Table of Contents:
Multitechnique Equipment for Gasification (Syngas)
The term gasification has been applied to a variety of technologies that take a solid “fuel" (or furnish, such as wood waste, silage, waste grasses, pulp mill black liquor, etc.) and convert it to gases. Incinerators operating under starved-air or sub stoichiometric combustion systems are often called gasifiers because they indeed are—and the term is more politically correct than incinerator.
The fuel (typically a biomass furnish) is converted in the process to a gas with the minimal application of oxygen, and then the resulting gas is separately converted to a fuel gas or combusted, thus producing heat. These sources may be equipped with cyclone collectors and dry precipitators or even baghouses. They may also use wet Venturi scrubbers or wet electrostatic precipitators. The final gas stream may or may not be released to the atmosphere, depending upon the process.
In other gasifiers, the furnish is converted under pressure and temperature (often using steam) to produce a syngas that is further catalytically converted to a fuel gas, dimethyl ether, or other product. In effect, the syngas producer is like a large, continuous pressure cooker. The furnish is converted to a syngas, and the syngas is cleaned and catalytically converted to other products (typically using a Fisher-Tropsch process). In some of these systems, a small portion of the syngas produced must be sufficiently cleaned so that it can be used to produce the heat to drive the gasification process. Depending upon the type of combustor used, the syngas stream must be cleaned at exceedingly high removal efficiency. Since water vapor is often just "along for the ride" and impacts the system size and cost, it is often removed through direct-contact condensation techniques. The water vapor content may also affect the catalytic conversion operation; thus, the water vapor content is carefully controlled.
Modern "pressure cooker" reformer type syngas gas cleaning equipment is truly a hybrid design. These applications typically use dry cyclone collectors for primary particulate separation followed by Venturi scrubbers for supplemental particulate separation and gas saturation, followed by counterflow gas coolers to squeeze out water vapor. Some systems may even include an additional wet scrubbing stage to remove sulfides (such as hydrogen sulfide) that might foul the downstream catalyst. These pressure cookers process gas cleaning stages are an inherent part of the process and are usually not considered an emission control system (the treated gases do not directly emit to the atmosphere); therefore, they come under the purview of the gasification system designer rather that the regulatory body. The specific design requirements are often set by the designer of the gasifier— not by code.
Figure 23.1 is a picture of the gas cleaning portion of a gasification system used to produce aviation-grade biofuel. Dry cyclones are shown toward the top of the photo, and a Venturi scrubber and condenser are shown to the left.
Typical Applications and Uses
To reduce our demand for fossil fuel, systems are being designed to convert waste materials into energy-producing products. The waste material typically is biomass such as wood waste (cellulosic), silage, grasses, sludges, and higher sulfur coal that, if the coal were to be burned, would produce excess SO, emissions. The biomass fuel is often called the furnish for the process. The furnish is typically solid but can also be liquid (such as pulp mill black liquor) that contains lignin or other organics that can be converted to syngas.
Though an oversimplification, the gasifiers used can be divided into three basic groups: starved-air gasification, plasma destruction, and reformer gasification.
The first is a gasifier that converts the furnish to syngas under starved-air (substoichiometric) conditions. Since minimal air is used, the resulting gas stream is typically high in carbon monoxide (CO) and low in CO,. The gas mixture properties typically reflect a higher molecular weight and higher specific heat than standard air. The gasifier itself may be of fluidized bed design or be a specially modified boiler.
To control the emissions, dry cyclone collectors to control particulate followed by dry or wet electrostatic precipitators may be used. Dry cyclones followed by a wet Venturi scrubber could also be applied.
Another method involves the application of an electric arc that produces a plasma that dissociates the furnish into component gases. Those gases are reformed into the syngas. These systems operate with little or no oxygen, and the gas stream is extremely high in hydrogen (H2). The high H, content reduces the molecular weight and dramatically increases the specific heat of the resulting gas stream. These effects combine to elevate the saturation temperature of the gas mixture.
If the furnish to the plasma unit contains chlorinated components, the system can be designed to remove the hydrochloric acid (HC1) that is formed and to increase the acid concentration in the blowdown by using multiple stage packed or tray tower absorption. In that type of acid gas recovery system, a dilute stage water-only absorber is preceded by one or more higher concentration absorbers. Make-up water is introduced to the last stage, and the blowdown is bled forward to the next upstream stage until the blowdown reaches the first stage. The concentrated acid is then bled from the first stage. For HC1 recovering, acid concentrations of up to about 18% wt. can be reached.
Figure 23.2 depicts the various components of a gas cleaning system as applied to a plasma type syngas gasifier. The technique is called hybrid because each stage uses a different technique to deal with the gas conditions at that stage. As the gas stream is cleaned, not only are pollutants removed but the gas is also cooled. Wet scrubbers have been successfully applied to plasma type systems. Dry cyclones may be used for primary particulate separation; however, the conveying velocities in these systems are so low that inert gases are usually removed through settling in the lower portion of the plasma unit.
The third common method is the use of a reformer. The reformer uses steam to react with carbon to produce H2 and CO when heat is applied.
In addition, the water reacts with the CO to produce additional H2.
The heat comes from the separate combustion of a portion of the produced fuel gas. The gas cleaning system takes the raw reformer gas and cleans it of particulate that might adversely affect the combustion stage. The gas stream water vapor content is carefully controlled by adjusting the cooling temperature of the condensing stage of the gas cleaning system. In the reformer circuit, the CO, essentially goes along for the ride and is maintained at as low a concentration as practical. Since the C02 is formed via the water shift reaction with the water and not from combustion with air, the C02 content is low. In the combustion (heat) zone, the fuel is syngas that is high in H2, so the stack C02 emissions are low.
In the reformer (or pressure cooker) systems, the operating pressure may be 10-15 psig up to 900 psig, depending upon the design. The furnish is mixed with pressurized steam and maintained under those conditions so that the H2 will form. It is not unusual for the reformer to take days of such "cooking" before H2 is produced. Thereafter, once stability is obtained, new furnish is added continually and inert material (ash, metallic oxides, etc.) is withdrawn. Since steam is used in the process, the gas stream characteristics at each stage must also be calculated since the extra water vapor in the gas density can vary greatly. The H2 content is typically lower with a reformer type system than with a plasma type system, but the condensing demand is higher given the presence of the steam. The CO content as measured at the gas cleaning system inlet is often higher than that seen in other gasification systems. The heat exchange circuit becomes a heat recovery stage, which improves the overall thermal efficiency as well as acts to control the water vapor content for the shift of the CO to H2 and carbon dioxide.
Plasma gasifier diagram. (Bionomic Industries, Inc.)
As mentioned previously, the gasification process produces gases of high specific heat and low molecular weight. The contaminants can include finely divided particulate, tars, and other condensable and hydrocarbons such as phenols and aldehydes. No single gas cleaning technique can remove all those contaminants.
Currently, the typical system includes the use of dry cyclones for primary particulate removal while the gas stream is hottest, followed by Venturi scrubbing with recirculated water (or a solvent) for control of smaller particulate and to initiate the condensing process. It is at or slightly behind the Venturi that tars can begin to condense and can complicate the equipment design. Often, these Venturis have large liquid passageways and avoid areas where tars can accumulate. They are also designed with additional access points to facilitate cleaning.
The droplets formed in the Venturi are usually cyclonically separated using devices that expose the minimum of surface area onto which tars can accumulate. Vane or baffle type separators are often avoided.
The gases, now at or near saturation, are then subcooled to remove water vapor. The subcooling causes further condensation and the possibility of further tar build-up. The water vapor usually must be controlled, however, given the catalytic processes that often follow.
If the gasifier produces acidic gases (say, SO,), an additional wet scrubber is used to absorb the SO, and neutralize it using an alkali (usually caustic).
All these devices may operate at high pressures (20 psig to more than 900 psig).
Primary Mechanisms Used
Cyclonic separation is commonly used to remove the dry particulate while the gas stream is at its hottest. Some systems reportedly use filtration. Venturi scrubbing (as described in previous chapters) is often used for fine particulate capture and to saturate the gas stream. In this application, the cooling also causes the condensation of tars.
Condensation is further applied to remove excess water vapor, since the goal is to produce syngas, not water vapor. In doing so, more tars can condense and some of the water-soluble hydrocarbons will be absorbed. The water from this stage is typically treated externally prior to being recycled.
These devices often run at a higher liquid-to-gas ratio than in more “mundane" applications.
Additives (both solid and liquid) may also be applied to sequester the tars. At this writing, various tar control techniques are being evaluated. In the meantime, the systems are designed for simplified maintenance access.
Gasification systems inherently deal with gas mixtures that exhibit low molecular weight and high specific heat. Also, if acid gases are evolved, upon dissolution a heat of solution/reaction occurs. To complicate things further, as pollutants and the carrier gas are cleaned and cooled, the gas properties change. Water of a molecular weight less than air, for example, is added and then is partially removed. Doing so changes the net characteristics of the gas mixture.
These gas streams are not like ambient gas streams. The molecular weight and specific heat must be calculated for the gas mixture at each stage. As mentioned in Chapter 1, "Air Pollution Control 101," psychrometric calculations are made for each stage. For these systems, the molecular weight and cp (specific heat) are most important. Table 23.1 shows a molecular weight and
Molecular Weight and cp Estimate
cp estimate for a gasification system gas stream for the gases as they enter the Venturi inlet.
This information is needed because many psychrometric programs allow you to adjust the cp and molecular weight, as these parameters determine the saturation conditions of the mixture. The saturation conditions in turn dictate the amount of water that needs to be added or removed, which determines the mechanical design of the system (pump sizes, heat exchanger duty, blowdown rates, etc.). Unlike many systems running with air mixtures, these calculations need to be performed for each stage (dry cyclone inlet; Venturi inlet; cyclonic separator inlet; condensing stage inlet and outlet; and if an acid control stage is used, its inlet and outlet).
If a compressor is used after the gas cleaning system, an accurate calculation of the gas properties entering the compressor is of critical importance since that device may be the prime mover of all gases through the system.
The system components must be designed for simplified service access since the formation of tars is a given and periodic maintenance is often required. These systems can be and are designed, however, for extended campaigns before cleaning despite the challenges.
Instrumentation that monitors pressure drops through the system is used to forewarn of buildup issues. If a Venturi scrubber is used for particulate control, the Venturi is often designed to be free of any spray nozzles that may plug. In addition, Venturi designs that minimize the wetted surface (upon which tars may accumulate) are often used. Extra access doors and sometimes clean in place steam or solvent lances are built into the design. For the condensing stage, oversize packing is used, and designs that tend to produce a dripping liquid surface rather than films are generally favored. Some systems use spray towers or agitated fluidized bed type devices for the condensing sections to reduce plugging. Some systems have the provision for the introduction of solvents or oils to cut tar buildup in the condensing stage.
Specific requirements for operating the system are dictated by the type of furnish used. Some furnish produces greater quantities of tars than others; therefore, the designer must adjust a basic design to suit the realities imposed by that furnish. Current practice assumes that tars will indeed accumulate over time; therefore, the facility design includes maintenance shutdowns.
If the system has particularly problematic buildup areas (say, in ductwork), the area is designed to be easily removed. An example would be to flange smaller ductwork sections so that those sections can be removed for cleaning.