Discusión de resultados

A. Gasifier End Use

For selecting an optimal integrated clean-up strategy, the intended end use (gas application) for the gasifier gas is a key consideration. The most important end uses, so far practiced commercially or under research study, can be summarized as follows:

• Close-coupled combustion (kilns, ovens, furnaces, dryers, “town gas” for local distribution, and boiler firing)

• Hydrogen fuel production

• External combustion for power: externally fired turbines, Stirling engines, steam engines, thermo-photovoltaic cells, catalytic oxidation, and thermo-electric systems

• Internal combustion (IC) diesel and Otto engines • Compressors

• Gas turbine internal combustion

• Fuel cells: molten carbonate, solid oxide, proton exchange membrane, and phosphoric acid • Chemical synthesis: methanol, ammonia, methane, Fischer-Tropsch liquids, other

oxygenates.

Specifications for contaminant levels that can be tolerated in these end-use applications are given in Table 5.1. Since Graham and Bain’s (1993) report published specifications have changed very little, but several new applications are under investigation, which require short- and long-term tests of contaminant toleration. The reader should view this information as representing a range of likely values, with the realization that few long-term tests with modern devices have been reported, and almost no studies in which the “tar” is well characterized. For specific applications the constructor of the engine or conversion device is the most reliable source of information. Close-coupled combustion applications for process heat are relatively insensitive to gas quality, and therefore the main concern is that the final combustion gas product (stack gas) meet emissions regulations. Aside from environmental considerations, the gas must be maintained above the “tar” dew point so that no condensation occurs in transport lines. Brown, quoted in Graham and Bain (1993), reports that maximum “tar” levels should be in the range of 60 to 600 mg/Nm (site-specific range) for3

Table 5.1. Contaminant Constraints Gas Application/

End Use mg/Nm , ppmw“Tar” Loading3 _Reference

Close-Coupled Combustion Limits are large Baker et al. 1986

“Town-Gas” for local distribution (a few miles)

50-500 ppmw Reed et al. 1987

Externally Fired

Stirling Engines Higher than for ICE

Tolerates raw producer gas

Cuda and Ziak 1995 Johansson et al. 1996 Steam engines Similar to boilers

Thermo-photovoltaic cells Unknown Broman and Marks 1995; Coutts and Benner 1994 Thermo-electric systems Unknown

Catalytic oxidation Unknown Järås and Johansson 1996

Externally fired turbines Kuehn 1995

Evans and Zaradic 1996

Internal Combustion Systems

SI and diesel Max of 100 mg/Nm3 _{BTG 1995a}

50-100 mg/Nm3 _{BTG 1995b}

10-50 mg/Nm3 _{Baker et al. 1986}

Less than 100 mg/Nm3

Preferably 50 mg/Nm3 Beenackers and_{Maniatis 1996}

Less than 20-500 mg/Nm3 _{Corella 1996}

Up to 30 mg/Nm3 _{Bridgwater 1995}

10-50 mg/Nm3 _{Brown et al. 1986b}

Less than 10 mg/Nm3 _{Bui et al. 1994}

Less than 30 mg/Nm3 _{Das 1985}

Total contaminates less than 10

Gas Application/

End Use mg/Nm , ppmw“Tar” Loading3 _Reference

Assumes less than 50 mg/Nm3

advisable Mukunda et al. 1994a Condensates that can be

consumed by engines need not be named “tars”

Parikh et al. 1987

World War II experience favors

less than 10 mg/Nm3 Reed 1985

Less than 100 mg/Nm OK but3

less than 50 preferable Stassen and Knoef 1995 (A direct comparison of “tar”

tolerances for spark ignition versus diesel has not been found.)

Direct-Fired Aero Gas Turbines Unknown

Direct-Fired, Industrial Gas Turbines

Compression is biggest problem. Tars OK if in vapor phase. Engines have higher tolerance to tars than turbines.

Bridgwater 1995

Tolerance for condensing tars

0.05–0.5 ppm Brown 1996 Tars may not be of concern for

BIG/BT. Keep in vapor phase. Williams and Larson1996 Tar and naphthalene,

0.5 mg/Nm3 Aigner 1996

Tar less than 5 mg/Nm ,3

C7+less than 0.01 vol % of gas. BTG 1995b

Compressors 50–500 mg/Nm3 _{Reed et al. 1987}

Ceramic Filters Unknown

Fuel Cells

MCFC-external reforming C H -tolerant; C H -less than2 6 2 4

0.25 vol.%; C H , less than 0.22 2

vol.%; benzene -0.5%;

Klinger and Kennedy 1987

Gas Application/

End Use mg/Nm , ppmw“Tar” Loading3 _Reference

MCFC-internal reforming Total contaminants less than 80

ppb. Anonymous 1997

Temperature is high enough to

reform hydrocarbons. Bain 1995 Saturated HC, less than

12 vol %; olefins less than 0.2 vol %; aromatics less than 0.5 vol %; cyclics less than 0.5 vol %.

Bossart et al. 1990

Tars are typical catalyst poisons. Heinzel et al. 1996 Benzene less than 1 vol.%,

naphthalene less than 0.5 vol.%. Ratcliff and Czernik1997 Steam/carbon for the reformer to

be set at 3.5 to avoid carbon formation.

Yasue et al. 1998

“Some external pre-reforming may be desirable to remove high molecular weight hydrocarbons from the fuel gas, which would otherwise crack to produce elemental carbon.”

Dicks 1988

Solid-Oxide, external reforming Unknown Clark et al. 1997 Solid-Oxide, internal reforming Unknown Clark et al. 1997

EPRI 1997 “Complete internal reforming can

lead to . . . carbon formation in the anode chamber.” Partial pre-reforming can avoid this problem.

Meusinger et al. 1998

Carbon deposition was a

problem unless air was added to the biogas.

Staniforth and Kendall 1998

Phosphoric Acid Unknown

Standard technology is available to ensure that a modern biomass gasifier coupled to a modern kiln or boiler (with a well-designed burner) will meet stringent environmental emissions guidelines and regulations. Therefore, there is no urgent need to further address close-coupled combustion in terms

ICE and methanol synthesis applications require that the gas be cooled before final use; therefore, it seems that gas cleaning would be greatly simplified by the use of proven, commercially available “cold” unit operations (filtration, direct scrubbing). However, there are many technical and economic reasons (thermal efficiency, environmental emissions compliance, non-condensible hydrocarbon gas removal, “tar”/effluent treatment costs) to justify catalytic cracking and reforming of the “tars” before cooling. However, if hot chemical conversion processes are adopted for gas conditioning, hot removal of particulates and aerosols must also be included. These constituents can cause catalyst fouling and poisoning, and deactivation in the cracking/reforming operations, and can result in excessive compressor erosion. Hot-gas cleanup (filtration and chemical conversion) is therefore relevant to “cold-gas” ICE and syngas end uses. Cold-gas cleanup unit operations may then be used in the final cleanup stages to ensure that technical specifications are fully met (CRE Group, Ltd. 1997).

ICE applications require that particles and “tars” be reduced before the producer gas can be effectively utilized. Limits of about 30 mg/Nm for particulate and 100 mg/Nm for “tar” are3 3

representative. For a turbo-charged engine the “tar” limit drops somewhat. Historically, gasifier/ICE/ generator sets (i.e., non-utility power plants as large as 1 MWe) have used direct scrubbers, versus filtration, and cracking, as a reliable, inexpensive means to condition gas. However, environmental considerations have rendered scrubbing largely unacceptable because these small non-centralized plants cannot afford to have individual water treatment and “tar” disposal facilities. Lately a new wet scrubbing option has been proposed to effectively clean the gas while reducing the wastewater consumption and final treatment needs by a factor of 20–30 (Abatzoglou et al. 1997a). The system has been developed at bench scale, scaled up to the pilot level, and tested with success. Commercial applications are under development.

Ideally, gas turbine fueling applications require that the hot gas be fully cleaned and remain hot (and under pressure) before use. It is not practical, or thermodynamically efficient, to cool down the gas at any point after production in the biomass gasifier. Because of the stringent gas quality requirements for turbine firing, any gas suitable for turbine applications, in terms of particulate removal, will be suitable for ICE applications. The range of the particulate concentration limit for gas turbines is 0.1 to 120 mg/Nm , depending on the design and the operating conditions. Alkalis3

are also critical contaminants, and the reduction of these to acceptable levels (usually below 0.1 mg/Nm3) remains one of the greatest challenges for successful commercialization. Very little has been published with respect to tolerable “tar” concentrations in gas turbine applications. A table of the latest information is provided in Nieminen et al. (1996). Alkali removal from hot gas is also possible. Various adsorpotion and absorption systems have been developed for coal gasification. According to Graham and Bain (1993), synthesis gas applications have high gas cleaning requirements. Before the gas enters the final synthesis loop, particulates should be less than 0.02 mg/Nm , and the “tar” concentration less than 0.1 mg/Nm . Hydrocarbons also pose potential3 3

problems for methanol synthesis processes. If the methane concentration is greater than 10%, the entire syngas stream must be reformed to CO and H . If less than 3%, no reforming is necessary. In2

B. Conclusions

There are very few well-defined and long-term data on the tolerance to “tar” of the great variety of energy conversion devices now being considered for gasifier output (boilers excepted). The older literature focused primarily on ICEs for automotive use. More recently, applications of the gas-to- fire turbines have been in the forefront. In almost no applications, with the exception of close- coupled boilers, have endurance tests or operations been carried out long enough to give valid projections of maintenance and systems costs. When such tests are done it will be most valuable if the offending organics are clearly identified so the results can be generalized. The studies going on in coal gasification, with coupling to turbines, engines, fuel cells, etc., should provide valuable information, particularly when highly cracked or tertiary “tars” are involved. Such “tars” are remarkably similar for biomass and for coal.

Recommendations:

Governments and developers should support long-term, well-controlled tests on engines, industrial as well as automotive; internally and externally fired turbines; fuel cells; and the variety of externally fired systems such as Stirling engines, where heat transfer materials and geometry differ from simple boilers. The nature of the “tar” involved in these tests should be well defined.

Gas Cleaning Technologies

Particles Cyclones Baffle separators

Cooling towers Venturi Wet Cyclonic separators

Flow disintegrators Demisters Droplet filters Coalescers Wet Scrubbing Particles Cyclones Cooling wet-dry contactors Absorption and /or Adsorption

on Solids

Baghouses ESP Viscous Cold Filters Dry or Wet-Dry Scrubbing

Particles Cyclones Granular / Deep-bed Ceramic Candles Ceramic Fibers Ceramic Fabrics Metallic Fabrics ESP Hot Filters Thermal or Catalytic Cracking Catalytic Reforming Gas Shift Hot Gas Conditioning Gas Conditioning

Options

Figure 6.1. Gas cleaning technologies.