Technical Field

The present invention relates to a process to produce heat, steam, or power from the combustion of sour gas using a calcium-based looping combustion process. More specifically, the present invention relates to a calcium-based looping

combustion of sour gas process involving the in-situ removal of sulfur from the sour gas.

Background

As the need for energy continues to grow worldwide, the need for additional energy sources also grows. Non-renewable energy sources remain the major supply of energy throughout the world as renewable energy sources remain too low in quantity to meet much of the demand, and their development has been slow and limited to specific localities. While established non-renewable energy sources (e.g., recoverable coal, conventional oil and natural gas) remain the major energy sources, energy producers must now look to other non-renewable energy sources such as unconventional oil resources (e.g., oil shale, tar-sands, and heavy crude) and unconventional natural gas resources (e.g., gas in pressurized aquifers and coal seams) to meet the increasing demand.

Among these unconventional non-renewable energy sources is sour gas fuel. Sour gas is a source of energy that must be utilized because it has a much longer lifetime as compared with other non-renewable sources. Sour gas is a colorless, flammable, and corrosive natural gas that contains significant levels of hydrogen sulfide (H ₂ S).

Combustion is a commonly used reaction in the field of power generation and one that, in certain instances, can utilize sour gas as fuel. Sour gas, however can be damaging to the mechanical parts of the combustion system at high temperatures and pressures under gas turbine system conditions, and is low in calorific value. Further, when H ₂ S is exposed to air, it easily oxidizes into a sulfur oxide (SOx) such as SO ₂ — an air pollutant. In some instances, direct combustion of sour gas in a gas turbine is not even possible due to the composition and the physical characteristics of the gas, particularly if the sour gas has an H ₂ S content above 5-20%. Thus, sour gas has not previously been considered a particularly useful fossil fuel for combustion processes.

The increasing worldwide demand for energy, however, has forced the use of sour gas as an energy source. There are many global undeveloped and

underdeveloped sour gas reservoirs. If these resources are to be used for the generation of power, the ability of a combustion process to handle quantities of H ₂ S is necessary in order to make such application possible. Not surprisingly, much effort is underway worldwide to reduce the economic impact of H ₂ S and sulfur management for sour gas fuels.

Conventionally, in order to avoid the corrosive effects and the pollution associated with combustion of sour gas, pretreatment of the sour gas was required to substantially remove the sulfur compounds from the gas stream— a process known as "sweetening." For example, an amine gas treating process can be used to "sweeten" the sour gas (i.e., remove the H ₂ S). The major shortcoming of processes for sweetening sour gas is that they are very complex and costly.

Thus, there is a need for a process that utilizes unconventional energy resources to produce energy in an efficient and cost-effective manner. In particular, there is a need for sour gas combustion processes with high efficiency in energy conversion, but without the costly pretreatment of H ₂ S.

Summary

The present invention is directed to process for the calcium-based chemical looping combustion (CLC) of sour gas to produce product gas for use in power generation. More specifically, the present invention relates to a CLC process of sour gas that features at least in part in-situ H ₂ S removal, and oxygen transfer media production.

In one embodiment, a bed of a calcium-based transfer media (e.g., CaO) is disposed within a lower portion of a fuel reactor and a stream of C0 ₂ and steam is then used to fluidize the bed. Once fluidized, the bed of CaO reacts with the H ₂ S of a sour gas feed injected into the fuel reactor to produce CaS (in-situ H ₂ S removal), which is then transported to an air reactor to be oxidized.

In the air reactor, air is injected and oxidizes the CaS to produce a calcium- based oxygen transfer media (e.g., CaS0 ). Further, limestone is injected into air reactor where, due to the operating conditions of the air reactor, it calcines producing CaO and C0 ₂ . The CaO produced in the air reactor is the circulated back to the bed in the lower portion of the fuel reactor, and the CaS0 ₄ produced in the air reactor can be circulated to the fuel reactor and/or the riser.

In the riser, the CH of the sour gas reacts with the injected CaS0 ₄ in a stream of C02 and steam to produce a product gas stream (e.g., syngas) and CaS. The product gas stream is then separated from the CaS using a gas-solid separator. The CaS particles from the reaction in the riser are then routed to a reactive hopper where a portion of them are oxidized into CaS0 ₄ . The resulting CaS0 ₄ and remaining CaS is then transported from the reactive hopper to the air reactor for further circulation throughout the system. The product gas stream can be utilized in other related applications such as power generation systems or gas turbine systems combined cycles.

This invention provides a distinct advantage over other CLC processes involving sour gas in that it allows for the full conversion of the sour gas, whereas most CLC sour gas processes will have a small portion of not fully converted gas products in the product stream. Additionally, the present invention reduces the reaction time in the fuel reactor, which allows for reductions in the size of the fuel reactor and the overall cost of the process compared with previous CLC processes.

Brief Description of the Drawing Figures

A more complete understanding of the invention and its many features and advantages will be attained by reference to the following detailed description and the accompanying drawing. It is important to note that the drawing illustrates only one embodiment of the present invention and therefore should not be considered to limit its scope.

Fig. 1 is a schematic of a calcium looping combustion process in accordance with at least one embodiment of the present application.

Detailed Description of Certain Embodiments

The present application relates to a calcium-based chemical looping combustion (CLC) of sour gas process involving the in-situ removal of sulfur from the sour gas. In a CLC process, a solid metal oxide oxygen carrier is typically used to oxidize the fuel stream in a fuel reactor. This results in the production of C0 ₂ and H ₂ 0. The reduced form of the oxygen carrier is then transferred to the air reactor, where it is contacted with air, re-oxidized to its initial state, and then returned back to the fuel reactor for further combustion reactions.

In general, the overall heat of a CLC process will be the sum of the two heat states, exothermic during oxidation and endothermic during reduction, which is equivalent to the heat released in a convention combustion reaction. Accordingly, one advantage of a CLC process is that minimal extra energy is required to capture C0 ₂ while still maintaining a combustion efficiency similar to direct combustion processes. More precisely, there is minimum energy penalty for C0 ₂ capturing in a CLC process, estimated at only 2-3% efficiency lost. Additionally, NO _x formation is reduced in the CLC process compared with direct combustion processes as the oxidation reaction occurs in the air reactor in the absence of fuel and at a

temperature of less than 1200°C— above which NO _x formation increases considerably. Thus, the lack of NO _x formation makes C0 ₂ capturing in CLC processes less costly compared with other combustion methods because C0 ₂ does not need to be separated from the NOx gas prior to capture. Further, in contrast with other combustion processes, C0 ₂ products in CLC processes can be separated from other gases and captured without the use of additional step or equipment.

The CLC process of the present application is designed to overcome the deficiencies associated with direct combustion processes and conventional CLC processes as they relate to the combustion of sour gas. In particular, the calcium- based CLC process of the present application allows for the in-situ removal of H ₂ S and the full conversion of sour gas during the combustion reaction. Further, the process of the present application is designed to produce heat, steam, and/or power from the combustion of a sour gas feed using a calcium-based looping process. Other advantages associated with the present application will be appreciated in view of the following description.

Fig. 1 illustrates an exemplary system 100 for performing the calcium-based chemical looping combustion of sour gas in accordance with the present application. Fig. 1 likewise shows an exemplary flow scheme that depicts a calcium-based CLC process in accordance with the present application. In one or more embodiments, as exemplified in the system 100 of Fig. 1 , the CLC system can have at least 3 different reaction zones: a first reaction zone defined by a fuel reactor 102, a second reaction zone defined by an air reactor 104, and a third reaction zone defined by a riser 106, which can be operatively connected to the fuel reactor 1 02 and the air reactor 104. The fuel reactor 102 can take any number of suitable forms and is designed to allow for the combustion of a fuel or the like therein. The fuel reactor 102 is thus defined by a structure (housing) that defines a hollow interior in which fuel combustion occurs. A bed material (fluidized bed) which provides the oxygen for the combustion in the fuel reactor 102 can be disposed within a portion 108 (e.g., a lower portion as illustrated) of the fuel reactor 1 02. It will be appreciated that oxygen carrier can take any number of different forms including those described herein.

Any number of different techniques can be used to contain and hold the bed material in place within the fuel reactor 102 including but not limited to placing the bed material on a substrate or support structure that is disposed within the lower portion 1 08. For example, a perforated horizontal plate can be provided in the lower portion 1 08 and the bed material rests along such perforated plate. The perforations in the horizontal plate allow for fluid flow through the plate and thus, allow for a fluid, such as a gas, to flow not only through the plate but also through the bed material resting thereon. This allows the bed material to act as a fluidized bed. The bed material serves as the oxygen carrier that provides oxygen for the combustion in the fuel reactor 102.

In one or more embodiments, the bed material in the fuel reactor 102 can be comprised of a calcium-based transfer media. In exemplary embodiments, the calcium-based transfer media in the fuel reactor 102 is calcium oxide (CaO), also referred to herein as "lime." In other variations, the calcium-based transfer media bed in the fuel reactor 102 can also be comprised of limestone (CaC0 ₃ ). The calcium-based transfer media is a reactive media that can transfer one or more elements, such as oxygen, under reaction conditions. The calcium-based transfer media bed can be fluidized by a suitable fluid, such as a stream of C0 ₂ or by another suitable fluid stream (gas stream). In one or more variations, a stream comprising both C0 ₂ and steam is used to fluidize the calcium-based transfer media bed. In at least one variation, the suitable fluid can comprise depleted air from the air reactor. In another variation, a part of the depleted air at an outlet of a steam turbine is connected to the air reactor to fluidize the lime bed in the fuel reactor. The depleted air can contain 2 to 5% of oxygen and can enhance the reactions in the fuel reactor 102 and the riser 1 06. Referring to Fig. 1 , the stream of C0 ₂ and steam is transported via transport line 1 10 and is used to fluidize the bed of lime within the lower portion108 of the fuel reactor 102 by flowing freely through the bed material.

It will be understood that the diameter of the fuel reactor 102 is preferably designed based on gas superficial velocities. The gas velocity needed for good fluidization gas distribution and good transfer media mixing in the bottom portion 108 of the fuel reactor 102 is between about 0.1 5 and 0.8 m/s, and preferably between about 0.25 and 0.5 m/s, at least according to one exemplary embodiment. The fluidization gas velocity will not exceed the terminal velocity of the calcium- based transfer media particles. Exemplary operating temperatures in the bottom portion 1 08 of the fuel reactor 102 are varied between 700 and 1200 °C.

In one or more variations, the fuel (e.g. sour gas) can be injected into an inlet 1 12 formed in the lower portion of the fuel reactor 102 that contains the calcium- based transfer media bed (e.g., lime). In one or more variations, the fuel is a sour gas feed. In one or more variations, the sour gas can be injected into a location in the fuel reactor 102 that is downstream of the calcium-based transfer media bed. In an exemplary embodiment, the H ₂ S of the sour gas reacts with the CaO (lime bed) in the lower portion of the fuel reactor 102 (e.g., in-situ H ₂ S removal) to produce a reduced calcium-based transfer media, (e.g., calcium sulfide [CaS]). The resulting CaS can be transported to the air reactor 104 via transport line 1 14 to be re-oxidized for recirculation in the system 100. In one or more variations, transport line 1 14 can be connected to a loop seal 1 16 to ensure gas tightness between the fuel reactor 102 and the air reactor 104. In variations in which depleted air is used to fluidize the bed of calcium-based transfer media, the depleted air can enhance the reactions of the calcium-based transfer media with the H ₂ S, thereby increasing H ₂ S removal.

The residence time of the sour gas feed in the fuel reactor 102 where it reacts with the lime can be between about 1 and 800 seconds, and preferably between about 100 and 500 seconds. It will be appreciated that these values are merely exemplary in nature. The reaction between H ₂ S and CaO is an exothermic reaction, which increases the total efficiency of the chemical looping unit. The sour gas stream can have an H ₂ S concentration of between about 0.1 and 75%, preferably between 0.1 and 50%. In one or more embodiments, the ratio of Ca to S can be between 1 and 3, preferably between about 1 .2 and 2.5.

In one or more variations, at least two reactions take place in the air reactor 104: calcination and oxidation. The CaO (lime) that forms a bed in the lower portion 108 of the fuel reactor 102 is produced in the air reactor 104 via calcination. To produce the lime in the air reactor 104, CaC0 ₃ (limestone) can be injected into the air reactor 104 via transport line 1 18. The particle size fraction of limestone particles injected into the air reactor 104 can be between 70 and 300 microns, preferentially between 100 and 250 microns. The operation temperature of the air reactor 104 can be between 800 and 1300°C, and preferably between 850 and 1300°C. In exemplary embodiments, the limestone in the air reactor 1 04 undergoes a calcination reaction because of the operating conditions of the air reactor 104 (e.g., temperature above 850 °C). As such, the limestone (CaC0 ₃ ) calcines (decomposes) into CaO (lime) and C0 ₂ .

Further, an oxidation reaction occurs in the air reactor 104, as the reduced calcium-based transfer media (CaS) transported from the fuel reactor 102 via transport line 1 14 reacts with the oxygen (0 ₂ ) present in the air reactor 1 04 to produce a calcium-based oxygen transfer media (CaS0 ₄ ) (which under reaction conditions is reduced to produce oxygen). The oxygen needed for the oxidation reaction in the air reactor 1 04 can be injected into the air reactor via transport line 120. The CaO and CaSO ₄ products produced in the air reactor 1 04 are then transported back to the fuel reactor and/or the riser. This portion of the CaO and CaSO ₄ products exit the air reactor 1 04 via transport line 122 and enter a gas-solid separator 1 24 (e.g., a cyclone). The CaO and CaSO ₄ solid particles exit the gas- solid separator 124 via transport line 126, while the excess gas (depleted air) exit the gas solid separator 124 via transport line 128.

A portion of the CaO and CaSO ₄ products in transport line 128 are then routed to the riser 106 and the fuel reactor 1 02 via transport line 130. The other portion of the CaO and CaSO ₄ products in transport line 128 are recirculated to the bottom portion 108 of the fuel reactor 102 via transport line 132. In an exemplary embodiment, transport line 132 can be connected to a loop seal 1 34 in order to ensure the gas tightness between the air reactor 104 and the fuel reactor 1 02. The ability of CaSO ₄ to be recirculated to the fuel reactor 102 and riser 1 06 as oxygen carriers allow for the full conversion of the sour gas to the product gas.

In one or more variations, SO ₂ byproducts can be present within the air reactor 104 as a result of the sour gas reactions in the fuel reactor 102 and/or the riser 106. The S0 ₂ byproducts are eliminated from the air reactor 104 via reaction with a suitable, calcium-based sorbent material that is introduced into the air reactor 104. In one or more variations, the sorbent material is limestone (injected via transport line 1 1 8). The reaction between S0 ₂ and the calcium sorbent in the air reactor 104 results in the formation of CaS0 ₄ , which can then be transported back to the fuel reactor 102 and/or the riser 104. In these variations, the injection of the calcium-based sorbent into the air reactor 1 04 prevents the potential pollution effects of S0 ₂ .

As explained above, a portion of the CaS0 ₄ (calcium-based oxygen transfer media) produced in the air reactor 104 is transported to the riser 106. In the riser 106, the CaS0 ₄ will serve as a non-metal oxygen transfer media or oxygen carrier. The sour gas injected via transport line 1 12 into the fuel reactor 102 flows from the fuel reactor 102 to the riser 106 in a stream of C02, steam, and or depleted air. In the riser 106, a combustion reaction occurs as the CaS0 ₄ reacts with the CH of the sour gas in the stream of C0 ₂ , steam, and/or depleted air. This combustion reaction produces CaS and the product gas stream, which can comprise CO, and H ₂ . In one or more variations, the product gas stream can also be comprised of CO ₂ , H ₂ O, N ₂ , and trace amounts of H ₂ S. In one or more variations, the product gas stream is syngas. In one or more variations, the product gas stream can be used for other applications, such as a power generation via a gas turbine combined cycle. In variations in which depleted air is used to fluidize the bed of oxygen carriers in the fuel reactor 102, the depleted air can enhance the reactions of the CaSO ₄ with CH , thereby increasing the production of product gas stream (e.g., syngas). The reaction between the CaSO ₄ and the CH of the sour gas in the riser 106 is slightly exothermic, and the temperature in the riser 106 will increase as a result. The operating temperature in the riser is varied between 700 and 1200°C.

In one or more variations, the fuel reactor and the riser can be integrated into a common structure and thus, while the drawings and description thereof discusses these two components separately, it is within the scope of the present invention that the two components can be integrated into one in some embodiments. As is known, the riser forms a riser channel in its interior and typically includes inlets (e.g., feed tubes) that allow material to be added to the riser as fuel (reactive components) that reacts in the riser with other components present therein (which can be formed as a result of combustion in the fuel reactor).

The CaS and the product stream exit the riser 106 via transport line 136. The CaS and the product gas stream are then routed to a second gas-solid separator 138 (e.g. a cyclone), where the CaS particles can be separated from the product gas stream. The product gas exits the second gas-solid separator 138 via transport line 140, which routes the product gas out of the system. After separation from the product gas stream, the CaS particles exit the separator 1 38 and are transported via transport line 142 to a reactive hopper 144 operated in a dense fluidized bed.

In one or more variations, air can be injected into the reactive hopper 144 such that at least a portion of the CaS is re-oxidized to CaS0 ₄ . In these variations, the injection of air in the reactive hopper 144 also helps to maintain the fluidization of CaS0 ₄ back to the riser. Further, in order to maintain pressure balance of the system 1 00, a portion of the CaS04 product in the reactive hopper 144 can be removed from the system 100 via a collection line 146 to a storing reactor 148. The remaining CaS particles in the reactive hopper 144 are then routed via transport line 150 (connected to loop seal 152) back to the air reactor 104 to be re- oxidized to CaS0 ₄ , thereby completing the calcium loop. In variations in which CaS0 ₄ is produced in the reactive hopper 144, the CaS0 ₄ particles can be transported along with the CaS particles to the air reactor 104 via transport line 150.

In certain variations, the system 100 can generate heat and steam as byproducts. The generation of heat and steam can occur from the outlet stream of the air reactor. The hot depleted air at 1 200 °C exiting the air reactor can be used to produce heat and steam and power using a steam turbine.

In one or more variations, the system 100 can also produce gypsum (CaS0 ) as a byproduct. At the bottom of the air reactor the gypsum can exit the system. The control of the mass, heat, and pressure balance of the system control the quantity of gypsum extracted from the loop. Thus, in at least one variation, the system can be coupled with a process to manufacture cement, as gypsum is a raw material for CSAB cement. Coupling the present system with a cement manufacturing process can be a benefit as it can reduce the overall emission of C0 ₂ typically associated with both processes, and therefore would result in cost savings.

EXAMPLE

The following example is provided to better illustrate an embodiment of the present invention, but it should not be construed as limiting the scope of the present invention. In this example, the calcium-based transfer media that is disposed in the lower portion 108 of the fuel reactor 1 02 is limestone, and it has a density of 2750 kg/m ³ . The sour gas feed injected into the fuel reactor has the following composition (wt %): CH ₄ (58.8%), H ₂ S (20.9%), N ₂ (13%), and C0 ₂ (7.3%). The amount of lime necessary for a fluidized bed reactor is calculated along with the flow rate of lime to the reactor. The lime reduces the H ₂ S content of the sour gas to the equilibrium value of 397 ppm, and with the inlet volume fraction of yH ₂ S = 0.209, this would correspond to a mean sulfur retention of 99.81 %.

The calculated time for complete conversion of the gas is about 360 seconds at 950 °C and atmospheric pressure. The residence time needed for a particle conversion of 0.7 is 420 seconds along with the lime (CaO) flow and inventory. The total flow of sour gas used to produce 450 MW _th is about 60,646 Kg/h. The limestone flow injected in the air reactor is approximately 21 .45 kg/s in order to handle the amount of H ₂ S in the sour gas. The corresponding lime (CaO) flow is approximately 1 2 kg/s. The temperature at the level of the lime bed (e.g., within the fuel reactor) is between about 700 and 1200°C, preferably between about 850 and 1040°C. The outlet temperature in the riser is increased to approximately 1050°C.

The ratio of Ca to S is between about 1 and 3, preferably between about 1 .2 and 2.5. The CaSO ₄ recirculated from the air reactor is approximately 1 .83 tonnes/second. In the riser, the recirculated CaSO reacts with CH from the sour gas to produce the syngas with the following specification (wt %): CO (27.0%), H ₂ (55.1 %), CO ₂ (3.4%), H ₂ O (8.5%), N ₂ (6.1 %), and H ₂ S (397.0 ppm).

As explained above, the present invention is directed to a calcium looping combustion of sour gas that includes in-situ H ₂ S removal, oxygen transfer media production, and syngas production. The method according to the invention allows full conversion of the sour gas in contrast with previous CLC processes. The method according to the invention reduces the reaction time in the fuel reactor which allows for the reductions in the size of the fuel reactor and the overall cost of the chemical looping unit. Additionally, the method according to invention allows for an exothermic reaction in the fuel reactor between the sour gas and the bed of lime, which increases the total efficiency of the calcium looping unit.

While the present invention has been described above using specific embodiments and examples, there are many variations and modifications that will be apparent to those having ordinary skill in the art. As such, the described variations and embodiments are to be considered in all respects as illustrative, and not restrictive. Therefore, the scope of the invention is indicated by the appended claims, rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.