Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 223

Gasification is the process of gaseous fuel production by partial oxidation of a solid fuel. This means in common terms to burn with oxygen deficit. The gasification of coal is well known, and has a history back to year 1800. The oil-shortage of World War II imposed an introduction of almost a million gasifiers to fuel cars, trucks, and busses. One major advantage with gasification is the wide range of biomass resources available, ranging from agricultural crops, and dedicated energy crops to residues and organic wastes. The feedstock might have a highly various quality, but still the produced gas is quite standardized and produces a homogeneous product. This makes it possible to choose the feedstock that is the most available and economic at all times.

Gasification occurs in a number of sequential steps: (i) drying to evaporate moisture, (ii) pyrolysis to give gas, vaporized tars or oils and a solid char residue, and (iii) gasification or partial oxidation of the solid char, pyrolysis tars and pyrolysis gases.

Not all the liquids from the pyrolysis are converted to syngas, due to physical limitations of the reactor and chemical limitations of the reactions. These residues form contaminant tars in the product gas, and have to be removed prior to a Fischer-Tropsch reactor. Other impurities in the producer gas are the organic BTX [benzene, toluene and xylene (benzene components with one or two methyl groups attached)], and inorganic impurities as NH₃, HCN, H₂S, COS and HCl. There are also volatile metals, dust, and soot. The tars have to be cracked or removed first, to enable the use of conventional dry gas cleaning or advanced wet gas cleaning of the remaining impurities. There are mainly three ways of tar removing/cracking: thermal cracking, catalytic cracking, or scrubbing.

Despite the long experience with gasification of biomass, there are some problems with large-scale reliable operation. No manufacturer of gasifier is willing to give full guarantee for the technical performance of their gasification technology. Though they are sold commercially, they are not delivered with the same kind of operational guarantee as e.g., a gas turbine. This shows the limited operational experience and lack of confidence in the technology, but in comparison with alternative routes to utilize cellulosic biomass gasification is well proven and one of the possible technologies to be introduced commercially as a major part of the energy route to biofuel.

The production of high-quality syngas from biomass, which is later used as a feedstock for biomass-to-liquids production, requires particular attention. This is due to the fact that the production of synthesis gas from biomass is indeed the novel component in the gas-to-liquids concept – obtaining syngas from fossil raw materials (natural gas and coal) is a relatively mature technology.

Various gasification concepts have been developed over the years, mainly for the purposes of power generation. However, efficient biomass-to-liquids production imposes completely different requirements for the composition of the gas because for power generation, the gas is used as a fuel, while in biomass-to-liquids processing, it is used as a chemical feedstock to obtain other products. This difference has implications with respect to the purity and composition of the gas.

The calorific value of the gas is the prime factor for power generation – the higher the value, the better. Hence, the availability in the gas of any compounds that increase calorific value is generally welcomed – product gas, which contains carbon monoxide (CO), hydrogen (H₂) and various hydrocarbon derivatives [methane (CH₄), ethylene (CH₂=CH₂), ethane (C₂H₆), tar, and char]. The presence of inert components [water (H₂O), carbon dioxide (CO₂), and nitrogen (N₂)] is also acceptable, provided it is kept within certain limits. In contrast, for biomass-to-liquids production, the amount of carbon monoxide and hydrogen is important. The presence of other hydrocarbon derivatives and inert components should be avoided or at least kept as low as possible.

The amount of components (primarily hydrocarbon derivatives) other than carbon monoxide and hydrogen can be reduced via their further transformation into carbon monoxide and hydrogen. This is, however, rather energy intensive and costly (two processes – gasification and transformation). As a result, the overall energy efficiency of syngas production and of biomass-to-liquids processing is also reduced.

The amount of various components can be minimized via a more complete decomposition of biomass, thereby preventing the formation of undesirable components at the gasification step. This approach seems to be more appropriate for energy efficiency. The minimization of the content of various hydrocarbon derivatives is achieved by increasing temperatures in the gasifier, along with shortening the residence time of feedstocks inside the reactor. Because of this short residence time, the particle size of feedstocks should be small enough (in any case – smaller than in gasification for power generation) in order that complete and efficient gasification can occur.

In gasification for power generation, typically, air is employed as an oxidizing agent, as it is indeed the cheapest among all possible oxidizing agents. However, the application of air results in large amounts of nitrogen in the product gas, since nitrogen is the main constituent of air. The presence of such large quantities of nitrogen in the product gas does not hamper (very much) power generation, but it does hamper liquids production. Removing this nitrogen via liquefaction under cryogenic temperatures is extremely energy intensive, reduces substantially the overall biomass-to-liquids energy efficiency, and increases costs. Among other potential options (steam, carbon dioxide, carbon monoxide), from a techno-economic point of view, oxygen appears to be the most suitable oxidizing agent for biomass-to-liquids manufacturing.

The air-blown direct gasifiers operated at atmospheric pressure and used in power generation – fixed bed updraft and downdraft and fluidized bed bubbling and circulating – are not suitable for biomass-to-liquids production. In addition, downdraft fixed bed gasifiers face severe constraints in scaling and are fuel inflexible, being able to process only fuels with well-defined properties.

Updraft fixed bed gasifiers have fewer restrictions in scaling (usually up to 10 MW), but the produced gas contains a lot of tars and methane. However, fluidized bed gasifiers generally do not encounter limitations in scaling and are more flexible concerning the particle size of fuels. Nevertheless, they still have limited fuel flexibility, due to a risk of slagging and fouling, agglomeration of bed material, and corrosion. The operating temperatures of air-blown fluidized bed gasifiers are therefore kept relatively low (800 to 1,000°C, 1,470 to 1,830°F), which implies incomplete decomposition of feedstocks, unless long residence times are used.

Fluidized bed gasifiers (especially the bubbling bed gasifiers) tend to contaminate the product gas with dust. The oxygen-blown atmospheric or pressurized circulating fluidized bed gasifiers and the steam-blown gas or char indirect gasifiers are better solutions for biomass-to-liquids processes. Both gasifying concepts significantly reduce the amount of nitrogen in the product gas. In the first case, it is achieved via substituting air with oxygen. In the second case, nitrogen ends up in the flue gas, but not in the product gas, because gasification and combustion are separated – the energy for the gasification is obtained by burning the chars from the first gasifier in a second reactor.

Nonetheless, both oxygen-blown circulating bed gasifiers and steam-blown indirect gasifiers still present some major weak points with regard to biomass-to-liquids processes. In the former case, the issues related to the necessity for further cracking of the unconverted hydrocarbon derivatives and with the high dust emissions are still relevant. In the latter case, these two drawbacks can be overcome, but at the expense of a significant increase in capital costs, since two reactors are needed instead of just one. In fact, the case of the gas indirect gasifier also involves a second reactor. Finally, indirect gasifiers carry a higher risk of malfunctioning and are less reliable, because of their more sophisticated configuration.

Considering the above reasons, the pressurized oxygen-blown direct entrained flow gasifier appears to be the most suitable gasification concept to obtain synthesis gas for later biomass-to-liquids processes. Entrained flow gasifiers do not encounter severe scaling restrictions, and their capacity can easily be of several hundred MW. They also represent a mature technology for coal (not for biomass!), which has been employed for years. Entrained flow gasifiers operate at elevated pressures and much higher temperatures (1,200 to 1,500°C, 2,190 to 2,730°F) than other gasifiers (usually below 900ºC, 1,650°F). The residence time of the fuel is also much shorter (a few seconds) compared to that in other gasifiers.

For a complete transformation of the feedstock into synthesis gas within such short residence time, its particle size also has to be smaller than that required for other gasifiers – not larger than 1 mm, typically below 0.1 mm (100 μm, 100 microns). With such extreme conditions, almost tar-free synthesis gas with high content of carbon monoxide and hydrogen is obtained. This high conversion rate is also facilitated by the high reactivity and volatility of biomass. Conversely, the maximization of the liquids yield, i.e., of the content of CO and H2 in the product gas, results in 10 to 15% lower total transformation efficiency (when other useful products from gasification are also counted) compared to other gasifying concepts.

Many feedstocks have high content of ash, which under high temperature turns into molten slag. Molten slag also retains some undesirable compounds of biomass, e.g., heavy metals. The removal of molten slag from the bottom of the reactor has to be incorporated into its design – a slagging-type entrained flow gasifier. In order to improve slag properties, the addition of fluxing material (silica sand or limestone) is necessary. In contrast, in non-slagging entrained flow gasifiers, the removal of molten slag is not foreseen. Hence, non-slagging gasifiers are fuel inflexible, suitable only for clean feedstocks with low mineral (ash) content (less than 1%), e.g., oils.

The energy efficiency of gasification is further reduced by the removal of large quantities of inert gas (usually carbon monoxide) from the product gas. Inert gas is employed as a medium for lock hopper pressurization and pneumatic feeding of pulverized material (a mandatory feeding option for finely pulverized fuels) into the reactor. The amount of inert gas depends on the bulk density of fuels – the lower the density, the larger the amount. The low bulk density of biomass implies large consumption of carbon dioxide, and alternative forms of biomass feedstock (via pre-treatment) have to be considered for entrained flow gasifiers. The most feasible biomass pre-treatment options are torrefaction, pyrolysis, and pre-gasification.

See also: Biomass – Gasification, Gasifiers, Synthesis Gas.