Process for pyrolysis of glycerol-containing feedstocks

The invention relates to a process for producing a hydrogen-containing product gas (product gas) from a glycerol-containing feedstock as well as a device for implementing the process.

In an attempt to reduce the introduction of carbon dioxide into the earth\'s atmosphere or at least to not let it increase further, and as alternatives to the dwindling natural oil and natural gas reserves, energy sources in enhanced form will be produced in the future from renewable raw materials. According to an EU guideline, at least 5.75% of the fuel requirement is to be covered by such energy sources in the European Union until 2010. In this connection, biodiesel, which even now is added at a concentration of up to five percent to the diesel fuel available at German gas stations, plays a tremendous role.

Biodiesel is a standardized fuel that is obtained from, e.g., rapeseed oil but also from other plant oils and fats. Plant oils and fats consist of triglycerides, i.e., fatty acid tri-esters of glycerol. This structure causes plant oils and fats to be viscous to solid at normal ambient temperatures, i.e., to have a much higher viscosity than the fuels for which a commercially available diesel engine is designed. Plant oils and fats behave differently in the injection process, and also the combustion proceeds less cleanly. These drawbacks can be only incompletely compensated for even by motor-driven interventions—such as, for example, preheating the plant oil.

Biodiesel is produced from plant oils and fats by the replacement of glycerol by methanol. Its viscosity corresponds to that of commercially available diesel fuel, which is why it can be easily consumed even in unmodified diesel engines.

The glycerol that is separated from the plant oils and fats in the biodiesel production is not obtained in pure form but rather accumulates as a portion of the mixture of materials, which contain larger amounts of contaminants in addition to glycerol. Such a mixture of materials is, for example, so-called crude glycerol, which has a glycerol content of 80-85%, but in addition also contains water, salts e.g. potassium salts, and organic contaminants (e.g., fatty acids or methanol) as well as residues from the production process in still larger amounts. According to the prior art, the crude glycerol is purified in expensive process steps by vacuum distillation, deodorizing and filtration, to the extent that it is sufficient for the strict requirements of the European Pharmacopeia and can be sold to the pharmaceutical industry with a purity of at least 99.5% as a pharmaceutical glycerol. At present, the entire amount of glycerol that accumulates in the biodiesel production can be used in this way. With the foreseeable expansion of the biodiesel production, this will be increasingly more difficult in the future, however, so that other ways of using crude glycerol must be sought.

It is therefore the object of the invention to provide a process of the above-mentioned type as well as apparatus for implementing the process, which make it possible to direct glycerol-containing by-products that accumulate in biodiesel production to a productive use.

This object is achieved according to the process side of the invention in that a product gas is produced from the glycerol-containing feedstock by separation of undesirable substances and pyrolysis of glycerol.

In this connection, pyrolysis is defined as the thermal decomposition of glycerol in volatile molecules, whereby the decomposition is carried out with the exclusion of oxygen and water or—deviating from the usual definition—in the presence of oxygen and/or water.

The invention is based on the experience that when glycerol-containing feedstocks, such as, for example, crude glycerol, are put to direct use by gasification, problems arise—which cannot be overcome or can be overcome only at great expense—owing to the high proportion of contaminants that are present in the feedstocks. Thus, salts lead to corrosion of system parts. Also, organic contaminants are controlled only with difficulty and can result in deposits and the formation of carbon black.

One embodiment of the process according to the invention calls for the separation of undesirable substances—already present in the feedstock and/or produced in the implementation of the process according to the invention—and the pyrolysis of glycerol to be implemented simultaneously in one process step.

Another embodiment of the process according to the invention calls for a product gas to be obtained from the feedstock in at least two successive process steps, whereby in each of the process steps, undesirable substances are separated and/or glycerol is reacted by pyrolysis.

To separate undesirable substances that are present in the feedstock, the feedstock according to the invention is preferably subjected to a distillation and/or a thermal drying and/or a filtering on activated carbon and/or a membrane and/or chromatography and/or an ion exchange and/or an ion exclusion and/or a precipitation.

In a suitable way, the water or steam content in the glycerol-containing fraction that was recovered by separating undesirable substances from the feedstock is set at a value by adding or removing water or steam, which makes it possible to implement a subsequent pyrolysis without the formation of carbon black and with simultaneously minimum energy input.

Another embodiment of the process according to the invention calls for the water required for pyrolysis to be fed in more than one step (in a stepped process), whereby the water is fed before and/or during pyrolysis at a suitable location. If pyrolysis is implemented in several successive steps (pyrolysis steps), it is useful for water to be added in each case before a pyrolysis step.

If the glycerol-containing fraction is fed in liquid form to pyrolysis, water is preferably fed in the form of steam, whereby the steam is sprayed into the glycerol-containing fraction or the glycerol-containing fraction is sprayed into the steam. A portion of the energy required for the subsequent pyrolysis is already introduced with the steam, which results in a reduced heating expense in the pyrolysis reactor and in a reduction of the equipment cost for the pyrolysis reactor.

Since it is possible to pressurize, at low cost, the glycerol-containing fraction in liquid form and to conduct pyrolysis at an increased pressure, the process according to the invention is suitable in particular for producing a product gas under increased pressure. Thus, an expensive compression of the product gas can be eliminated.

In this connection, thermal drying is defined as the feedstock being introduced into a thermal drying system and being subjected there to a thermal treatment. Volatile components, such as water and glycerol, are evaporated and form a gas fraction possibly with other gaseous substances, while solids, such as, for example, salts, are converted into a largely anhydrous solid fraction. Solid and gas fractions are then separated to a large extent from the thermal drying system, which is equipped for this purpose with a suitable system for separating dust and gas, such as, for example, a gravity separator and/or a cyclone and/or a filtering system and/or a water scrubber.

At sufficiently high temperatures, glycerol is thermally decomposed, i.e., pyrolyzed into a gas containing hydrogen. Further developing the process according to the invention, it is therefore proposed that the thermal drying of the feedstock be implemented at temperatures in which at least part of the glycerol contained in the feedstock is pyrolyzed. Depending on how much of the glycerol is pyrolyzed in the thermal drying, the gas fraction is subjected to further pyrolysis downstream of the thermal drying.

An advantageous embodiment of the process according to the invention calls for the heat that is required for thermal drying of the feedstock to be removed from the hot product gas.

Preferably, for thermal drying of the feedstock, fluidized-bed granulators and/or fluidized-bed dryers and/or drum dryers and/or fluid-bed dryers and/or suspension dryers and/or paste dryers are used.

Variants of the process according to the invention call for the pyrolysis to be implemented while water and/or steam and/or an oxidizing agent are being fed in, whereby the oxidizing agent is air or oxygen-enriched air or oxygen.

The application of oxygen during the pyrolysis step refers to another reaction option. The glycerol will react partly with the oxygen to steam and CO/CO₂. The benefit is the internally generated heat which is used for the pyrolysis of the remaining glycerol. This results in decreased operating costs owing to an improved heat transfer/balance. This variant will lower the requirement of energy from the externally applied heat (usually methane) but will use some of the glycerol and the applied oxygen to form CO/CO₂.

Based on the process, which is selected for separating undesirable substances from the feedstock, an aqueous mixture (waste water), in which the separated substances are present in dissolved and/or suspended form and whose material cannot be used without additional treatment, can accumulate when the process according to the invention is implemented. The waste water represents a waste that has to be shipped to a hazardous waste site at a dump. To keep dumping costs low, an attempt is made to keep the volume of waste to be dumped as small as possible. An embodiment of the process according to the invention therefore calls for the waste water to be subjected to a treatment in which the volume of waste to be deposited is reduced. The waste water is preferably subjected to drying in a thermal drying system, whereby a largely anhydrous solid fraction and a gas fraction are produced. In the most advantageous case, the thus obtained solid fraction can be used economically (e.g., as fertilizer), so that the volume of waste to be dumped drops to zero.