Patent Application
20140109468
The present invention relates to the field of fuel pellets production based on various sources of feedstock, e.g. biomass or waste.
Bergman and Kiel, “Torrefaction for Biomass Upgrading”, 14th European Biomass Conference & Exhibition, 17-21 Oct. 2005, discloses a process of torrefaction of biomass, separation of the volatiles, and cooling of the torrified biomass. It is suggested that the torrified biomass may undergo size reduction and pelletization.
Bergman et al., “Torrefaction for biomass co-firing in existing coal-fired power stations—“Biocoal””, ECN-C-05-013, Energy research Centre of the Netherlands (ECN), 2005, discloses a process of torrefaction of biomass at a temperature of about 280° C., wherein the torrified biomass is cooled and the torrefaction gas is combusted and used for drying the biomass and as a heat supplement for the torrefaction process.
Bergman, “Combined torrefaction and pelletisation—The TOP process”, ECN-C-05-073, Energy research Centre of the Netherlands (ECN), 2005, discloses a process of torrefaction of biomass at a temperature of 250-300° C. and subsequent pelletization.
In Gilbert et al., “Effect of process parameters on pelletisation of herbaceous crops”, Fuel 88 (2009), 1491-1497, a study of pelletisation under various conditions is reported. It was concluded that torrifaction of grass was not an attractive pre-process as the pellets were very brittle and possessed little mechanical strength and reduced bulk density. It was mentioned that heavy pyrolysis oil has a potential for use as a binding material which can significantly increase the strength and durability of the pellets.
WO 2010/129988 A1 discloses a process for the preparation of fuel pellet, wherein a feedstock is subjected to torrefaction and/or partial pyrolysis at temperature in the range from 250 to 500° C., whereby a solid char and a volatile fraction are obtained. The volatile fraction is used for heating of a mixer vessel. The condensed tar may subsequently be combined with the solid char.
EP 2,287,278 A2 discloses torrefaction of biomass, whereby a solid fraction is directed to a cooler. A rotary valve ensures that the volatile is not allowed to enter the cooler, but is instead fed to a combustion unit.
US 2009/007484 A1 discloses an apparatus and process for converting biomass feed materials into reusable carbonaceous and hydrocarbon products. The biomass may be torrified and the volatile fraction is condensed in one or a series of condensors. The solid material may be pelletized.
In this text char is defined as biomass or waste with a high organic fraction that has been exposed to a temperature of minimum 200° C.
The present invention provides a process for providing fuel pellets based on biomass or waste that can be optimized for use in power plant boilers (grate, fluid bed or suspension fired), district heating boilers, small pellet stoves, industrial process furnaces, kilns and boilers, small-scale heating devices, and barbeque grills. The pellets may be utilized as a global trading product. Process steps to control pellet heating value density, pellet milling properties, particle size in pellet and pellet ash properties may be included. From the end-user's point of view, the following pellets properties are attractive, namely i) a high heating value density to minimize transport costs, ii) a high pellet stability and hydrophobic properties of the pellets which make handling simple, minimized dust problems, and thereby reduce the risk of self-ignition and provide the option of out-door storage even in wet climates; iii) the option of easy grinding of the pellets in a mill e.g. a coal mill to obtain a small particle size; and iv) acceptable pellet ash properties so that ash deposition, corrosion and flue gas cleanings equipment interference are minimized and residual product utilization is possible.
Hence, the present invention provides a process for the preparation of fuel pellets, said process comprising the steps of
As mentioned above, the present invention provides a process for the preparation of fuel pellets in which a feedstock undergoes torrefaction and partial pyrolysis, and wherein produced tar is condensed onto solid char followed by pelletization.
The process of the present invention may be applied using a wide variety of feedstock, e.g. a biomass material or waste, including herbaceous biomass such as straw and grains, wood biomass including hard and softwood, as well as in principle all waste types with a significant (>10 wt %) organic fraction, or any mixtures of such feedstock. Preferably, the feedstock has an organic content of at least 15 wt %, such as at least 20 wt %, e.g. at least 40 wt %, or at least 60 wt %.
In one currently preferred embodiment, the feed stock is a biomass material. Many preferred biomass materials have an organic content of at least 80 wt %, such as at least 90 wt %.
Preferred types of feedstock include straw, grains, hard wood, soft wood, and dried sewage sludge. In some embodiments, the feedstock is wood (typically ash content 0.3 to 3 wt %), annual biomass (typically ash content 4 to 10 wt %), or more variable organic waste materials such as waste wood or dried sewage sludge.
Preferably, the water-content of the feedstock is reduced to 2-15 wt % prior to the torrefaction and partial pyrolysis process of step a) below. Reduction of the water content may be obtained in the first process step by steam drying, heating, compression or centrifugation.
Hence, in one embodiment of the process, step a) (see immediately below) is preceded by a drying step wherein the water-content of the feedstock is reduced to less than 10 wt %.
After a possible drying the first step of the process includes a combined torrefaction and partial pyrolysis process (see also Process A in
The torrefaction process is carried out by heating the feedstock in a suitable reactor in an inert atmosphere or an atmosphere with less than 0.5 Vol % O2 up to a temperature from 200° C. to 300° C. The atmosphere typically consists of the evolved volatiles, N2, CO2, steam or a flue gas depleted of oxygen. The residence time of the feedstock in the reactor at temperatures for torrefaction is typically between 0.5 seconds and 2 hours.
Typically, a solid char product yield after torrefaction of 50 to 90 wt % is obtained containing 70-90% of the feedstock heating value. The residual product is a volatile fraction (a gas) rich in CO, CO2 and water with smaller contents of H2 and some light hydrocarbons, and possible small amounts of tar.
At higher temperatures, i.e. from 300° C. to 500° C., the process is defined as a partial pyrolysis process. The residence time of the feedstock in the reactor at temperatures for partial pyrolysis is typically from 0.5 second to 1 hour.
The solid char product yield after partial pyrolysis is typically from 15 to 85 wt % depending on process conditions (temperature, heating rate, residence time). The evolved volatiles (i.e. the volatile fraction) contain both a gas and a condensable fraction of tar rich in oxygenated hydrocarbons. The tar yield can be in the range from 2 to 65 wt % of the feedstock depending on operation conditions.
It should be understood that the border-line between torrefaction and partial pyrolysis is somewhat theoretical, because it is found that the volatile fraction of a torrefaction process already from about 250° C. may comprise tar.
The present invention combines the torrefaction and partial pyrolysis so as to obtain a suitable amount of tar. Hence, the optimal torrefaction/pyrolysis reactor operation temperature is a compromise between two objectives. The temperature shall be sufficiently high to obtain a sufficient yield of tars and thereby to obtain pellets with adequate quality. Also the char yield shall be as high as possible to obtain a maximum of the feedstock energy content transferred to the fuel pellets. Generally, the char yield decrease and the tar yield increase with increasing reactor temperature. It is not possible to define a generally applicable optimal reactor temperature for all types of feedstock. However, previously conducted studies indicate that the optimal temperatures may be in the range of 250 to 500° C. The actual optimal reactor temperature is dependent on the applied feedstock and reactor type.
However, in some preferred embodiments, the torrefaction and partial pyrolysis involves that the feedstock is subjected to a maximum temperature in the range from 250° C. to 500° C., such as from 260° C. to 490° C., e.g. from 270° C. to 480° C., or from 280° C. to 475° C., or from 290° C. to 470° C., or from 300° C. to 460° C., preferably from 310° C. to 450° C., or from 320° C. to 450° C., or from 330° C. to 450° C., or from 340° C. to 450° C., or from 350° C. to 450° C. In other embodiments, the feedstock is subjected to a temperature in the range from 250° C. to 400° C., such as from 260° C. to 390° C., e.g. from 270° C. to 380° C., or from 280° C. to 360° C., or from 290° C. to 350° C.
The combined torrefaction and partial pyrolysis is typically allowed to proceed for a total period from 2 seconds to 2 hour, such as from 10 seconds to 90 minutes, such as from 4 minutes to 90 minutes, or from 6 minutes to 70 minutes, e.g. from 8 minutes to 50 minutes.
A possible method to control the quality of the obtained pellets could be to use an instrument that determines the amount of condensable products in the volatiles fraction. The instrument could determine the amount of condensed material by cooling the volatile fraction to e.g. 110° C.
The torrefaction process and the partial pyrolysis process may be run as separate processes in the same or separate reactors, however, preferably, the processes are run sequentially, e.g. by using a temperature gradient. The processes may be implemented with co-current flow conditions (as illustrated in
Hence, in some embodiments, the feedstock is heated for up to 2 hours. Within this embodiment, the exit temperature at completion of the torrefaction and partial pyrolysis process typically is in the range from 300° C. to 450° C.
A heat source is needed to facilitate the torrefaction and partial pyrolysis process. Heat may be supplied by heat transfer through a metal wall, by an intermediate heat carrier such as sand, ceramic, concrete or metal balls, steam, CO2 or by a flue gas nearly depleted of oxygen. Heat can be generated by using the gas developed in process step a), by using heat from other processes or by using a separate fuel supply.
A possible size reduction of the char may be performed (see Process B in
The output stream from step a) (see Process A (and Process B) in
One interesting fraction of the volatile fraction is the tar fraction, which will be discussed further in connection with step b) below.
The torrefaction/pyrolysis process (possibly also including the subsequent condensation step, see step b)) can be implemented by use of a range of different reactors, some examples are provided:
An essential feature of the step b) (see Process C in
The tar-rich fraction is typically condensed when cooling from the exit temperature of step a) (i.e. the torrefaction/partial pyrolysis temperature (such as about 350° C.)) to a temperature of 20-150° C., such as 50-150° C.
Water can be condensed upon cooling to a temperature below 100° C. i.e. below the water drew point temperature. In some embodiments, it is desirable to allow water to become condensed together with the tar-rich fraction in that the presence of water will facilitate the pellet formation (step c)).
Hence, in one embodiment, the cooling is conducted by cooling the volatile fraction from the exit temperature of step a) to at temperature of around 100° C. (e.g. in the range from 50° C. to 150° C.).
Another feature of step b) is to combine the tar-rich fraction (possibly including water) with the solid char. It has been found, that the combination of solid char and the tar-rich fraction will provide benefits with respect to the pelletization and with respect to the properties of the final fuel pellets.
By controlling the cooling temperature of the complete product stream from step a) (i.e. the solid char and the volatile fraction), a controlled mixture of the solid char and the tar-rich fraction (possibly including water) can be obtained. Hence, by cooling of all products in one step tar and water can be efficiently mixed with the char.
Thus, in one preferred embodiment, solid char and the volatile fraction are simultaneously cooled, whereby the solid char is combined with at least a part of the tar-rich fraction. This is e.g. illustrated in
Hence, in another embodiment, the cooling is conducted by cooling the solid char and the volatile fraction from the exit temperature of step a) to a temperature of around 100° C. (e.g. in the range of 50° C. to 150° C.) while allowing the condensed tar-rich fraction to become mixed with the solid char.
In some embodiments, any gasses from the volatile fraction from which the tar-rich fraction is condensed, may be combusted so as to provide energy to any drying of the feedstock or to the torrefaction and partial pyrolysis process. Hence, the evolved gas may be used to provide heat for, e.g., process step a).
The cooling step could depending on temperature be utilized for power or heat production, e.g. by heat exchange with appropriate water or steam cycles.
For some types of feedstock (typically alkali rich feedstock) and for some applications of the fuel pellets, it may be advantageous to combine additives (see Process D in
Hence, in some embodiments, it is—for the purpose of making optimal pellets for different combustion and gasification units—advantageous that the pellets are formulated by addition of additives (see Process D in
Examples of pellets formulations may include:
Any additives may be combined with the solid char before, in combination with, or after combination of the solid char with the tar-rich fraction. In some embodiments, the additives may even be fed together with the feedstock.
In step c) of the process (see Process E in
Keeping of the material at a temperature in the range from 50° C. to 100° C. of the pelletizing process may increase pellet stability and hardness.
The pelletizing is conducted using conventional equipment, e.g. an Andritz sprount pellet mill, using conventional conditions.
The pelletizing may be followed by a curing step in order to harden the pellets, e.g. by curing the tar.
Preferably step a) and step b) of the process are run as a continuous process. In some interesting embodiments hereof, step a), step b) and step c) of the process are run as a continuous process.
In one embodiment, the invention relates to a process for the preparation of fuel pellets, said process comprising the steps of
The product can advantageously be stored and transported with high stability and the pellets can be used as fuel in a pulverized fired power plant boiler.
The primary demands for an adequate pellet quality is a pellet that is hydrophobic and does not fragment significantly during transportation. Hence, the pellets should also have suitable mechanical strength, e.g. defined as the tensile strength thereof. The tensile strength can be measured using a tensometer for compression of a pellet in the radial direction, cf. the method described by da Rocha SSHF, “Mechanical evaluation for the quality control of biomass pellets and briquettes. In: Proceedings of the second world conference on pellets, Jönköping, Sweden; 2006, 183-187.
It appears that useful pellets preferably have a tensile strength of at least 100 kPa, such as at least 200 kPa, e.g. at least 300 kPa. Very attractive pellets are those having a tensile strength of at least 400 kPa, such as at least 500 kPa, or at least 600 kPa, or at least 700 kPa.
Hence, it is believed that the pellets obtained by the above process are novel as such. Hence, the present invention also provides a fuel pellet comprising a solid char, tar, and, optionally, one or more additives, said solid char and said tar being obtained by torrefaction and partial pyrolysis of a feedstock at a temperature from 250° C. to 500° C. Preferably, the pellet has a tensile strength of at least 100 kPa.
The pellets prepared according to the invention can be grinded with low energy consumption and is thereby optimal to use in suspension fired boilers. Moreover, the pellets can be stored under out-door conditions on moist regions of the world, e.g. in the Scandinavian countries.
The pellets prepared according to the invention can be provided to a national or an international market with end-uses in: power plant boilers (grate, fluid or suspension fired), district heating boilers, small pellet stoves, industrial process furnaces, kilns and boilers, small-scale heating devices, and barbeque grills.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11169556.5 | Jun 2011 | EP | regional |
| 11169562.3 | Jun 2011 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DK2012/000073 | 6/11/2012 | WO | 00 | 12/10/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61495471 | Jun 2011 | US |
