Gasification

Waste Heat Recovery

The product of the partial oxidation reaction is a raw synthesis gas at a temperature of about 1300°C that contains particles of residual carbon and ash. The recovery of the sensible heat in this gas is an integral feature of the SGP process.

Primary heat recovery takes place in a syngas cooler generating high-pressure steam (up to 120 bar) steam in which the reactor effluent is cooled to about 340°C. The syngas cooler is of a Shell proprietary design discussed in more detail in Section 6.6.

Secondary heat recovery takes place in a boiler feed-water economizer immedi­ately downstream of the syngas cooler.

Carbon Removal

The partial-oxidation reactor-outlet gas contains a small amount of free carbon. The carbon particles are removed from the gas together with the ash in a two-stage water wash. The carbon formed in the partial oxidation reactor is removed from the system as a carbon slurry together with the ash and the process condensate. This slurry is subsequently processed in the ash-removal unit described in the following. The product syngas leaves the scrubber with a temperature of about 40°C and is essentially free of carbon. It is then suitable for treatment with any commercial desulfurization solvent.

Carbon Management

Over the course of its development SGP has gone through three distinct stages in its approach to management of the carbon produced in the gasification section.

The early plants were equipped with the Shell Pelletizing System, an extraction pro­cess using fuel oil as extraction medium. The fuel oil was put in contact with the car­bon slurry in the pelletizer where carbon pellets of about 5-8 mm were formed leaving a clear water phase. These pellets were separated from the water on a vibrating screen.

The pellets could be burned directly or mixed in with fuel oil to make a liquid fuel known as carbon oil. The carbon oil could in part be used as feedstock for the gasifier, thus providing partial recycle of the carbon. This process had the advantage of being cheap and simple to operate. However, in the extraction process with fuel oil, the sep­aration of soot from the heavy metals (vanadium and nickel) from the gasifier feed­stock was poor so that any attempt at 100% carbon recycle brought an unacceptable buildup of metals in the system. Furthermore, there was some water slip with the pel­lets in the carbon oil, so the carbon oil mixing process could not be operated above 100°C without causing foaming. This limitation meant that this process became unus­able with the increasingly heavier feedstocks appearing on the market.

The next development was therefore to substitute the fuel oil with naphtha as extraction medium. This development was known as the Naphtha Soot Carbon Recovery process. The principle of extraction in a mixer to increase the size of the agglomerates as well as mechanical sieving was maintained so as to achieve a low naphtha/slurry ratio. The equipment was now operated under pressure, however. The naphtha-soot pellets are mixed with the main feedstock at whatever temperature is required to achieve the desired viscosity. The naphtha is then distilled off from the feed and recycled to the extraction stage leaving the soot behind in the feed (Brejc 1989). The use of naphtha as an intermediate allows the use of heavier, more viscous feedstocks than in the case of pelletizing with fuel oil. Also, an improvement in the separation between carbon and ash allows 100% carbon recycle. Nonetheless, an ash buildup factor of about 3:1 can be observed under 100% recycle conditions. These improvements are bought, however, at a cost in investment and operating expense. Furthermore, the ash buildup still places a limit on ash content in the feedstock.

The third generation of soot management now employed by Shell is based on filtration of the carbon slurry and subsequent handling of the soot-ash filter cake and goes under the name of Soot-Ash Removal Unit (SARU; Figure 5-29). The carbon slurry leaves the SGP under pressure at a temperature of some 125 °С and is flashed into an intermediate slurry storage tank at atmospheric pressure. Thence it is cooled before water and filtrate are separated in a membrane filter press. The clear filtrate is mostly recycled to the SGP scrubber as wash water. The overall water balance produces a surplus, however, which is treated in a sour water stripper to remove dissolved gases such as H2S, HCN, and ammonia before being sent to a biotreater.

The filter cake contains typically about 75-85% moisture, but nonetheless behaves for most purposes as a solid. It is then subjected to thermal treatment in a multiple hearth furnace (Figure 5-30). The carbon is burnt off under conditions that prevent the formation of liquid vanadium pentoxide, which has a melting point at about 700°C. In this type of furnace, which is used extensively in the vanadium industry, the filter cake is fed from the top of the furnace in counter-current to the combustion air/flue gas. Rakes, mounted to the central air-cooled shaft, rotate slowly drawing the solid material to downcomers, which are located on alternate hearths at the center and the periphery of the furnace. In the upper hearths the rising flue gas dries the filter cake. In the lower hearths the filter cake is gently burnt off. The bottom product has less than 2 wt% carbon and, depending on the metals in the SGP feedstock, can contain typically 75% V205. The soot combustion is under the prevail­ing conditions not quite complete, so that the off-gas contains not only the water vapor from the moisture in the filter cake but also carbon monoxide. In addition it contains traces of H2S contained within the filter cake. This off-gas is incinerated either as part of the SARU facility or centrally depending on the site infrastructure.

SGP is a reliable process that has been proved in many applications worldwide. This reliability is based on the use of proven equipment in critical duties. Typical life­times are listed in Table 5-12 (Higman 1994).

SGP employs a sophisticated automatic start-up and shutdown system. Since But- zert’s description of the main characteristics (Butzert 1976), further developments include, for example, automated reactor heat-up and a system for minimizing flaring of sulfur-containing gases during start-up.

Process Performance

Table 5-13 provides some information on typical process performance with differ­ent feedstocks.

5.4.1 Lurgi’s Multipurpose Gasification Process (MPG)

Lurgi has maintained a leading position in coal gasification since the 1930s, but for many years worked as contractor and licensing agent for the Shell SGP process for partial oxidation of liquids and gases. In 1998 Lurgi announced that it would now be marketing its own technology under the name of LurgiSVZ multipurpose gasifica­tion (MPG; Figures 5-31 and 5-32). This technology had been in existence since 1969 at what is today SVZ Schwarze Pumpe (Hirschfelder, Buttker, and Steiner 1997). It was developed originally out of a Lurgi moving-bed gasifier to process tars produced in the other twenty-three Lurgi gasifiers at the location, which produced town gas from lignite.

Recently the start-up after the revamp of an existing 60 bar 16 t/h asphalt feed reactor has been reported (Erdmann, Liebner, and Schlichting 2002).

Process Description

The gasification reactor is a refractory-lined vessel with a top-mounted burner. The burner has a multiple-nozzle design that allows it to accept separate feed streams of otherwise incompatible materials.

Waste Heat Recovery

MPG is offered with two alternative syngas cooling configurations, quench and heat recovery. The criteria for selection of the cooling configuration are listed in Table 5-14.

Carbon Management

The carbon is washed out of the gas with a conventional water wash. Lurgi’s carbon management process for MPG, the metals ash removal system (MARS), is a filtra­tion-multiple hearth furnace process. The flowsheet is very similar to that of Shell’s SARU described in Section 5.4.2. Differences are a matter of detail in equipment design and selection. Lurgi uses its own proprietary design of multiple-hearth furnace, which already had a long track record in the vanadium industry before find­ing application in the field of residue gasification. Lurgi also propagates the use of belt filtration. This has the advantage of being a continuous process with easier operation and maintenance. In order to achieve the same dewatering performance as a membrane filter press, flocculants are required.

Process Performance

A particular feature of MPG is its multinozzle burner, allowing a wide range of feedstocks. The Table 5-15 lists operational ranges and maximum concentrations of base components and contaminants as experienced with MPG.

Typical product gas quality is in Table 5-16.