely break down toxic compounds to their elemental constituents, making them easily neutralized, and the gas is mixed with oxygen and steam inside the gasifier. The organic compounds in the fuel are converted to synthesis gas, similar to the other gasification technologies, and any residual materials are captured in a rock-like mass which is highly resistant to leaching. With this technology, all known contaminants can be easily contained, making it ideal for municipal solid waste applications where feedstock composition is sometimes unclear. Also, after the initial electricity required at startup for the plasma gasifier, these systems are self-sustained by running off the electricity produced by firing the synthesis gas in a gas turbine.
Thus, gasification processes can accept a variety of feedstocks but the reactor must be selected on the basis