PROCESS FOR REPLACEMENT OF FOSSIL FUELS IN FIRING OF ROTARY LIME KILNS

Abstract

A method and system for the generation of a medium-Btu, clean and renewable fuel gas to replace fossil fuels in existing lime kilns with minimal retrofitting without significantly compromising the kiln capacity. A steam-blown dual fluidized bed gasifier produces renewable fuel gas from a carbonaceous feedstock such as woody biomass. A gas cleanup process purifies the raw fuel gas, resulting in a clean fuel gas for mitigation of lime contamination and environmental issues. The adiabatic flame temperature and flue gas volume/GJ for the combustion of the renewable fuel gas are similar to values for natural gas, making retrofitting of fossil fuel-fired lime kilns relatively straightforward.

Claims

1. A system for manufacturing of a renewable fuel gas for use in a rotary lime kiln or in a limestone calcination plant, comprising: a dual bed gasification apparatus, said dual bed gasification apparatus comprising: a gasifier, such as a bubbling fluidized bed gasifier, providing partial oxidation of a biomass to generate a hot raw syngas and char; a char combustor, such as a circulating fluidized bed char combustor comprising one or more stages, connected to the gasifier generating a char combustor flue gas and providing energy for the gasification process; a heat carrier circulating between the said gasifier and char combustor to transfer heat from the char combustor to the gasifier; at least one gasifier cyclone, connected to said gasifier for recovery of coarse particulates from said hot raw syngas; optionally a heated tar reformer for elimination of tar in said hot raw syngas to provide a hot tar reformer exit syngas; at least one gasifier heat exchanger connected to said gasifier cyclone or said optional hot tar reformer, for cooling of the said hot raw syngas or said hot tar reformer exit syngas to about 200? C. and provide a resultant cool raw syngas; a baghouse or other separation devices for removal of fine particulates from the cool raw syngas and provide a resultant cool syngas; a liquid scrubber or other separation devices for removal of tar and/or moisture from the cool syngas; optionally, an ammonia scrubber or other ammonia separation devices for removal of NH.sub.3 from the cool syngas; optionally, an amine unit or other carbon dioxide separation devices for removal of carbon dioxide from the cool syngas; optionally, a sulfur scrubber or other sulfur separation devices for removal of sulphurous species from the cool syngas; optionally, a chloride scrubber or other chloride separation devices for removal of acid chloride species from the cool syngas; a gas passage with the means to deliver the cool syngas to a kiln burner at ambient temperatures for combustion; at least one char combustor cyclone for recovery of coarse particulates from the char combustor flue gas; at least one char combustor heat exchanger connected to said char combustor cyclone, for cooling of the char combustor flue gas, providing recovered heat and a cool flue gas; a char combustor baghouse or other separation devices to remove fine particulates from the cool flue gas.

2. (canceled)

3. A system according to claim 1, further comprising a steam source for the gasifier, wherein steam to biomass ratio is greater than 0.9 (w/w).

4. (canceled)

5. A system according to claim 1, wherein the gasifier is operated at 750-850? and/or the char combustor is operated at 850-950? C.

6. (canceled)

7. (canceled)

8. A system according to claim 1, wherein the char combustor utilizes one or more of a stream of organic scrubber liquid with tar, a stream of recycled clean syngas and a stream of biomass as auxiliary fuels.

9. (canceled)

10. (canceled)

11. A system according to claim 1, wherein the cool syngas delivered to the kiln burner is free of particulates and tars.

12. A system according to claim 11, wherein the cool syngas contains moisture below 8% vol, for example, below 4% vol.

13. (canceled)

14. A system according to claim 11, wherein the lower heating value of the syngas is a minimum of 9 MJ/Nm.sup.3.

15. A system according to claim 1, further comprising a dryer, upstream of the gasifier, for reducing the moisture content of the biomass where biomass moisture content exceeds 20%.

16. A system according to claim 15, wherein the dryer utilizes heat recovered from the cooling of the hot raw syngas in the gasifier heat exchanger, and/or heat recovered from the cooling of the char combustor flue gas in the char combustor heat exchanger.

17. A system according to claim 1, wherein the thermal based volumetric flow rate (m.sup.3/GJ) of flue gas generated in the kiln burner is no larger than that when the kiln burner is fed with natural gas.

18. A method of manufacturing a renewable fuel gas for use in a rotary kiln or in a limestone calcination plant, comprising: partially oxidizing a biomass in a gasifier, such as a bubbling fluidized bed gasifier, to generate a hot raw syngas and char; burning said char in a char combustor, such as a circulating fluidized bed char combustor comprising one or more stages, to generate a char combustor flue gas and energy for the gasification process; circulating heat and char between said gasifier and said char combustor with a heat carrier; recovering coarse particulates from the hot raw syngas utilizing a gasifier cyclone; optionally reforming the tars in the hot raw syngas to provide a hot syngas; cooling the hot raw syngas or hot syngas to about 200? C. in a gasifier heat exchanger resulting in a cool raw syngas and heat; removing fine particulates from the cool raw syngas utilizing a baghouse or other separation devices, resulting in a cool syngas; removing tar and/or moisture from the cool syngas with a liquid scrubber or other separation devices; optionally, removing NH.sub.3 from the cool syngas with an ammonia scrubber or other ammonia separation devices; optionally, removing carbon dioxide from the cool syngas with an amine unit or other carbon dioxide separation devices; optionally, removing sulphurous species from the cool syngas with a sulfur scrubber or other sulfur separation devices; optionally, removing acid chloride species from the cool syngas with a chloride scrubber or other chloride separation devices; delivering the resultant cool syngas to a kiln burner at ambient temperature through a gas passage for combustion in said kiln burner; recovering coarse particulates from the char combustor flue gas in a char combustor cyclone; cooling the char combustor flue gas in a char combustor heat exchanger, resulting in a cool flue gas and heat; removing fine particulates from the cool flue gas in a char combustor baghouse or other separation devices.

19. (canceled)

20. The method according to claim 18, further comprising feeding steam to the gasifier for use in the oxidizing step, wherein the steam to biomass ratio is greater than 0.9 (w/w).

21. (canceled)

22. The method according to claim 18, wherein the gasifier is operated at 750-850? C. and/or the char combustor is operated at 850-950? C.

23. (canceled)

24. (canceled)

25. The method according to claim 18, wherein the char combustor utilizes one or more of a stream of organic scrubber liquid with tar, a stream of recycled clean syngas and a stream of biomass as auxiliary fuels.

26. (canceled)

27. (canceled)

28. The method according to claim 18, wherein the cool syngas delivered to the kiln burner is free of particulates and tars.

29. The method according to claim 28, wherein the cool syngas contains moisture below 8% vol, for example, below 4% vol.

30. (canceled)

31. The method according to claim 28, wherein the lower heating value of the syngas is a minimum of 9 MJ/Nm.sup.3.

32. The method according to claim 18, further comprising drying the biomass to reduce the moisture content of the biomass prior to oxidizing the biomass in the gasifier, in situations where biomass moisture content exceeds 20%.

33. The method according to claim 32, wherein the dryer utilizes heat recovered from the cooling of the hot raw syngas in the gasifier heat exchanger, and/or heat recovered from the cooling of the char combustor flue gas in the char combustor heat exchanger.

34. The method according to claim 18, wherein the volumetric flow rate of flue gas generated in the kiln burner is no larger than that when the kiln burner is fed with natural gas.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0085] In the following, the invention will be described in more detail with reference to the appended drawings, in which:

[0086] FIG. 1 shows a schematic representation of a prior art rotary lime kiln.

[0087] FIG. 2 shows a schematic process flow diagram of an embodiment of the process for the replacement of fossil fuels in the firing of rotary lime kilns according to the present invention.

[0088] FIG. 3 shows a schematic process flow diagram of a further embodiment of the process according to the present invention.

[0089] For the sake of clarity, the drawings only show the details necessary for understanding the invention. The structures and details that are not necessary for understanding the invention but would be understood to a person skilled in the art have been omitted in the figures in order to emphasize the characteristics of the invention.

DETAILED DESCRIPTION OF INVENTION

[0090] FIG. 2 illustrates a simplified flow diagram for the generation and purification of the medium Btu renewable fuel gas and its application in the lime kiln, according to one embodiment of the present invention. In this embodiment, the woody biomass waste, such as bark or sawdust (labelled BIOMASS in FIG. 2), is gasified in a steam-blown indirect gasification system, including a bubbling fluidized bed (BFB) gasifier 2 and a circulating fluidized bed (CFB) riser combustor 3. The configuration of the BFB gasifier 2 and CFB riser combustor 3 can be configured generally as described previously In the art, including for example, in our previous patent publications WO 2012/065272, U.S. Pat. No. 9,364,812, and U.S. provisional patent application 63/298,990, all incorporated herein by reference. Heat carrier (not shown), such as olivine sand, silica sand, aluminum oxide or engineered catalyst, is re-circulated between the BFB gasifier 2 and the CFB combustor 3heated in CFB combustor 3 and releasing heat in BFB gasifier 2.

[0091] Optionally, and preferably for biomass with moisture content significantly exceeding 20%, the wet biomass may be dried before being fed to the BFB gasifier 2, for example and as shown in a belt dryer 1, preferably to roughly 18% or less moisture content. The belt dryer 1 may use air preheated with the heat recovered within the system, from flue gas and syngas cooling, or indirectly with hot water heated from the same source. For certain biomass, a dryer may not be required, in which case sized and pre-dried biomass may be fed directly into gasifier 2, which is operated under bubbling fluidization conditions, typically at a superficial gas velocity of 0.2-0.5 m/s and a temperature of 750-850? C. and pressure of about 23 psia.

[0092] Super-heated steam generated using the heat recovered from syngas and flue gas cooling enters the bottom of gasifier 2 at about 200? C. A moderately high H.sub.2 content and H.sub.2/CO ratio improve flame stability when the gas is burned in the kiln. This is achieved by operating the gasifier at relatively high steam/biomass mass ratios. It is found beneficial to operate the gasifier at a Steam/Biomass mass ratio of around unity. H.sub.2 increases and CO decreases with increasing steam/biomass ratio, leading to an increase in the H.sub.2/CO ratio. The maximum H.sub.2 yield could be reached at an increased steam/biomass mass ratio. It is noted that other Prior Art medium BTU gasifiers, which have been used for combined heat and power applications but not for rotary lime kilns, are operated at much lower steam/biomass ratios of 0.4 to 0.7.

[0093] The heat carrier, of the size of about 100 to 300 ?m, flows from the combustor cyclones into the BFB gasifier 2 bed at 900? C. to provide heat for the gasification. Raw product gas exits the BFB gasifier 2 and passes through the gasifier cyclone 4 to remove coarse entrained solids. The product gas is then cooled to about 180? C. in heat exchanger 6. The heat from hot syngas can be used for steam generation, which can be used to heat BFB gasifier 2. The cooled product gas enters a baghouse 7 and/or a filter (not shown) to remove the remaining fine ash.

[0094] The CFB riser combustor 3 is connected to BFB gasifier 2 by non-mechanical devices at the top and the bottom, as described in U.S. provisional application 63/298,990, incorporated herein by reference. In certain embodiments, the non-mechanical device connected to the top of combustor 3 is a standpipe with a loop-seal or an L-valve to the gasifier 2. In other embodiments, the non-mechanical device connected to the bottoms of gasifier 2 and combustor 3 is a U-bend. CFB combustor 3 operates in fast fluidization with a superficial gas velocity of ?5-m/s and a top temperature of about 930? C. To heat the sand particles transferred from the gasifier 2, it burns the char particles and auxiliary fuels. The auxiliary fuel comprises one or more of a stream of recycled tar with some contaminated scrubber liquid, a stream of recycled clean syngas, and a stream of biomass. The CFB combustor 3 may also include a second stage (not shown) to ensure complete combustion of the char. Air for CFB combustor 3 is pre-heated, typically utilizing heat recovered from flue gas cooling. The flue gas exiting the combustor cyclone 10 is cooled by passing through heat exchangers 11. The cooled flue gas flows through a baghouse 12 to remove fine particles and then to the stack.

[0095] The syngas from baghouse 7 enters scrubber 8a, contacting with cold organic scrubbing liquid to remove the tar and condense most steam in syngas. Alternatively, the tar may be removed by a reformer (FIG. 3) followed by a Venturi scrubber 8b using water instead of organic solvent. After the scrubber 8a or 8b, the syngas may be purified with some optional processes represented by element 9, which may include [0096] (1) NH.sub.3 removal to reduce NO.sub.x emissions, [0097] (2) chloride removal to mitigate facility corrosion, [0098] (3) H.sub.2S removal in the case of pellet lime kilns to remove sulfur and therefore avoid resultant sulphur contamination of the product CaO, [0099] (4) CO.sub.2 removal to increase the RFG heating value and adiabatic flame temperature, and potentially reduce the GHG emissions with CO.sub.2 sequestration or utilization processes.

[0100] Finally, the clean syngas is cooled to ambient temperature (?25? C.) or lower to reduce the syngas moisture content to about 3% and sent to lime kiln 13, and to combustor 3 if needed to heat the recirculating heat carrier in the combustor 3.

[0101] The lime kiln itself, and the equipment downstream thereof, can be a typical prior art rotary kiln arrangement as described in FIG. 1, or may be any other type of rotary lime kiln, or stationary vertical lime kiln.

[0102] The syngas is burned in the lime kiln 13 for calcination. The off-gas is drawn out of the kiln by the ID fan 14 to the dust scrubber 15 for cleanup and then to the stack. The adiabatic flame temperature with the medium-Btu syngas utilizing this method is substantially equal to that with natural gas under the same burner conditions (Table 1), which means the syngas can be delivered to the burner at ambient temperatures (25? C.) without sacrificing the adiabatic flame temperature. Furthermore, the matching of thermal based flue gas volumetric flow rates (m3/GJ) enables the same fan and dust scrubber to be used after the change in fuel from natural gas to the clean syngas of this invention.

[0103] Low-temperature syngas allows more gas cleanup procedures to remove one or more of the syngas particulates, tar, NH.sub.3, most moisture, H.sub.2S, chlorides and CO.sub.2. Drawback 1 is at least partially overcome.

[0104] A large amount of energy can be recovered during the syngas and char combustor flue gas cooling by a variety of means including thermal oil or steam, which can be used to produce steam for gasifier 2 and preheat air for combustor 3 or feed dryer 1. In the event that the moisture content of the biomass is below a certain level (e.g., 22%,) a dryer 1 may not be needed. In a separate embodiment, the thermal oil loop can be replaced by a more conventional water/air cooling system. Typically, the waste heat from syngas and combustor flue gas is sufficient for the feedstock dryer 1 and steam generator (not shown) feeding superheated steam to gasifier 2. Thus less or no fossil fuels or extra biomass are needed, which at least partially overcomes Drawback 4.

[0105] The retrofitting of existing natural gas fired lime kilns is dramatically simplified with the hereindescribed system and method. The thermal based volumetric flow rate of the kiln flue gas with the syngas of the invention is only slightly lower than that with natural gas, which means neither the ID fan nor the dust scrubber is required to be re-sized to maintain the original lime kiln capacity. Low-temperature syngas transport from the gasifier unit to the lime kiln means the pipeline material used for natural gas will also serve for the syngas, and replacement by high-grade materials is not needed. Meanwhile, the gasification plant doesn't have to be located near the kiln to minimize heat loss, which means more choices in the location of the gasifier. These features help avoid Drawbacks 2, 3, 5 and 6.

[0106] With the advantages of the invention mentioned above, natural gas can be entirely replaced with syngas of the invention without jeopardizing the kiln throughput or the quality of the lime, or significant retrofitting, which decreases the capital cost and increases the environmental benefit. Drawback 7 is overcome.

[0107] Example 1: Table 2 shows how renewable HEI syngas can meet the conditions to replace fossil natural gas in a lime-mud kiln in the pulp and paper industry. HEI syngas has an LHV value of ?11 MJ/m.sup.3, giving an adiabatic flame temperature essentially the same as natural gas (within 5? C.), and has a slightly lower flue gas volume/GJ than a kiln that uses natural gas.

[0108] Table 2 compares the representative properties and combustion performance of different fuel gases for lime kiln applications.

TABLE-US-00002 TABLE 2 Comparison of Representative Properties of Natural Gas, Medium- Btu Syngas of this Invention and Low-Btu Syngas of the Prior Art Fuel Gas Present Prior Art Invention Low-Btu Natural Medium-Btu Syngas Gas Syngas [11] LHV (MJ/Nm.sup.3) 37.0 10.8 5.3 Moisture (% mol) 0.002-0.004.sup.1 3.1 15 Adiabatic Flame 1,920.sup.3, 5 1,920.sup.3, 5 1,520.sup.3, 5 Temperature (? C.).sup.2 1,920.sup.3, 5 Fuel flow rate (Nm.sup.3/GJ) 27 93 189 Air/fuel Ratio (v/v) 10.8 2.6 1.3 Flue gas flow rate (Nm.sup.3/GJ) 289 277 368 .sup.1dry natural gas before moisturization and odorization .sup.210% excess air .sup.3The fuel gas and combustion air are delivered to the burner at 25? C. .sup.4The fuel gas and combustion air are delivered to the burner at 700? C. and 350? C., respectively. .sup.5The adiabatic flame temperature values are rounded off to the nearest 10 degrees.

[0109] Example 2: Table 3 shows the impact of gasification operating conditions on the syngas compositions and combustion properties. Case 1 is a typical operation of this invention. The feed biomass is gasified at 830? C., with olivine sands as the gasifier bed heat carrier material. Case 1a is Case 1 with 90% CO.sub.2 removal from the syngas, resulting in higher syngas LHV and adiabatic flame temperature, and lower thermal based flue gas volumetric flow rate. Running the gasifier at a lower temperature with the same bed heat carrier material in the gasifier (Cases 2-3 and 5-6) will increase the syngas LHV and adiabatic flame temperature as the methane content in the syngas is raised, and vice versa (Case 4). At the same gasifier temperature, replacing semi-catalyst olivine sands with non-catalytic silica sands as the heat carrier particles in the gasifier system (Cases 5-6) raises the syngas LHV and adiabatic flame temperature. However, olivine sands, while giving slightly lower LHV and adiabatic flame temperature than with silica sand, catalyze tar reforming reactions, reducing tar concentration and associated operating problems. Similarly, reforming catalyst as the gasifier bed will decrease the syngas LHV and adiabatic flame temperature (Case 12 vs. 11) as the CH.sub.4 content decreases. Likewise, downstream reforming or water-gas shift process will reduce the syngas LHV and adiabatic flame temperature (Cases 7-10).

[0110] Example 3: Cases 1-7 and 9-10 in Table 3 all give syngas adiabatic flame temperatures over 1900? C. and flue gas flow rates below 280 Nm.sup.3/GJ, indicating suitability for replacing fossil natural gas. From the published literature, the composition of syngases from three competing companies designated here as S, N and E were used to calculate their LHV and adiabatic flame temperature values (Table 4). It is noted that syngases from gasifiers S and N contain substantial percentages of nitrogen (being in part air-blown gasifiers), leading to flame temperatures of 1,675 and 1,439? C., respectively. These would not meet the criteria of the flame temperature (1,750? C.). Similarly, Gasifier E, which has a high concentration of CO.sub.2 but a low concentration of N.sub.2, would not match the adiabatic flame temperature of any of the HEI syngases in Table 3. The syngas with high inert N.sub.2 or CO.sub.2 also causes flue gas flow rates over 300 Nm.sup.3/GJ.

TABLE-US-00003 TABLE 3 Impact of Operating Conditions on the Syngas Compositions and Combustion Properties Operating Conditions (Gasifier Temperature, Bed Case Material, and Syngas Composition, mol % LHV AFT.sup.1 Flue gas.sup.1 No. Further Steps) H.sub.2 CO CH.sub.4 CO.sub.2 N.sub.2 H.sub.2O (MJ/m.sup.3) (? C.) (Nm.sup.3/GJ) 1 830? C., olivine 40.5 27.5 8.2 18.2 2.4 3.1 10.8 1,919 277 1a 830? C., olivine 90% CO2 removal 49.0 33.3 9.9 2.2 2.4 3.1 13.1 2,051 262 2 750? C., olivine 36.0 31.2 10.0 17.2 2.4 3.1 11.4 1,929 277 3 800? C., olivine 38.8 28.9 9.1 17.7 2.4 3.1 11.1 1,924 277 4 850? C., olivine 40.9 27.2 7.9 18.4 2.4 3.1 10.7 1,917 277 5 800? C., silica 29.4 39.9 11.2 13.9 2.4 3.1 12.3 1,965 274 6 850? C., silica 32.9 36.8 10.1 14.6 2.4 3.1 11.8 1,958 274 7 830? C., olivine with downstream 48.2 25.0 3.5 17.8 2.4 3.1 9.6 1,929 272 tar reforming 8 830? C., olivine with downstream 43.5 20.7 7.8 22.4 2.4 3.1 10.1 1,870 282 water-gas shifting 9 830? C., silica 34 35 13 18 0 0 12.8 1,956 274 10 830? C., silica with downstream 60.8 18.5 <0.2 20.6 0 0 8.9 1,942 265 tar & methane reforming 11 Gasification with sand as the bed 36.5 34.4 11.5 13.5 0 0 14.9 2,012 269 material.sup.2 12 Gasification with reforming catalyst 60.6 10.6 3.2 25.5 0 0 9.0 1,870 275 as the bed material.sup.2 .sup.1Adiabatic flame temperature, 10% excess air; fuel gas and air at 25? C. .sup.2bio-oil as gasifier feedstock; Syngas contains 4.2 mol % C.sub.2H.sub.4. .sup.3bio-oil as gasifier feedstock

TABLE-US-00004 TABLE 4 Fuel Gas Compositions and Combustion Properties from Published Literature Case Types of Fuel Gas Composition, mol % LHV AFT.sup.1 Flue gas.sup.1 No. Fuel Gas H.sub.2 CO CH.sub.4 CO.sub.2 N.sub.2 H.sub.2O (MJ/m.sup.3) (? C.) (Nm.sup.3/GJ) 13 S Gas 21.4 24.2 0.1 10.2 43.2 0 5.7 1,675 334 14 N Gas.sup.2 8.0 20.1 2.0 12.0 57.2 0 4.1 1,439 405 15 E Gas 25.3 22.2 7.1 41.4 4.0 0 8.1 1,686 310 16 Producer Gas.sup.3 15.9 25.1 2.0 6.3 50.3 0 5.6 1,667 342 17 Blast Gas 1.0 27.5 0 11.5 60.0 0 3.6 1,371 443 .sup.1Adiabatic flame temperature, 10% excess air; fuel gas and air at 25? C. .sup.2Containing 0.7 mol % O.sub.2 .sup.3Containing 0.4 mol % O.sub.2

REFERENCES

(All Incorporated Herein by Reference)

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