Patent Application
20140166465
The present invention pertains to the field of renewable energy and, in particular, to a system and process for conversion of organic matter (such as biomass feedstock) into a torrefied product.
Torrefaction is a thermal process used to convert biomass in an oxygen-deficient environment into a high-quality biochar. Torrefaction as a process to generate renewable fuel has long been sought after. The concept that this process can transform biomass into a product that is stored, handled, burned and has properties similar to coal is extraordinary. Torrefaction has been around for a long time, however, to use this process to create a renewable fuel is a new phenomenon. Over the past decade, many inventors have been tinkering with this process in order to create renewable fuel and the key challenges they face when trying to make this process work to generate a fuel are scaleability, mass and energy balance, and cost effectiveness. This patent encompasses a torrefaction solution that is scaleable, does not use more energy to generate the fuel than the fuel can create, and offers a cost effective solution.
The torrefaction process requires a heat source operating in a range of 200-400° C. in a low oxygen environment to minimize the release of toxic emissions and to avoid complete combustion of the feedstock. The process removes moisture within the feedstock through chemical reactions. The end product is a black organic material known as biochar (resembles charcoal).
During the initial heating, the biomass is dried and continues with further heating in which more moisture is removed due to the chemical reactions occurring within the hemicellulose and cellulose of the biomass through a thermo-condensation process. As the temperature increases past 160° C., CO2 is formed. Between 180° C. and 270° C., the reaction becomes exothermic and the hemicellulose breakdown continues (Tumuluru, Sokhansanj, Wright and Boardman, 2010). When the operation reaches the torrefaction temperatures, hemicellulose is the primary lignocellulosic material within the feedstock that is degraded into gases (including H2, CH4, aromatics, CO,CO2 and CxHy), condensable liquids (including acids, ketones, furans, alcohols, terpenes, phenols, waxes, tannins, water), and solids (including char, new and existing sugar structures, and new polymers and ash (NRCan-Madrali, 2011).
As the hemicellulose continues to degrade and lose moisture, a colour change occurs. The feedstock becomes darker due to the loss of water and the amount of acetic acid and phenols released. Both the lignin and cellulose may also be depolymerized and devolatilised during the process to a lesser extent.
For the torrefaction process, it is not recommended that the temperatures reach above 300° C. to prevent the reaction from becoming a pyrolysis process. Torrefaction is considered to be a mild form of pyrolysis. Table 1 compares the products generated from dry wood using different modes of pyrolysis. With torrefaction, 82% of the product is a solid char and 18% gases are released during the process.
)
)
)
indicates data missing or illegible when filed
The end product of torrefaction has many advantages over its original feedstock. Table 2 summarizes the additional value of a torrefied product.
The product will maintain 50 to 70% of the mass of the feedstock, but 90% of the energy. The moisture content of the product is in the range of 2%. If 20 kg of feedstock with an energy value of 12,000 kJ/kg (20*12000=240,000 KJ net) go into the kiln, the product, assuming 50% mass retention, will weigh 10 kg and have an energy value of 22,000 KJ/Kg (220,000 KJ net).
The properties of the feedstock biomass are improved through limited devolatilisation that occurs under these conditions. The end product is a biochar that has the potential to replace coal. This biochar end product has increases in caloric value, hydrophobicity, and energy density when compared to the raw feedstock. All of the biomass undergoes dehydration reaction which destroys the —OH groups that are responsible for hydrogen bonding with water therefore the absorption of water is reduced in the densified torrefied product.
As previously mentioned, fuel characteristics of torrefied biomass are enhanced through the torrefaction process. Table 2 provides the data of key fuel characteristics of a variety of feedstocks (wood versus coal versus charcoal versus torrefied pellets). As can be seen the torrefied pellets have similar caloric value (GJ/t), bulk density, and volumetric energy density as charcoal and coal which is significantly higher than the raw wood.
There are several methods of heating the feedstock in the process of torrefaction, but the processes using a heat carrier (gas, solid, liquid) will either be direct or indirect. Using an indirect method, the gases are heated in a burner that is external to the reaction chamber and circulated around the feedstock chamber so that there is no direct contact of the heated gases and the feedstock. Using a direct method, the feedstock is in direct contact with the heated gases. Typical direct fired approaches involve fluidized bed, and other mechanical processes which emulate a fluidized bed. The indirect heating method has typically been combined with a batch type process. The present invention does not utilize the heat carrier as a feedstock carrier, which reduces the risk of explosion and ensures the residence time is the same for all feedstock particles—irrespective of particle size.
The majority of current technologies (Weisselberg U.S. Pat. No. 8,161,663, Paoluccio U.S. Pat. No. 7,942,942, Paoluccio US2008/0223269, Yvan U.S. Pat. No. 4,553,978, Reed US2003/0221363, Weisselberg U.S. Pat. No. 8,161,663, Budarin US2011/0219679) use indirect sources of heating to vaporize the volatiles and moisture within the biomass feedstock, which can lead to, among other problems, fluctuations in processing temperatures. Indirect heating is less energy efficient. The greater the number of steps between the flame and the feedstock, the greater the potential for energy loss. Also indirect heating can have more associated equipment, therefore costing more to build, run, maintain, and repair.
Current technologies, whether direct or indirect-fired, have tight restrictions on the physical properties of the feedstock. Some of the restrictions include substantially dry feedstock, narrow particle size distributions (similar to sawdust), and in some cases the feedstock must be agglomerated prior to torrefaction (Paoluccio U.S. Pat. No. 7,942,942, Paoluccio US2008/0223269, Yvan U.S. Pat. No. 4,553,978, Weisselberg U.S. Pat. No. 8,161,663, Budarin US2011/0219679).
In Budarin US2011/0219679, a method for heating the feedstock using microwaves is presented. One disadvantage of microwave-based torrefaction processes is that while they may be quick, they require substantially dry feedstock at elevated temperatures as the input. This requires significant energy in the processing of the biomass. Furthermore, Budarin proposes the agglomeration of feedstock into either pellets or extruded logs which adds costs to the construction and operation of the process.
There is therefore a need for a torrefaction system that can accommodate substantially high moisture feedstock direct from the source without additional pre-treatment processing.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.
An object of the present invention is to provide a system and process for conversion of organic matter into torrefied product. In accordance with an aspect of the present invention, there is provided a system for conversion of organic matter feedstock into a torrefied product, the system for conversion of organic matter into a torrefied product, the system comprising a direct fired rotary kiln, the kiln having a first end and a second end, the kiln being tilted at an angle such that the first end is lower than the second end, a burner assembly located at the first end of the kiln, a feedstock input means and a gas conduit located at the second end of the kiln, and a torrefied product outlet at the first end of the kiln.
In accordance with another aspect of the present invention, there is provided A process for conversion of organic matter feedstock into a torrefied product in a direct fired rotary kiln, the process comprising the steps of: introducing the feedstock into the rotary kiln; applying heat to the feedstock for a residence time sufficient to convert the feedstock to the torrefied product; and collecting the torrefied product.
The term “organic matter” is used to define biomass feedstock, including but not limited to wood products (bark, chips, slash), switchgrass, paper products, human and/or animal wastes, de-inking sludge, agricultural residues, construction and demolition waste, and other organic materials.
The terms “low oxygen” and “low O2” are used to define environments comprising less than 10% oxygen.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
The present invention provides a torrefaction system comprising a direct-fired, refractory-lined, continuously operating rotary kiln. Accordingly, the present torrefaction system also comprises a burner assembly comprising a primary combustor and one or more gas inlets, wherein the burner assembly produces hot, low oxygen content combustion gases that come in direct contact with the feedstock.
With the presently disclosed system, the use of a direct-fired rotary kiln allows a more efficient process in the conversion of the biomass. The typical conversion rate for torrefaction process is a 3 to 1 ratio (input to product), while the conversion ratio of the present system is 2.5 tonnes of input (raw feedstock) to produce 1 tonne of biochar product.
The control of the internal environment of the kiln creates a steady and stable process that is highly repeatable. The use of the kiln removes critical scaling issues faced by alternative technologies. The rotary kiln design allows for continuous input of the feedstock, and by rotating it, the feedstock is well mixed. The degree of the incline and the speed of rotation control the mixing and the residence time of the feedstock and ensures that there is a consistent end product generated. The presently disclosed system therefore also provides continuous operation.
The primary combustor can be fired by a variety of fuels (such as natural gas, propane, diesel or designed to accommodate use of some of the biomass feedstock as a fuel source) and is used primarily for heating of the kiln during initial start-up. Flue gas recirculated from the afterburner at the exhaust of the kiln will provide hot gases during normal operation that will reduce the load on this burner.
Direct contact between the hot gases passing through the kiln, the heated refractory, and the feedstock biomass in a controlled low O2 environment provides very efficient heating of the feedstock and causes the moisture and volatile organic compounds (VOCs) in the feedstock to vaporize and produce a low to medium BTU gas. This gas can be combusted in an afterburner at the kiln exhaust and the resulting flue gas can supply some of the heat and maintain the low O2 environment required to support the torrefaction process in the kiln.
Accordingly, in one embodiment of the present invention, the system further comprises an afterburner assembly, immediately downstream of the kiln exhaust, which operates at a slightly negative pressure to prevent flue gas leakage, and completes the burnout of any residual tars and/or other volatiles released during the torrefaction process. The resulting flue gas can then be recirculated through a flue gas recirculation (FGR) system to the input of the kiln to supply heat and maintain the low O2 environment required to support the torrefaction process in the kiln.
The presently disclosed system optionally employs a flue gas recirculation (FGR) loop using these produced gases. By utilizing the FGR, the low O2 environment can be maintained, and the internal kiln temperatures can be controlled (hot gases circulating through the kiln between 500-950° C.; and refractory temperature maintained between 200-300° C.). The flue gas then is passed through a heat-exchanger (to cool the gases to protect the gas clean-up equipment) and gas clean-up train downstream of the afterburner. After the flue gas has passed through the gas clean-up train, energy generated in the afterburner assembly during combustion of tars and/or other volatiles and organics is transferred to the flue gas prior to recirculation back to the kiln as shown in
The torrefied end product of the presently disclosed process is an energy-dense carbon neutral fuel that resembles coal in a number of its properties (black charcoal pieces). The product is more energy-dense, more resistant to water, and more chemically and physically uniform than the feedstock used to make it. The coproduct, or fines, produced also has potential value as a soil amendment product that also acts as a carbon sequestration process, returning carbon to the forest floor or agricultural field.
The presently disclosed process is scaleable, and can be used with both a wide variety of feedstocks, and feedstocks with wide variations in physical and chemical properties.
The excellent control of the internal environment allows for a stable process that is highly repeatable and easily scalable. This allows a variety of feedstock inputs and fine-tuning to produce a more consistent and homogeneous product. The rotary kiln also allows continuous input of feedstock with physical variability (i.e. different input forms and sizes) that will require minimal or no preparation before entering the process. This is a major advantage, as most prior art systems and processes rely on feedstock being a specific size and humidity, and operate only in batch or semi-continuous production.
The rotary kiln 100 is formed of steel and insulated refractory material, and is provided with internal baffles to ensure mixing of the feedstock when rotated.
The kiln is oriented at an angle, wherein the feedstock input assembly is located at the raised end, and the torrefied product output is located at the opposite, lower end. The orientation (i.e., tilt) and the rotational speed of the kiln are controlled to ensure that there is sufficient residence time of the biomass feedstock within the kiln to complete the torrefaction process. The rotational speed and tilt are adjustable, and therefore the residence time is customizable according to the specific biomass feedstock.
As the kiln rotates, the feedstock travels through the kiln through the different temperature zones (
Once the material has travelled through the length of the kiln, the torrefaction process is complete, and the product is discharged through the torrefied product outlet 110 located at or adjacent to the burner end of the kiln. The outlet is configured to minimize oxygen ingress to the kiln, thereby maintaining a low O2 environment conducive to the torrefaction process.
The feedstock is introduced to the kiln through a feedstock input assembly 140. The feedstock input assembly can be configured for continuous inputs, which allows the process to be run in a continuous manner. The rate at which the feedstock is introduced is also controllable. It is also within the scope of the presently disclosed system to provide the feedstock in a batch-wise manner, as long as it is in a sufficient frequency and volume that the torrefaction process in the kiln is allowed to proceed in a continuous manner. The feedstock input assembly 140 can employ a screw-type or auger mechanism, conveyor belt system or any other suitable mechanism for continuous feedstock inputs.
Located at the opposite end of the kiln from the feedstock input assembly is a burner assembly 120 that produces the hot combustion gases that come in direct contact with the feedstock to drive the torrefaction process. The burner assembly 120 is in communication with a heated gas inlet 180 located at the burner end of the kiln 100. The burner assembly 120 is comprised of a primary combustor fuelled by an external fuel such as natural gas; and optional flue gas injection ports located at the burner combustion zone which permit flue gas to be introduced to the kiln through the burner assembly. For example, natural gas and oxygen inlets are provided to control the amount of heat provided by the burner, which controls the temperature in the kiln. The burner is designed to operate with fuel and air flows that will maintain the appropriate heating, but also a low gas velocity, in the kiln. In addition, optional injection of flue gases, which include non-oxidizing gases such as CO2 and N2, allows control of the composition of the gases in the kiln to maintain the low O2 environment required for torrefaction (i.e. minimizing the amount of oxidizing gases present). The recirculated flue gas also provides heat to the kiln, minimizing the use of external fuel to support the torrefaction process. Inert gases such as CO2 or N2 can also be introduced through dedicated gas inlets located at or near the burner to maintain the internal kiln environment in a safe, non-combustible mode until feedstock can be removed from the kiln during upset conditions.
The burner assembly 120 is configured in such a manner that the material inside the kiln will be shielded from direct exposure to the burner flame. This is done to prevent local heating and combustion of feedstock particles at the incoming end of the kiln. The presently disclosed process achieves this by including angled piping between the primary combustor and the heated gas inlet of the kiln. In one embodiment, the burner assembly comprises a 90-degree angle in the burner assembly piping prior to the hot gases entering the kiln. Other configurations and piping shapes which prevent direct exposure of the burner flame to the interior of the kiln are within the scope of the present invention. Other methods of preventing direct exposure include incorporation of internal baffles or shielding at the burner end of the kiln to absorb the radiation from the burner flame.
Combustion gases from the burner travel through the kiln, picking up the moisture and volatiles from the feedstock. The resulting gaseous mixture then moves through a transition box/gas conduit 150 to the afterburner 160, where the volatiles are burned. The resulting flue gas is then processed through a flue gas recirculation system, which includes heat-exchange equipment (to cool the gas to protect the emission control equipment) and emission control equipment downstream of the afterburner (as shown in
In one embodiment schematically depicted in
In one embodiment, the emission control equipment comprises a cyclone, baghouse filter, an ID fan, and a stack as shown in
In one embodiment, the gas conditioning suite includes heat exchanger(s) and a water vapour condenser(s). The water vapour condenser is employed to remove water vapour in the flue gas before returning the flue gas to the kiln. The heat exchanger(s) function is to first cool the flue gas to the required temperature to permit it to enter the emission control equipment without damaging it, and then to reheat the dry, clean flue gas returning from the water vapour condenser before injecting it back into the kiln at the re-injection ports in the burner assembly. Heat exchangers may exist in multiples and their arrangement will depend on the numbers of emission control equipment in the FGR.
In one embodiment, the present torrefaction system further comprises a control subsystem, which includes, for example, sensors throughout for measuring kiln rotation speed, fuel flow, temperature at burner, temperature in kiln (gas and refractory), O2 sensor, and standard emissions monitoring (e.g. particulate matter, CO, NOx, SOx, etc.). The kiln rotation speed is used to control material residence time. The fuel flow controls the burner heat input and the gas temperature entering the kiln, and therefore also the temperature of the refractory. The gas monitoring, including O2 levels, helps ensure the system is sealed and operating safely. The emissions monitoring equipment ensures that the emissions control equipment is working as required. Other sensors are provided to determine gas composition at various points throughout the system.
The invention will now be described with reference to specific examples. It will be understood that the following examples are intended to describe embodiments of the invention and are not intended to limit the invention in any way.
The first test was performed April 21, after several days of becoming acquainted with the system and test-firing the new burner. The primary objective was to try a set of conditions and to see what came out the other end. After the test, additional insulation was added to the outside of the burner and a screen was installed to prevent accidental contact with the hot equipment. The test lasted 3 hours and the difference from one end of the kiln to the other was 125° C. Torrefaction occurred only to a very limited extent. The product was not remarkably different than the feedstock. We concluded that the temperature was too low. The highest refractory temperature reached during this test was less than 200° C., and the retention time was approximately 45 minutes.
The second test aimed to improve upon the disappointing product from the first test by increasing the temperature of the flame and kiln refractory. The test lasted 3.5 hours and the difference from one end of the kiln to the other was 161° C. The product was darker than the product of test 1, but still nowhere near the appearance of coal. The highest refractory temperature reached during this test was less than 245° C., and the retention time was approximately 45 minutes.
Test 3 incorporated the techniques figured out in the first two tests, and further increased the temperature of the flame and kiln refractory. The firing rate was manipulated in order to create a darker product. The product of test 3 is shown in
The fourth and final test considered in this report was designed as a culmination of the techniques learned in the first three tests, and used to design future experiments. In the previous three tests, the darkest product was obtained after several hours. In test 4, the kiln was pre-warmed to reduce the time required to reach steady state. The residence time was increased by decreasing the speed of rotation of the kiln. The product of test 4 is shown in
Test 1 yielded a product that looked almost unchanged compared to the feedstock. It did not appear to be torrefied at all, but rather had a dirty and grey appearance. The product created in test 2 was darker than what was produced in test 1, but still did not look very different than the feedstock. In test 2, some darker pieces were starting to appear. Products from tests 1 and 2 were not analysed because it was determined visually that the target product had not been successfully created.
Tests 3 and 4 were also successively darker (
For analytical purposes, a half-hour period of steady state was selected for comparisons between the tests. This half-hour period was used in order to eliminate any inconsistencies that may result from start-up and shutdown operations. Means were calculated using the descriptive statistics tool in MS Excel (Table 3).The elapsed time from the beginning of the test to the start of the half-hour steady state sample's beginning is noted in the third column of Table 3. In each case, it was approximately 2.5 hours after the test started.
The temperatures for the flue gas at the point where it enters the kiln were averaged for the four tests since there was so little variation. The temperature of the refractory lining of the kiln was greater with each successive run (
There is a positive correlation between refractory temperature and the degree to which the material was torrefied. Tests 1 through 3 were initiated several hours after the kiln and burner were started. However, test 4 began after the kiln had been heated overnight. This pre-warming accounts for the higher starting refractory temperature for test 4.
Natural gas was used to fuel the torrefaction process. Flue gas recirculation was simulated by injecting CO2 and N2 into the burner very near the natural gas inlet. Natural gas was also used to fuel the afterburner. The flow of natural gas across the whole system (burner and afterburner) was similar during steady state of all four experiments (Table 4). However, the quantity of fuel used by the burner declined sharply from one test to the next. Test 4 used the least natural gas and N2, and it had a significantly reduced rotation speed. Test 4 accepted the same quantity of feedstock during the steady state period as test 3 (Table 4), despite the longer retention time. In addition, it had a lower temperature than test 3 and produced a more torrefied product. The amount of energy in BTU used by the burner in the 30-minute steady state sample period for each test was calculated by multiplying the cubic feet of natural gas by 1015 because there are 1015 BTU per cubic foot of natural gas.
Feedstock (pine chips) and the products from tests 3 and 4 were analyzed using calorimetry (Table 5). Calorimetric analysis was used to determine if there was a significant increase in the energy value of the product compared to the feedstock. Initial estimates were that an increase of 100% was possible. This was not achieved when compared on a dry basis. However, the dried product from test four had double the BTU value of the “as received” wood chips (Table 5).
Analytical results from test 3 show that the BTU value of the product was not much greater than the feedstock (Table 5). This was significantly improved upon in test 4. The moisture content of the product from test 4 (38.5%) is much higher than that of test 3 (4.12%) because it was captured in a bin of water during experimentation. This was necessary because of combustion upon exposure to air when leaving the kiln through the discharge trap.
The feedstock had more moisture than expected, and therefore it is useful to note that a feedstock that has greater than 40% moisture can be successfully torrefied with the present system.
Sieve analysis separated the product from test 4 into five standard ASTM size categories (ASTM D4749). Each size underwent calorimetric analysis to test the thoroughness and uniformity of torrefaction. Most pieces of product were found to be smaller than a half inch, with only about 13% exceeding ½″. A significant quantity of very small pieces made it through, supporting the claim that the draft inside the kiln is not causing all small particles to be blown into the afterburner. The size distribution analysis results and the corresponding energy values for the test 4 product are presented in Table 6.
Proximate analysis was performed to determine the reduction in volatile matter and the increase in fixed carbon (Table 7). The ash content more than doubled from the feedstock to test 3. The fixed carbon was significantly higher, from 13.24% to 29.94% between the feedstock and test 4's product. Sulfur content was reduced by more than half, although it was already extremely low at 0.02% by dried mass of feedstock. The main concern with these comparative results is the increase in ash content. Test 4 resulted in a nearly 7-fold increase in the ash compared to the feedstock. The feedstock and test 3 had similar volatile matter content, but there was a sharp decrease in volatile matter in test 4.
Ultimate analysis was conducted to chemically qualify the feedstock and the products from tests 3 and 4 (Table 8). Based on the proximate analysis, it is surprising that there was not a greater increase in the percentage of mass represented by carbon. Test 4 resulted in a notable decrease in hydrogen
Additional feedstocks, including switchgrass, construction waste, hog fuel/bark, and de-inking sludge, were tested using the torrefaction process and system of the present disclosure. These additional tests have demonstrated that the process and system of the present disclosure can be used to convert a wide range of feedstocks to a useful torrefied product.
It is obvious that the foregoing embodiments of the invention are examples and can be varied in many ways. Such present or future variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2012/050457 | 7/6/2012 | WO | 00 | 2/21/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61505340 | Jul 2011 | US |
