BURNERS
Most burner designs are confidential, although a fair amount of knowledge can be gleaned from the patent literature. Most of the burners (sometimes known as combustors or feed injectors) used are of the со-annular type where the reactants are fed through axisymmetrical annular openings at the front of the burner. The burners have to be protected from the hottest part of the reactor. Hence, having the oxygen come in contact with the hot syngas in the reactor near the burner opening must be avoided. Very large capacity burners are only possible when the coal also enters the reactor via an annular slit (van der Burgt 1990). In virtually all cases the reactants come into contact with each other inside the reactor. Premixing the reactants is not recommended because of the dangers associated with precombustion.
Special attention has to be paid to cooling the burner front. Water-cooling is applied in most cases. For safety reasons, the pressure of the water should be higher than in the gasifier so that the gas can never enter the cooling water system. This has the advantage that there is a large degree of freedom for selecting the cooling water temperature. In general, this temperature should not be too low, as then the steam present in the blast could condense inside the burner unless precautions are taken to insulate the steam tract in the burner.
For pressurizing gasifiers, special attention must be given to the heat-up procedure since the gas flows during this operation differ considerably from the normal design case. Where the reactor has an insulating refractory wall it is possible to heat up this wall with an atmospheric pressure gas or oil burner, exchange the burner for a coal burner, and ignite the coal/blast mixture on the hot brick wall. In the case of membrane walls with their very limited heat capacity, this is not possible as they will cool in less than a minute, and burner changes cannot be made in this short period. Therefore, in the case of a membrane wall gasifier, the ignition burner must remain lit until the first coal burner ignited. During this period there must be a continuous flame while the pressure is increased from atmospheric to operating pressure. This is particularly complicated for reactors with multiple burners or where the heat-up burner is not integrated into the main burner.
The relatively long time that is required for this operation is one of the reasons why IGCC power stations are not generally considered suitable for peak shaving duty. On the other hand, where a reactor is kept on hot stand-by, a quick start is possible, particularly since in most cases the procedure is at least semi — if not fully automatic.
Burners for oil service are also generally of a water-cooled со-annular design (Pelofsky 1977; Weigner etal. 2002). The design of such burners, which are centrally top — mounted, can include a removable gas-fired start-up burner with internal igniter. Weigner etal. (2002) describes an automatic temperature ramp system integrating firing of the start-up and main burners during reactor heat-up. The start-up burner is removed at 1100°C prior to ignition of the main feedstock. With such a burner, a turndown ratio of 60% is achievable.
A similar arrangement can be seen in the top-fired Noell reactor, which incorporates a central gas flow to the pilot burner surrounded by annular slits for oxygen that incorporate a swirler and an outer slit for fuel (Schingnitz et al. 2000).
Burner lifetime for coal service, particularly for slurry feeds, continues to be a source of concern. Typical lifetimes of between two and six months have been reported (Clayton, Stiegel, and Wimer 2002). Burners in oil service achieve a service life of over one year (Higman 1994; Weigner etal. 2002), which is generally considered acceptable even if a “long-term goal of two years” would be desirable (Clayton, Stiegel, and Wimer 2002).