Case Study: A renewable energy plant converting 1000 TPD (dry) urban green waste can generate 600 BPD of Fischer-Tropsch fuels and 16.5 MW of export electricity to yield 466 bbl/day of "green diesel" or syngas and 166 bbl/day of Naphtha.

The following process and key components are described for the purpose of meeting the above requirements.

The Fischer-Tropsch gasification process consists of a Gasifier Feeding System, Fluid Bed Gasifier, Refractory Lining for Syngas and Olivine Sand, Gasifier Cyclones, Combustor Seal Pot, Combustor, Refractory Lining for Flue Gas and Olivine Sand, Combustor Cyclones, Gasifier Seal Pot, Flue Gas Cleaning System, and Ash Handling System.

Gasifier Feeding System

The gasification system operates in a dual fluidized bed mode that circulates sand between the two primary system components, the gasifier and the combustor.

The dried feed is fed by conveyor to two Live-Bottom Bins, each with 48 tons (1 hour) of storage capacity at an average density of 10 lb/ft3. Each bin is dedicated to a separate feed system for the gasifier to eliminate feeding problems as a common source of unscheduled shutdowns. Each live bottom system is capable of continuous retrieval of 95,200 lb/hr = 9,500 cfh with a variable speed drive. Each bin is fitted with a filtered vent system for filling at ambient pressure.

Each of two pressure sealing rotary valves is located at the discharge of each live bottom retrieval system, capable of the same continuous retrieval capacity, with a constant speed drive. Design pressure differential across each valve is 40 psig. Two 100% constant-speed auger feeders operate without a pressure differential to feed the gasifier and maintain >90% availability goal for continuous operation of the gasification system. These feeders are installed at 15 degrees above horizontal to prevent hot sand from approaching the feeder drive seals.

Fluid Bed Gasifier

Feedstock from the feeders, high temperature sand from the combustor, and fluidizing steam are fed to the bottom section of the gasifier. The heat provided by the sand drives the endothermic gasification reactions in the gasifier to form a raw product gas primarily comprised of syngas (H2 and CO), methane, tars and char. The gasifier is designed to the following criteria:

Sample Gasifier Design Criteria

Condition

Gasifier

Shell Design Pressure, psig

40

Shell Design Temperature, °F

500

Inside Refractory Design Temp, °F

1,500 (syngas, char & sand)

I.D. of Shell

68"

Tangent Length

528"

Distributor Design

ACFM = 3,400

Superficial velocity in the gasifier is more than 35 fps to elutriate the sand and char to the gasifier cyclones. The gasifier vessel is constructed of low-alloy carbon steel, rated to minimize hydrogen induced cracking (HIC). The arrangement at the bottom of the gasifier is shown in Figure 1.

Gasifier Lower Section

Figure 1: Gasifier Lower Section with On-line Tramp Removal

Refractory Lining for Syngas and Olivine Sand

The gasifier shell is insulated with double-layer refractory lining. The layer exposed to the process flow is designed for erosion resistance, no less than 2” thick. The layer of refractory between the erosion-resistant liner and the shell is insulating to protect the shell from over-temperature, no less than 8” thick. The design temperature of the inner face of all shells is 250°F to avoid carbonic acid corrosion. Acid-resistant coatings on the inside of the shells will be considered during detail design. Insulating refractory in the gasifier, gasifier cyclones and related piping is two times thicker than calculated for air to protect the shell from over-temperature due to hydrogen permeation and hydrogen attack.

Gasifier Cyclones

The fluid bed gasifier is designed to discharge all of the olivine sand, char and gas-phase tar to two stages of cyclone separators where char and the sand are separated from the product gas. The char is composed of ash and carbon. The secondary gasifier cyclones are designed to separate olivine sand, tar and char remaining in the offgas from the gasifier primary cyclones to minimize the quantity of solids entering downstream equipment. The raw product gas from the gasifier secondary cyclones is conducted to the gas conditioning equipment in the gas conditioning system. The gasifier cyclones are designed with the same refractory lining as the gasifier. Target pressure drops for each set of cyclones and filter are less than 1 psi each. The gasifier cyclones are constructed of low-alloy carbon steel, rated to minimize hydrogen induced cracking (HIC).

Combustor Seal Pot

The gasifier cyclones operate at a higher pressure than the combustor, and thus must be sealed in order to prevent product gas from leaking into the combustor. The separated solids from both primary and secondary gasifier cyclones are discharged to the combustor seal pot that feeds the fluid bed combustor. The combustor seal pot maintains a hydraulic head that feeds the combustor and seals the gasifier from air in the combustor. The down-legs from both sets of gasifier cyclones drop vertically downward into a single Combustor Seal Pot. The Combustor Seal Pot provides a positive pressure seal between the Gasifier and the Combustor, using a fluidized bed of sand. The fluidized bed of sand acts much like a liquid, and provides the same sort of pressure seal as a liquid with a submerged dip tube. The combustor seal pot is designed to the following criteria:

Combustor Seal Pot Design Criteria

Condition

Combustor Seal Pot

Shell Design Pressure, psig

40

Shell Design Temperature, °F

500

Inside Refractory Design Temp, °F

1,500 (steam & sand)

I.D. of Shell

84"

Tangent Length

348"

The sand and char are directed to the bottom of the combustor through a slanted side overflow. The slanted overflow is fluidized with air to provide movement to the fluidized material.

Combustor

Combustor receives the mixture of char and sand from the Combustor Seal Pot where the un-reacted carbon in the char is oxidized with air to heat the olivine sand. The char is a combustible material comprised mainly of carbon and small amounts of hydrogen and oxygen, with traces of other elements. The sand and char are immediately contacted by preheated air from Combustor Blower and Fired Heater through a distributor similar to the Gasifier Distributor. The fired heater uses a fuel gas duct burner to raise the temperature of the air to 1,000°F and to superheat fluidizing steam for the gasifier. The char immediately combusts in the hot air, creating large volumes of CO2 and a small amount of water. This voluminous production of gas entrains the sand and remaining ash out the top of the Combustor at a velocity above 35 fps, and into the Combustor Cyclones. The combustor is design to the following criteria:

Combustor Design Criteria

Condition

Combustor

Shell Design Pressure, psig

40

Shell Design Temperature, °F

500

Inside Refractory Design Temp, °F

1,700 (syngas, char & sand)

I.D. of Shell

84"

Tangent Length

795"

Distributor Design

ACFM = 34,800

Refractory Lining for Flue Gas and Olivine Sand

The combustor shell is insulated with double-layer refractory lining. The layer exposed to the process flow is designed for erosion resistance, no less than 2” thick. The layer of refractory between the erosion-resistant liner and the shell is insulating to protect the shell from over-temperature. The design temperature of the inner face of all shells is 250°F to avoid carbonic acid corrosion. Acid-resistant coatings on the inside of the shells will be considered during detail design.

Combustor Cyclones

The sand and ash from the combustor are discharged to the primary combustor cyclones. The primary combustor cyclones are designed to separate the sand for recirculation to the gasifier while minimizing carryover of sand to the secondary combustor cyclones and minimizing ash in the sand. Ash collected with the sand by the combustor primary cyclones will recirculate with the sand. The secondary combustor cyclones are designed to separate the sand remaining in the offgas and the ash from the combustor primary cyclones to minimize the quantity of solids entering downstream equipment.  The solids discharged from the combustor secondary cyclones will be discharged from the plant, so loss of olivine sand is minimized.

The design should include consideration of alternate arrangements, such as multi-clone units, should this reduce the impact of the cyclone height on the “stack-up” and minimize capital costs, O&M costs, exposure of downstream equipment, power requirements and sand losses.

The combustor secondary cyclones minimize the particulate loading in the flue gas entering the dryer and the flue gas treatment system. The ash and sand separated in the combustor secondary cyclones are sent to an ash cooler and then mixed with solids from the gas treatment system for disposal or beneficial use. The sand separated in the combustor primary cyclones falls vertically into Gasifier Seal.

Gasifier Seal Pot

Gasifier Seal Pot is located under the primary combustor cyclones. This seal pot performs the same pressure sealing function as the combustor seal pot, but in the opposite direction. Its function is to prevent gasses from the gasifier running up into the bottom of the combustor cyclones, and allowing flammable gases to mix with flue gas that contains oxygen.

The gasifier seal pot provides a seal by raising the mildly fluidized sand “liquid surface” well up into the dip leg – above the top of the fluidized surface inside the seal pot. Thus, like a liquid sucked up into a straw, the level of the fluidized sand goes up inside the dip leg until it is well above the “liquid level” of fluidized sand inside the seal pot. This fluidized “surface” is level with the overflow line.

The gasifier seal pot provides a negative pressure seal, with the entering sand coming down from the combustor cyclones being at a lower pressure than the gasses entering the gasifier seal pot through its overflow leg. The overflow leg is attached to the gasifier, which operates at a higher pressure than the combustor cyclones.

Fluidization in the gasifier seal pot is provided by low pressure steam, since the fluidizing gases have a direct route to the gasifier. The gasifier seal pot is designed to the following criteria:

Gasifier Seal Pot Design Criteria

Condition

Gasifier Seal Pot

Shell Design Pressure, psig

40

Shell Design Temperature, °F

500

Inside Refractory Design Temp, °F

1,700 (steam & sand)

I.D. of Shell

96"

Tangent Length

132"

Flue Gas Cleaning System

After cooling the flue gas and reducing any NOX to acceptable levels in the feed dryer, the flue gas enters Flue Gas Cleaning System.

The flue gas cleaning system meets requirements for sulfur and PM2.5 emissions limits. An optional activated carbon injection system can be added to mitigate compounds that may cause smog outside the renewable plant. The by-product consists of sand, ash and slaked lime that can be combined with ash and sand from the combustor secondary cyclone to meet Toxicity Characteristic Leaching Procedure (TCLP) requirements for disposal and combined with cement for beneficial use as a building material. The stack is fitted with a continuous emissions monitoring (CEM) system to assure compliance with regulations.

Ash Handling System

In general terms, the ash cooled by Ash Cooler is transported to the Ash Hopper, where it is combined with by-product from the flue gas cleaning system. The ash and sand is discharged through Ash Bin Rotary Feeder into Ash Conditioning Screw for final mixing. Intermittent operation of the rotary feeder and conditioning screw allow the cementitious ash and sand mixture are dropped into a truck at a controlled moisture content for delivery to a local cement plant or disposal site.