This integrated gasification combined cycle power plant (see Figure 7.3) is operated by Tampa Electric [6]. In 2001/2002, the plant used about 1.5 wt% woody biomass harvested from a five-year-old eucalyptus grove along with coal to test whether biomass can be converted to fuel gas and whether a fuel handling system accommodates this change. The original system was not designed to handle softer fibrous biomass.

The Polk Power Station used old ChevronTexaco IGCC technology that is now owned by General Electric. In this process, 60-70% coal-water slurry is fed to the gasifier at the rate of 2,200 tons (on dry basis) of coal per day. The normal feed is a blend of coal and petroleum coke, the solid residue from crude oil refining. The fresh feed is mixed with unconverted recycled solids and finely ground in rod mills until 98% of the particles are less than 12 mesh in size. The slurry passes through a series of screens before being pumped into the gasifier. The slurry and oxygen are mixed in the gasifier process injector. The gasifier is designed to convert 95% of carbon per pass, and it produces syngas of 250 BTU/Scf heat content.

A schematic of the Polk Power Station plant is shown in Figure 7.3. As shown in the diagram, the syngas coming out of gasifier is cooled in a series of steps, each recovering heat in the form of saturated high-pressure steam. The first syngas cooler, called the "radiant syngas cooler" (RSC), produces 1,650 psig saturated steam. The gas from RSC is split into two streams and they are sent to parallel convective syngas coolers (CSC) where the pro­cess of cooling and generating additional high-pressure steam (at lower

FIGURE 7.3

Polk Power Station co-gasification configuration. (From Ratafia-Brown et al. 2007. Assessment of Technologies for Co-converting Coal and Biomass to Clean Syngas-Task 2 Report (RDS), NETL report (May 10); and McDaniel,
temperature) is repeated. The gases then further go through a simultaneous cooling and impurity removal (particulates, hydrogen chloride) process. A final trim cooler reduces the syngas temperature to about 100°F for the cold gas clean-up (CGCU). The CGCU system is a traditional amine scrubber sys­tem, and it removes sulfur which is then converted to sulfuric acid and sold to the local phosphate industry.

The eucalyptus feedstock used in this power plant contained about 1/3 of heating value per pound at about half the bulk density of coal. The character­istics of the mixed feedstock for the Polk Power Plant are shown in Table 7.11a. These numbers indicate that even a modest concentration of this biomass will require a massive and expensive feed system. Although the combined characteristics of the mixed feedstock are not significantly different from the baseline, it increases hydrogen, oxygen, and ash content by 4.6, 11, and 3.4%, respectively. The CO2 discharge is reduced by 0.87%. Biomass used in the Polk plant did not lend itself to size separation and screening, and it caused minor plugging of the suction to one of the pumps [6]. The results indicate that for a slurry system, feed preparation must be tailored to the nature of the biomass in order to prevent any malfunction by the slurry pump as well as downstream gas cleaning and turbine operation. Typical test results for the Polk Power Station are described in Tables 7.11a and b. The experience of the Polk Power Station can be extended to coal and other materials.

Polk Power Station Biomass Co-Gasification Test Results

Source: Ratafia-Brown, et al., 2007. "Assessment of Technologies for Co-converting Coal and Biomass to Clean Syngas-Task 2 Report (RDS)", NETL report (May 10); and McDaniel,

J., 2002. "Biomass gasification at Polk Power Station-Final Technical Report," DOE award DE-FG26-01NT41365 (May).