DE102021001242A1 - Process for obtaining energy and raw materials from biomass - Google Patents

Abstract

Climate change is u. a. caused by the extraction of energy and raw materials from fossil sources, combined with the emission of fossil CO2. The separation of CO2 from fossil power plant exhaust gases (CCS) is currently being investigated as a way out of this dilemma. However, this, like the attempt to separate CO2 directly from the outside air, is not a technically and economically feasible way to stop climate change. Converting the fossil-based raw material extraction to non-fossil carbon cannot be realized with these approaches in the foreseeable future. The method proposed here completely avoids the use of fossil carbon and the unacceptably high effort of separating CO2 from the exhaust gases of fossil-fired systems and the outside air. With the help of oxyfuel combustion of biomass and simple separation through condensation of the water vapor produced during combustion, the biogenic carbon contained in the CO2 of the exhaust gas is made available as a raw material for further processing. The O2 used for the oxyfuel combustion is obtained by means of hydrolysis during hydrogen production from regeneratively generated electricity. Coupling the CO2 production to the hydrogenation with the H2 from the hydrolysis results in sustainable approaches for a climate-neutral, secure energy and raw material supply. Of particular advantage is the generation of electricity by means of a hot gas turbine in the CO2 cycle of the furnace. Thanks to the low gas temperatures after condensation of the water vapor, this process control for the gas turbine process results in high efficiency for power generation even at low outputs.

Description

State of the art and task

The current state of technology, industry and the world economy is the predominant use of fossil resources in the form of coal, crude oil and natural gas for the production of energy and raw materials. In the course of dwindling fossil reserves, but above all due to the ever-increasing demand to combat climate change triggered by the release of fossil carbon compounds (CO2, methane, etc.), the search for non-fossil substitute sources for energy and raw material production is becoming increasingly important . The material and energetic use of biomass is seen as a sensible and sustainable solution to the energy and raw material issue. Carbon, which is always present in the biosphere as floral carbon compounds in plant growth, still has a climate-neutral effect if this carbon is converted, e.g. B. by rotting or incineration, is released back into the atmosphere as CO2.

The development of biomass as a source of raw materials and energy for the entire civilization does not contribute to the disruption of the ecological balance. In contrast, this balance has been disturbed in recent decades by the excessive use of fossil carbon carriers. The conversion from fossil to biogenic carbon carriers can therefore make a decisive contribution to averting the climate change that is already underway and its disastrous effects on human life.

In addition to the aforementioned considerations on the use of biomass, the possibility of separating CO2 from (fossil) combustion gases (carbon capture and storage CCS) and even from the outside air is also being discussed and, on a case-by-case basis, also investigated in comprehensive research and development projects. There is no foreseeable success for either method.

The technical and economic effort involved in the CCS of exhaust gases from coal-fired power plants as well as in the separation of CO2 from the outside air is so high that a comprehensive and comprehensive application is ruled out for the time being.

The method presented here for the material and energetic use of biomass represents a solution to this problem, which can already be implemented both technically and economically with far less effort. The absorption of atmospheric CO2 and its decomposition takes place in nature through interaction between the atmosphere and the biosphere. The carbon is used by nature as a building block for plant growth in complex organic compounds and the released O2 is “breathed out” again. The intended use of the biomass here is aimed at the plant, i. H. to make "floral" carbon available as a carbon compound for material and energetic use. This avoids the almost unmanageable effort of separating CO2 from the outside air or from the exhaust gases from fossil fuel furnaces. The production of energy and raw materials is placed on a new, sustainable basis.

• • Begrenzter Aufwand bei Bau, Unterhaltung und Betrieb der Anlage
• • Hohe Gesamteffizienz bei Energie- und Rohstoffgewinnung
• • Gewinnung von biogenem CO2 mit sehr geringen Fremdstoffanteilen bzw. mit sehr hohem Reinheitsgrad
• • Skalierung der Anlagenleistung innerhalb eines sehr großen Leistungsbereiches ab einer elektrischen Leistung von ca. 100 kW bis zu einer Größe von 10 MW und mehr
• • Eignung für den Betrieb als Kraft-Wärme-Kopplungs-Anlage

The task of the new process for obtaining energy and raw materials from biomass is that the following properties should be met:

• • Limited effort in construction, maintenance and operation of the facility
• • High overall efficiency in energy and raw material production
• • Production of biogenic CO2 with a very low proportion of foreign matter or with a very high degree of purity
• • Scaling of the system output within a very large output range from an electrical output of approx. 100 kW up to a size of 10 MW and more
• • Suitability for operation as a combined heat and power plant

Description of the procedure

In contrast to conventional combustion chambers, this biomass combustion is not operated with air, but with pure oxygen as the oxidizing agent. The process of combustion with pure oxygen instead of air as the oxidizer is referred to as "oxyfuel" in combustion technology and has been theoretically and practically investigated for the combustion of fossil fuels for a long time. The main purpose of this deviation from conventional combustion was z. T. very high formation of environmentally and climate-damaging nitrogen oxides during combustion, which is mainly caused by the nitrogen contained in the combustion air with a volume fraction of 79%. In the large-scale application of the process, the provision of pure oxygen is only possible with the aid of air separation plants, in which the oxygen is separated out of the air with very high construction and energy costs.

In the course of the now starting production of hydrogen by hydrolysis from water using electricity from renewable sources len, e.g. B. solar and wind power, new perspectives arise for the provision of pure oxygen. This occurs as a by-product of the hydrogen generated by hydrolysis. Decentralized hydrolyzers, e.g. B. at an inland wind farm, decentralized biomass furnaces can serve as oxygen suppliers. Together with the generated hydrogen, an advantageous composite material can be realized with which the high-purity CO2 produced in the biomass furnace is hydrogenated and converted into a hydrocarbon used as a basic material in the chemical industry, e.g. B. methanol, is further processed.

Combustion of biomass with pure oxygen instead of air requires fundamental changes in process control. The method presented here incorporates these changes into the process flow in a beneficial way. The exhaust gas produced during combustion largely contains CO2, with the addition of water vapor, which is produced on the one hand by burning the hydrogen content of the biomass and on the other hand the natural water content of the biomass. The energy and mass balance of this furnace results in a high final combustion temperature, which would be significantly higher compared to normal heat generation with heat emission in the case of an adiabatic process control and would lead to thermal damage to the system that could no longer be controlled. In order to avoid these high temperatures, exhaust gas recirculation with previously cooled exhaust gas is essential. This is the only way to limit the combustion chamber temperatures to a level that the materials can still tolerate.

In the simplest case, the heat generated is used by decoupling the heat into a heating network and supplying it to heat consumers. Heat at a higher temperature can also be used to cover process heat consumers in industry. High temperatures in the exhaust gas from the combustion chamber also enable the heat to be converted into electricity in a hot gas turbine process. The combustion of biomass with oxygen instead of air offers these possibilities of energy conversion and use while at the same time efficiently producing high-quality biogenic CO2.

• • eine atmosphärische oder mit einem gegenüber der Atmosphäre erhöhten Druck betriebene Biomassefeuerung mit Zufuhr von reinem Sauerstoff als Oxidationsmittel und einer hohen Abgasrückführungsrate zur Begrenzung der Brennkammertemperaturen,
• • weiterhin eine - des durch die energetische Nutzung abgekühlten - zum weit überwiegenden Teil aus CO2 bestehenden Abgases Restabkühlung in einer Kältefalle zwecks Auskondensation des Wasserdampfes und der Rückführung des größten Teils des Abgases in die Feuerung
• • ein Rekuperator zur Erhöhung der Effizienz des Prozesses, in dem das kalte, weitgehend wasserdampffreie Gas mit einem Teil der im Heißgas enthaltenen thermischen Energie wieder aufgeheizt wird,
• • Heißgasturbinen, in denen ein Teil der in der Biomassefeuerung erzeugten Wärme in elektrische Energie umgewandelt wird.

The main components of the procedure are

• • Atmospheric or pressurized biomass firing with the addition of pure oxygen as an oxidant and a high exhaust gas recirculation rate to limit combustion chamber temperatures,
• • Furthermore, a residual cooling of the exhaust gas - which has been cooled down by the energetic use - consisting largely of CO2 - in a cold trap for the purpose of condensing the water vapor and the recirculation of the largest part of the exhaust gas into the furnace
• • a recuperator to increase the efficiency of the process, in which the cold, largely water vapor-free gas is reheated with part of the thermal energy contained in the hot gas,
• • Hot gas turbines, in which part of the heat generated in the biomass furnace is converted into electrical energy.

The proportion of CO2 in the exhaust gas produced during the combustion of biomass is preferably discharged from the circuit with a concentration of almost 100% before it enters the recuperator and is therefore available for further material use.

The basic scheme of biomass combustion based on this principle is shown in 2 shown. The recirculated gas is fed to the combustion chamber (1). The energetic use of the partly high-temperature heat depends on the local and regional demand for electricity and heat. In the simplest case, a waste heat boiler (2) is installed downstream of the recuperator (6) on the hot gas side, in which heat that can be used externally is extracted at a low and medium temperature level and is supplied to the corresponding heat consumers by means of a heat network. After leaving the waste heat boiler, the exhaust gas enters a cooling device (3), in which it is cooled in several stages to far below the exhaust gas dew point in order to extract the water vapor produced during combustion from the exhaust gas by condensation. The condensate is pumped out of the circuit (4). A CO2 fan (5) conveys the gas through the entire circuit. Before entering the recuperator, the amount of CO2 (9) that was created in the biomass by combustion is removed from the circulating gas stream. The O2 (11) required for combustion is added to the remaining mass flow, consisting of the CO2 (10) to be returned. The gas flow is then heated in the recuperator.

The method is also suitable for use at higher temperatures, e.g. B. in the form of high-pressure steam, hot water and thermal oil, with which industrial customers can be supplied efficiently and CO2-neutrally. In the large-scale application, a steam power process with a steam generator, back-pressure turbine and process or heating steam extraction is also conceivable.

The biomass (7) supplied to the combustion chamber can be pre-dried to a residual water content of 10%, for which purpose part of the heat extracted in the cooler (3) can be used. The ash (8) that remains after the biomass has been burned can be returned to the ground for the purpose of returning the minerals contained in the biomass or applied to agricultural land.

The coupling of the CO2 cycle with a hot gas turbine process according to 3 and 4 also opens up the possibility of efficient power generation on a small and medium technical scale for a predominantly decentralized supply infrastructure based on regionally available biomass. Combustion chambers for the conventional combustion of biomass can already be successfully coupled with hot gas turbines. If a hot gas turbine process is used in the CO2 cycle described here, there is another possibility of generating electrical energy while at the same time generating usable heat and CO2 as a carbon carrier for further uses.

At the acc. 3 built-up system, heat is extracted from the hot exhaust gas exiting the furnace (1) in a recuperator (5) on the primary side as a first cooling stage and this is heated on the secondary side to the mixture consisting of oxygen and recirculated gas before entering the turbine (12) to a suitable turbine inlet temperature . In further cooling stages, after leaving the recuperator, heat is extracted from the exhaust gas in a waste heat boiler (2) for external heat supply. In the downstream cooler (3), cooling below the dew point takes place with condensation of the water vapor (4). The practically water-free exhaust gas is then sucked in at the lowest possible temperature by the compressor (11), which compresses it to an increased pressure, from which the exhaust gas in the turbine is expanded again to the initial pressure in front of the compressor. The mechanical energy thus obtained is fed to the generator (13).

In 4 a process control for the application of the inverse gas turbine process (Inverse Brayton Cycle IBC) is shown. The hot exhaust gas from the combustion chamber (1), which is under ambient pressure, is fed directly to the turbine (12). In the turbine, it is expanded from ambient pressure to a negative pressure, at which point it is routed to the recuperator (5), from there to the waste heat boiler (2) and on to the exhaust gas cooler (3). The compressor (11) compresses the cooled exhaust gas from negative pressure to ambient pressure, at which point it is fed into the furnace (1) after the excess CO2 (8) produced during combustion has been removed and the O2 (10) required for combustion has been added becomes.

5 contains a schematic representation of the interaction of biomass combustion with the generation of electricity, heat and CO2 acc. 3 and 4 by means of hydrolysis and a CO2 hydrogenation. The CO2 (16) obtained from biomass combustion (1) and thus climate-neutral is converted to liquid or gaseous energy carriers, e.g. Methane and methanol (17), processed and preferably used in regional energy and fuel supply. The waste heat from the hydrolysis (10), together with the heat (11) extracted from the biomass combustion, is released to on-site heat consumers by means of heating networks (6). Temporary surpluses of solar and wind power (7) are fed into the medium-voltage grid. Any temporary additional power requirement (9) for the operation of the hydrolysis can be taken from the medium-voltage network (5).

Advantages of the method over the prior art

• • Combustion of the biomass with oxygen as an oxidizing agent enables easy production of biogenic CO2, which is taken from the exhaust gas cycle with a concentration of almost 100% and used for other purposes, e.g. B. in the hydrogenation with hydrogen to form methanol. A complex separation of the CO2, as would be necessary with exhaust gases from other combustion systems, is not required.
• • By using a recuperator to preheat the recirculated gas, the overall efficiency of the combustion can be significantly improved.
• • When equipped with hot gas turbines, the process also supplies electrical energy in addition to heat and CO2.
• • The process enables the efficient generation of electricity, heat and CO2 that is suitable for further material use, even in small, decentralized systems for the use of regional biomass.
• • Combustion of the biomass with oxygen instead of air results in the generation of electrical energy by means of hot gas turbines due to the strong cooling and condensation of the water steam is a decisive advantage over other methods of generating electricity from biomass or using gas turbine processes in general. The very low temperature of the gas at the compressor inlet results in a significantly lower power requirement in the compressor, which significantly increases the proportion of mechanical energy on the turbine shaft that reaches the generator as excess.
• • Decentralized biomass furnaces for power and heat generation, in which concentrated CO2 is produced due to combustion with O2, can be coupled with regional regenerative power generation in solar fields and wind farms in connection with hydrolysis and hydrogenation plants. This results in high overall efficiency through the use of regional, on-site biomass resources, low-loss heat supply in heating networks close to consumers and the production of biogenic energy sources and fuels close to consumers, combined with minimal transport routes and logistical expenses.

Further development of the procedure

A further embodiment of the process consists in simultaneously obtaining biochar or vegetable charcoal in addition to the CO2. In this case, an O2 quantity that is reduced compared to the stoichiometric requirement is supplied. Due to the resulting incomplete combustion of the biomass, part of the carbon contained in the biomass remains partially burned in the combustion residue and is used as biochar or biochar for further use together with the basic materials contained in the ash for plant growth, such as e.g. B. phosphorus, for soil improvement of degraded arable land or for stable storage in the earth's crust as a permanent carbon sink for the purpose of direct removal of CO2 from the atmosphere (Carbon Dioxide Removal CDR).

The process control can also be modified and optimized by raising the overall pressure level. In the procedures according to 1 until 4 the main mass flow is guided in an almost closed circuit. The biomass is z. B. by means of a rotary valve from the ambient pressure of the furnace operated with overpressure. The O2 is compressed in a gas compressor to the pressure level prevailing at the feed point. When the exhaust gas expands to a pressure that is higher than that of the atmosphere, after the exhaust gas has cooled far below the exhaust gas dew point, the volumetric water content in the exhaust gas is lower than in the case of expansion to ambient pressure. When the exhaust gas pressure is increased from 1 bar to 2 bar, the water content is reduced from e.g. B. 1.0% by volume to 0.5% by volume. Accordingly, the degree of purity of the CO2 increases from 99.0 to 99.5%.

Furthermore, the combustion chamber outlet temperature and the degree of burnout can be set to a lower value by means of a regulated bypass of the recirculated exhaust gas mass flow around the combustion system. Any CO produced as a result of incomplete combustion can be oxidized to CO2 in an afterburner behind the furnace or in an afterburning zone within the furnace.

character description

• legend to 1

  1

  Biomass combustion chamber

  2

  waste heat boiler

  3

  Aftercooler and exhaust gas condenser

  4

  condensate removal

  5

  CO2 fan

  6

  biomass feed

  7

  ash discharge

  8th

  CO2 removal

  9

  CO2 recirculation

  10

  O2 admixture

• legend to 2

  1

  Biomass combustion chamber

  2

  waste heat boiler

  3

  Aftercooler and exhaust gas condenser

  4

  condensate removal

  5

  CO2 fan

  6

  recuperator

  7

  biomass feed

  8th

  ash discharge

  9

  CO2 removal

  10

  CO2 recirculation

  11

  O2 admixture

• legend to 3 and 4

  1

  Biomass combustion chamber

  2

  waste heat boiler

  3

  Aftercooler and exhaust gas condenser

  4

  condensate removal

  5

  recuperator

  6

  biomass feed

  7

  ash discharge

  8th

  CO2 withdrawal

  9

  CO2 recirculation

  10

  O2 admixture

  11

  compressor

  12

  turbine

  13

  generator

• legend to 5

  1

  biomass firing

  2

  hydrolysis

  3

  CO2-H2 hydrogenation

  4

  wind and solar power plants

  5

  medium voltage grid

  6

  heat network

  7

  wind and solar power

  8th

  Electricity from biomass firing with hot gas turbine

  9

  Electricity from medium-voltage grid

  10

  heat from hydrolysis

  11

  Heat from biomass firing

  12

  biomass

  13

  H2O for hydrolysis

  14

  O2 from hydrolysis for biomass firing

  15

  H2 from hydrolysis for hydrogenation

  16

  CO2 from biomass firing

  17

  Products from hydrogenation (methane, methanol, etc.)

  18

  H2O (condensate)

  19

  ash, biochar

Claims (6)

Biomass combustion with an atmospheric or pressurized combustion space or combustion chamber for the production of energy and CO2 and combustion with pure oxygen as the oxidizing agent is claimed, characterized in that the exhaust gas from the furnace is cooled in a cooling section, which consists of a recuperator for preheating of the supplied gas mixture of CO2 and O2, at least one waste heat boiler for the extraction of usable heat and at least one cooling stage with cooling of the exhaust gas far below the water vapor dew point of the water vapor contained in the exhaust gas and from which the largely water vapor-free exhaust gas of the furnace with increased temperature in the recuperator and with is returned to the admixed O2 required for combustion. procedure after claim 1 , characterized in that the largely water vapor-free exhaust gas cooled in the recuperator in at least one waste heat boiler and at least one cooling stage is compressed to an increased pressure in the compressor of a hot gas turbine system, the excess CO2 is removed on the way to the recuperator and the O2 required for combustion is added, in The recuperator is heated to the maximum possible temperature and then expanded to ambient pressure in the turbine, with simultaneous decoupling of mechanical energy, and fed to the firing system. procedure after claim 1 , characterized in that the exhaust gas exiting the furnace is immediately expanded in a turbine from ambient pressure to a negative pressure, cooled in a recuperator and then in at least one waste heat boiler and in at least one cooling stage downstream of the waste heat boiler to well below the water vapor dew point, compressed by the compressor to ambient pressure, after removing the CO2 produced during the combustion and adding the O2 required for the combustion, it is heated in the recuperator and fed to the furnace. Procedure according to claims 1 until 3 , characterized in that a part of the amount of carbon contained in the biomass remains unburned in the combustion residue due to a reduced O2 supply compared to the stoichiometric requirement and the resulting incomplete combustion of the biomass and as biochar or vegetable charcoal for further use together with the Ash contained raw materials for plant growth, such. B. phosphorus, for soil improvement of degraded arable land or for stable storage in the earth's crust as a permanent carbon sink for the purpose of direct removal of CO2 from the atmosphere (Carbon Dioxide Removal CDR). Procedure according to claims 1 until 4 , characterized in that in the entire circuit, consisting of furnace, recuperator, formed from one of one or more waste heat boilers and exhaust gas coolers cooling section, CO2 fan or turbomachinery and the connecting channel sections on a compared to the in the claims 1 until 4 described pressures increased level is raised. Procedure according to claims 2 until 4 , characterized in that the O2 supplied for combustion is generated in a hydrolyser, which is preferably heated with electrical energy from solar and wind power systems, and the CO2 generated in the furnace is combined with the hydrogen generated in the hydrolysis to form a carbon-containing liquid or gaseous energy and raw material carrier is hydrogenated.

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