Method and apparatus for low-temperature biomass pyrolysis and high-temperature biomass gasification

The invention relates to a technology for transforming combustible materials into a clean and highly efficient synthetic gas, and more particularly to a method and a system for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification.

Gasification technology of combustible materials has achieved an amazing development in the later twentieth century, especially the gasification technology of combustible coal, which has been very mature. Researchers have successfully developed a process for gasifying coal that is widely applicable, highly efficient in gasification, and pollution free. Gasification technology of biomass, like tree twigs, straws, and other agriculture and forest wastes, is a new technology for comprehensive energy utilization in the 21th century. The conventional biomass gasification technology includes: fixed bed gasification, fluidized bed gasification, and two stages gasification, all of which are direct gasification technologies. The processes of direct gasification technologies are characterized in that the heat produced by part of the biomass supplies energy resource for gasification, the air, oxygenized air, or a combination of the oxygenized air and water vapor is functioned as an oxidant during the gasification reaction. However, studies have shown that technologies of direct gasification of the biomass are disadvantageous in the following aspects:

First, the components and the heat value of the biomass fuels are unstable, the biomass has low fire point and fast combustible reaction, thus, explosion easily occurs. When part of regions are superheated and coked, the operating temperature of the gasifier is very difficult to control.

Second, when the air works as an oxidant in which the content of the inactive gas of N₂ is prominent, it results in a higher content of N₂, a lower content of effective gas (CO + H₂), and a lower ratio of H₂/CO, besides, the heat value of the synthetic gas is low and unstable, which only maintains at 5000 KJ/Nm³ below and hardly meets the need of the later industrial utilization.

Third, when the oxygenized air works as an oxidant, although the content of N₂ is relatively lowered, an additional air separating device is necessitated. Because of a large capacity and high energy consumption of the air separating device, such a process largely increases the production cost.

Fourth, when the oxygenized air and the water vapor work as both oxidants, although the content of N₂ in the synthetic gas is lowered, and the content of H₂ is increased, the water vapor working as a reacting medium still consumes a large amount of heat energy, plus the energy consumption in the air separation, the process largely maximizes the production cost.

Fifth, about 15-20% of the biomass is necessitated to self-ignite for providing the energy resource for gasification, but at the same time a large amount of CO₂ is produced in the combustion, correspondingly, the content of effective gas (CO + H₂) is lowered. Furthermore, the high temperature synthetic gas and the mixed air carry a large amount of sensible heat, and thus, the conversion of the heat energy into the chemical energy is largely minimized, and the efficiency of the cooled gas is also lowered, which is generally 70% below and no higher than 80% in exceptional conditions.

Sixth, the operating temperature of the gasifier is generally controlled at 800-1200°C, at such a temperature, the gasification of the biomass produces a large amount of tar which is difficult to remove, and too much of tar aggregated in the device and pipes is apt to cause pipe blocking and device contamination.

Seventh, the gash produced in the gasification of the biomass contains a prominent content of alkali metal oxides comprising K and Na, which is general 20-40 wt. % of the total ash. However, at a temperature higher than 800°C, the alkali metal oxides is apt to be gasified and mixed into the synthetic gas, which not only affects the property of the synthetic gas, but also adheres to the pipes and devices together with the tar, thereby resulting a serious corrosion on the devices and pipes.

In view of the above existing problems, technologies of direct gasification of biomass are difficult to be applied in practical production. Thus, a method for gasifying the biomass which can be applied in industrial production and converted to commercial benefits is desired.

In view of the above-described problems, it is one objective of the invention to provide a method and a system for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification. The method features easy control, energy saving, and low cost. The produced synthetic gas has a high efficiency and high heat value, with absence of tar or alkali metal dioxides.

To achieve the above objective, there is provided a method for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification. The method employs a superheated water vapor as an oxidant and an energy carrier, conducts biomass pyrolysis and gasification at different temperature ranges, and finally produces clean synthetic gas. The method comprises the following steps:

a) Grinding the biomass, feeding the biomass into a pyrolysis furnace, spraying a low temperature superheated water vapor into the pyrolysis furnace, controlling the pyrolysis furnace at an operating temperature of 500-800°C, contacting the biomass with the low temperature superheated water vapor for conducting a pyrolysis reaction to yield crude synthetic gas and an ash comprising a coke. Because the operating temperature of the pyrolysis furnace is below sublimation points of alkali metal oxides comprising K and Na, the alkali metal oxides exist in the ash comprising the coke, and the crude synthetic gas comprises no tar or minor tar.

b) Cooling the ash comprising the coke generally to a temperature of 150°C below, and separating the coke from the ash. The coke is used for producing synthetic gas in the following step, and the ash comprising the alkali metal oxides are transported to an ash storehouse.

c) Transporting the crude synthetic gas and the coke into a gasifier, spraying a high temperature superheated water vapor into the gasifier, controlling the gasifier at an operating temperature of 1200-1600°C, contacting the biomass with the high temperature superheated water vapor for conducting a gasification reaction and acquiring primary synthetic gas. Because the operating temperature of the gasifier is above a temperature to form tars, the crude synthetic gas and the coke are fully gasified, and the acquired primary synthetic gas comprises no tar.

d) Cooling, dust removing, deacidifying, and desiccating the primary synthetic gas to yield clean synthetic gas. The process of cooling down not only is a necessity in the whole process for production of the synthetic gas, but also recovers a large amount of sensible heat for comprehensive utilization. The process of dust removal separates the dust from the crude synthetic gas, and lowers the dust concentration of the gas to 50 mg/Nm³ below. Harmful ingredients like H₂S, COS, HCL, NH₃, and HCN are removed from the synthetic gas in the deacidification process. After desiccation, the primary synthetic gas is transformed into the clean synthetic gas, which is stored for latter industrial application.

The ground biomass in step a) has a particle size of 20 mm × 20 mm below and a water content of 40 wt. % below. Biomass of such a particle size and water content fully contacts with the high temperature superheated water vapor, so that processes of desiccation, separation of volatile matters, pyrolysis, and evaporation are stably conducted, and the operating temperature of the gasifier is easy control, cokes do not form in the pyrolysis furnace.

In step a), a nitrogen atmosphere is provided at a feed inlet of the pyrolysis furnace in case of fire and explosion caused by leakage of the crude synthetic gas from the pyrolysis furnace.

In step a), a preferable operating temperature of the pyrolysis furnace is controlled at 500-650°C, an operating pressure of the pyrolysis furnace is controlled at 105-109 kPa. An input speed of the low temperature superheated water vapor into the pyrolysis furnace is 35-50 m/s; a retention time of the crude synthetic gas in the pyrolysis furnace is 15-20 s, and an output speed of the crude synthetic gas from the pyrolysis furnace is 15-20 m/s. Thus, the pyrolysis furnace operates at a normal pressure, and no special pressure device is needed, thereby lowering the production cost. The biomass in the pyrolysis furnace is fast desiccated, separated from volatile matters, and pyrolyzed during the contact with the crude synthetic gas and the low temperature superheated water vapor. Furthermore, the operating temperature of the pyrolysis furnace is much lower than sublimation points of the alkali metal oxides, which are about 800°C, so that the alkali metal oxides are removed from the crude synthetic gas. The relatively lower output speed from the pyrolysis furnace prevents the ash from aggregating in the outlet of the pyrolysis furnace and the gas pips.

In step c), a preferable operating temperature of the gasifier is controlled at 1200-1400°C, and a preferable operating pressure of the gasifier is controlled at 105-109 kPa. An input speed of the high temperature superheated water vapor into the gasifier is 35-50 m/s; and a retention time of the crude synthetic gas in the gasifier is 15-20 s, and an output speed of the primary synthetic gas from the gasifier is 15-20 m/s. Thus, the gasifier operates at a normal pressure, and no special pressure device is needed, thereby lowering the production cost. A high input speed of the high temperature superheated water vapor into the gasifier largely improves the contact and mix of the crude synthetic gas and the coke. The operating temperature range of the gasifier is suitable, which ensures a total gasification of the crude synthetic gas and the coke during the contact with the high temperature superheated water vapor, the acquired primary synthetic gas comprises no tar; at the same time the energy consumption is lowered as much as possible, and the performance of the gasifier is largely improved.

In step d), the primary synthetic gas is cooled down to a temperature of 260-320°C, and then cleaned. As the temperature of the primary synthetic gas output from the gasifier is still high, about 120-1400°C, the cooling process is not only conducive to the later dust collection, deacidification, and desiccation, but also helpful to recover the sensible heat in the primary synthetic gas, thereby achieving a comprehensive utilization of the exhaust heat.

A system for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification according to the above method, comprises: the pyrolysis furnace, the gasifier, a low temperature plasma torch heater, a high temperature plasma torch heater, a water storage tank, a water pump, and a heat exchanger.

The water storage tank is connected to a water inlet of the heat exchanger via the water pump. A vapor outlet of the heat exchanger is at the same time connected to a vapor inlet of the low temperature plasma torch heater and a vapor inlet of the high temperature plasma torch heater. A vapor outlet of the low temperature torch heater is connected to a vapor nozzle of the pyrolysis furnace. A vapor outlet of the high temperature plasma torch heater is connected to a vapor nozzle of the gasifier.

A gas outlet of the pyrolysis furnace is connected to a gas inlet of the gasifier, an ash outlet of the pyrolysis furnace is connected to an ash inlet of an ash cooler, and an ash outlet of the ash cooler is connected to a feed inlet of an ash-coke separator. A gas outlet of the gasifier is connected to a gas inlet of the heat exchanger; and a gas outlet of the heat exchanger is connected to a dust collector, a deacidification tower, and a desiccator in series.

The plasma torch heater is advantageous in ultra-high temperature heat, fast transfer of heat and mass, high efficiency, and adjustable heat power, when it is used to heat the water in the water storage tank, a high temperature superheated water vapor can be effectively, successively, and stably produced. The high temperature superheated water vapor is functioned as not only an oxidant but also an energy carrier, so that the gasifier is maintained to work stably. The heat exchanger effectively recovers a large amount of the sensible heat of the primary synthetic gas. The water in the water storage tank is preheated and transformed into a saturated water vapor due to the sensible heat, and the saturated water vapor is then transported to the plasma torch heater, thus, the energy consumption of the plasma torch heater is lowered, and comprehensive utilization of heat energy is achieved.

A nitrogen protecting device is connected to a feed inlet of the pyrolysis furnace. A nitrogen sealing layer prevents the crude synthetic gas from leaking out of the gasifier, and keeps the air outside the gasifier, the fire and explosion are eliminated and the property of the crude synthetic gas is assured.

A coke outlet of the ash-coke separator is connected to a coke inlet of the gasifier via a coke transporter. For example, a screw feeder is employed to directly transport the coke to the gasifier, so that the intermediate manual transportation is saved, which improves the stability and the succession of the gasifier.

The vapor nozzles arranged on the pyrolysis furnace and the gasifier are grouped into 2-4 height levels, respectively, and the vapor nozzles of each level are evenly and tangentially arranged along a circumferential direction. Thus, the superheated water vapor is sprayed into the pyrolysis furnace and the gasifier from different levels, and an even and stable temperature filed is maintained at different height levels, resulting in a fully contact between the superheated water vapor and the reactants.

Based on the inherent characteristics of the water, ash, volatile matters, and ash fusion point of the biomass, and combined with the operating features of the gasifier, the method of the invention employs the superheated water vapor, rather than the conventional oxidant air or oxygenized air, to produce synthetic gas from biomass by low temperature pyrolysis and high temperature gasification. Advantages of the invention are summarized hereinbelow:

First, the superheated water vapor is employed to indirectly gasify the biomass. The superheated water vapor is not only an oxidant but also an energy carrier, so that the oxidant air or oxygenized air is not necessary, which means a highly energy consumed air separating device is not necessitated, and the energy consumption in the whole process and the total production cost are largely minimized.

Second, no self-ignition occurs in the biomass during the pyrolysis and the gasification, thereby effectively solving the problems in conventional gasify process, such as fuel explosion in the pyrolysis furnace or the gasifier, regional cokings, and difficulties in controlling each process. Because the air or the oxygenized air is not necessary in the reaction anymore, the synthetic gas has a high ratio of H₂/CO, and a high content of the effective gas (CO + H₂), which is 85% above, thus, the heat value of the synthetic gas is largely improved, and the use of the synthetic gas is much wider.

Third, the main reaction devices are the pyrolysis furnace and the gasifier. The biomass is at first pyrolyzed into the crude synthetic gas and the coke at a low temperature, and both the productions are gasified at a high temperature. Since the temperature ranges are suitably set, the produced crude synthetic gas comprises no alkali metal oxides, the tar and coke are all transformed into the primary synthetic gas, so that the carbon conversion is very high, the acquired primary synthetic gas is absent of impurities that are dirty and corrosive to the devices and pipes, and the later cleaning process becomes much simpler.

Fourth, the plasma torch heater produces all the heat energy which is necessary for biomass gasification by the superheated water vapor outside the gasifier, the heat energy of the biomass fuel is all transformed into a chemical energy, and the efficiency of the cooled gas is 88% above, which is 8% higher than that of the conventional.

Fifth, the plasma torch heater has a high heat efficiency, and adjustable input power, when the components of the biomass fuel changes, the power of the plasma torch heater can be adjusted, so that it is very convenient to control the temperature of the superheated water vapor, and maintain the gasifier work stably, and assure a stable output of the primary synthetic gas and a stable property.

Tests have shown that, the method and the system of the invention is applicable to different kinds of biomass fuels, and is especially applicable in industries of the integrated biomass gasification cycle combination and the biomass liquid fuel.

FIG. 1 is a structure diagram of a system for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification of the invention.

A method and a system for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification is specifically described with accompanying drawings:

As shown in FIG. 1, a system for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification, comprises: a belt conveyer 1; a hopper 2; a screw feeder 3; a pyrolysis furnace 5 and a gasifier 9 for biomass pyrolysis and gasification, respectively; a low temperature plasma torch heater 8 and a high temperature plasma torch heater 10 for supplying superheated water for the pyrolysis furnace 5 and the gasifier 9, respectively; a water storage tank 17 and a water pump 16 for supplying water to the low temperature plasma torch heater 8 and the high temperature plasma torch heater 10; a heat exchanger 11 for comprehensive utilization of heat energy; and a dust collector 12, a deacidification tower 13, and a desiccator 14 for later cleaning of the synthetic gas.

An output end of the belt conveyer 1 is arranged above an inlet of the hopper 2, an outlet of the hopper 2 is connected to a feed inlet of the screw feeder 3, and a feed outlet of the screw feeder 3 is connected to a feed inlet of the pyrolysis furnace 5.

As a key device for a first stage of biomass processing, the pyrolysis furnace 5 comprises a casing comprising an air cooled jacket or a water cooled jacket, and is thermal insulated at a normal pressure. The feed inlet of the pyrolysis furnace 5 is arranged on an upper part or an upper end; to assure an even biomass addition and a stable flow field inside the pyrolysis furnace, the number of the feed inlet is two or four. A nitrogen protecting device 4 is connected to the feed inlet of the pyrolysis furnace 5, so that a nitrogen sealing layer is formed for effectively separating the crude synthetic gas from the air. A gas outlet of the pyrolysis furnace 5 is arranged on the upper part or a lower part, and is connected to a gas inlet of the gasifier 9 via a pipe, so that the crude synthetic gas is transported to the gasifier 9. The pyrolysis furnace 5 comprises an ash outlet arranged at a bottom; the number of the ash outlet is one or two. An ash discharged from the ash outlet is in a liquid state. The ash outlet is connected to an ash inlet of an ash cooler 6 for cooling the ash comprising a coke. An ash outlet of the ash cooler is connected to a feed inlet of an ash-coke separator 7 for separating the coke from the ash. Preferably, a coke outlet of the ash-coke separator 7 is connected to a coke inlet of the gasifier 9 via a coke transporter 19, which is energy saving compared with the manual transportation and assures a stable and continuous operation of the gasifier 9.

As a key device for a second stage of biomass processing, the gasifier 9 also comprises a casing comprising an air cooled jacket or a water cooled jacket, and is thermal insulated at a normal pressure. The coke inlet of the gasifier 9 is arranged on an upper part or an upper end. To assure an even coke addition and a stable flow field inside the gasifier 9, the number of the coke inlet is one or two in compliance with the capacity. An ash outlet of the gasifier 9 is arranged at a bottom, from which an ash is discharged in a liquid state; the number of the ash outlet is one or two in compliance with the capacity. A gas outlet of the gasifier 9 is arranged on the upper part, or a lower end, and is connected to a gas inlet of the heat exchanger 11, a gas outlet of the heat exchanger 11 is connected to the dust collector 12, the deacidification tower 13, and the desiccator 14 in series, and an outlet of the desiccator 14 is connected to a gas storage tank 15.

The superheated water vapor sprayed into the pyrolysis furnace 5 and the gasifier 9 is transformed from soft water or desalted water in the water storage tank 17 by heating. An outlet of the water storage tank 17 is connected to a water inlet of the heat exchanger 11 via the water pump 16. The heat exchanger 11 is usually a scrapped boiler. A vapor outlet of the heat exchanger 11 is at the same time connected to a vapor inlet of the low temperature plasma torch heater 8 and a vapor inlet of the high temperature plasma torch heater 10. A vapor outlet of the low temperature plasma torch heater 8 is connected to a vapor nozzle of the pyrolysis furnace 5 via a pipe. A vapor outlet of the high temperature plasma torch heater 10 is connected to a vapor nozzle of the gasifier 9 via a pipe. Preferably, the vapor nozzles arranged on the pyrolysis furnace 5 and the gasifier 9 are grouped into 2-4 height levels, respectively, and the vapor nozzles of each level are evenly and tangentially arranged along a circumferential direction. Thus, an even and stable vapor filed is maintained, and a fully contact between the superheated water vapor and the reactants is achieved.

The system also comprises an ash storehouse 18, and the solid ash from the ash-coke separator 7 and the liquid ash from the gasifier 9 are transported to the ash storehouse 18 by a manual or mechanical mode.

A method for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification using the above system is specifically described as follows:

A) Ground biomass is transported to the pyrolysis furnace 5 via the belt conveyor 1, the hopper 2, and the screw feeder 3 in turn, at the same time nitrogen is input from a nitrogen protecting device 4 into a feed inlet of the pyrolysis furnace 5. When the biomass is a gray straw, for example twigs and roots of trees, a particle size of the biomass is controlled at 20 mm × 20 mm below, and a water content of the biomass is controlled at 40 wt. % below. When the biomass is yellow straw, for example stalks of threshed grain, thatch, stalks of corns, the particle size of the biomass can be relatively large.

B) The desalted water is output from a water storage tank 17 to a water inlet of the heat exchanger 11 via a water pump 16, and the desalted water exchanges heat with primary synthetic gas input from a gas inlet of the heat exchanger 11, and a sensible heat is extracted by the desalted water, during which 0.4-0.6 Mpa of saturated vapor is produced. The saturated vapor is output from a vapor outlet of the heat exchanger 11 to the low temperature plasma torch heater 8 and the high temperature plasma torch heater 10 and transformed into superheated water vapors at different temperatures.

C) The low temperature superheated water vapor produced from the low temperature plasma torch heater 8 is at a temperature of 500-800°C, and is input into the pyrolysis furnace 5 via the vapor nozzles. Operating parameters of the pyrolysis furnace 5 are: 500-650°C of a temperature, and 105-109 kPa of a pressure. An input speed of the low temperature superheated water vapor into the pyrolysis furnace 5 is controlled at 35-50 m/s, so that the biomass is fully contacted with the low temperature superheated water vapor and pyrolyzed into the crude synthetic gas and the ash comprising the coke. The crude synthetic gas is maintained in the pyrolysis furnace 5 for 15-20 s, and an output speed of the crude synthetic gas from the pyrolysis furnace 5 is controlled at 15-20 m/s.

D) The crude synthetic gas at the temperature of 500-650°C is output from the pyrolysis furnace 5 to the gas inlet of the gasifier 9 via the pipe; and the ash comprising the coke at the temperature of 500-650°C is transported from the ash outlet of the pyrolysis furnace 5 into the ash cooler, after the heat recovery, the temperature of the ash comprising the ash is cooled down to 150°C below. The coke is separated from the ash by the ash-coke separator 7. The coke is then transported to the coke inlet of the gasifier 9 via the coke transporter 19, and the ash from the ash-coke separator 7 is transported to the ash storehouse 18.

E) The high temperature superheated water vapor produced from the high temperature plasma torch heater 10 is at a temperature of 1200-1600°C, and is input into the gasifier 9 via the vapor nozzles. Operating parameters of the gasifier 9 are: 1200-1400°C of a temperature, and 105-109 kPa of a pressure. An input speed of the high temperature superheated water vapor into the gasifier 9 is controlled at 35-50 m/s, so that the crude synthetic gas is fully contacted with the high temperature superheated water vapor and gasified into the primary synthetic gas. The primary synthetic gas is maintained in the gasifier 9 for 15-20 s, and an output speed of the primary synthetic gas from the gasifier 9 is controlled at 15-20 m/s.

F) The liquid ash at the temperature of 1200-1400°C is output from the ash outlet of the gasifier 9 and transported to the ash storehouse 18 for comprehensive utilization. The primary synthetic gas at the temperature of 1200-1400°C is transported from the gasifier 6 to the gas inlet of the heat exchanger 11 via the pipe. After being cooled down to a temperature of 260-320°C by the desalted water, the primary synthetic gas is output from the gas outlet of the heat exchanger 11 to the dust collector 12. Dust in the primary synthetic gas is arrested by the dust collector 12, and a dust concentration of the primary synthetic gas at the outlet of the dust collector 12 is 50 mg/Nm³ below.

G) After dust removal, the primary synthetic gas is transported to the deacidification tower 13, in which harmful ingredients like H₂S, COS, HCL, NH₃, and HCN are removed.

H) After deacidification, the primary synthetic gas is transported into the desiccator 14, in which the water is removed, and clean synthetic gas is acquired. The clean synthetic gas is transported into a gas storage tank 15 and is stored for later industrial application.

After many times of tests and data detections, main components and characteristics thereof of the clean synthetic gas are shown in Table 1. As shown in Table 1, the clean synthetic gas produced by the method comprises 90% of a total content of (CO+H₂), a ratio of H₂/CO is equal to or larger than 1, a heat value of the synthetic gas is 12.5-13.4 MJ/Nm³, and an efficiency of the cooled gas is about 88%. Thus, the synthetic gas can bring great commercial benefits, and is especially applicable in industries of the integrated biomass gasification cycle combination and the biomass liquid fuel.