PROCESS FOR CONVERTING BIOGAS TO A

PIPELINE GRADE RENEWABLE NATURAL

GAS

Michael N. Funk

[0001] This application claims the benefit of U.S. Provisional Application No.

60/874, 120, entitled Process for Converting Biogas to a Pipeline Grade Renewable Natural Gas, filed December 1 1 , 2006.

Background of the Invention

[0002] The present invention relates in general to employing all of the necessary elements into a scrubbing process that will allow for the economic optimization of producing pipeline grade methane or a gas that meets D.O.T. specification from biogas.

[0003] Biogas can be derived from a number of different sources. The predominate sources of economically viable streams of biogas are generally produced through anaerobic digestion. Anaerobic digestion can occur naturally in landfills or in controlled environments that enhance the biological degradation of sewage waste, foodstuff waste and animal waste.

[0004] Biogas in general is a low Btu gas that is contaminated with hydrogen sulfide and carbon dioxide. The gas is also highly saturated with water.

[0005] Based upon the increased value of all energy products and the drive for renewable and distributed sources of energy the invention can be deployed nearly anywhere in the country where a economically viable biogas waste stream exists.

[0006] In general the waste streams that are used in the anaerobic digestion process that converts waste to energy are a nuisance in their unaltered

state. Anaerobic digestion in general converts waste streams to safer more useful products through the destruction of pathogens and the conversions of organic nutrients to inorganic nutrients in animal waste.

[0007] The general purpose for purifying the methane is to clean the product to the point where it meets pipeline grade natural gas specifications and/or D.O.T. specifications and can therefore be directly injected into a pipeline carrying natural gas or transported to a pipeline injection point.

[0008] It is known that carbon dioxide and hydrogen sulfide can be absorbed from methane by passing the stream containing the three gases countercurrent to water. The water absorbs the carbon dioxide and the hydrogen sulfide. The effectiveness of the process is based upon three key elements: gas pressure, water volume and contact time of the water and gas. Wide ranges of these variables have been employed to absorb carbon dioxide and hydrogen sulfide from methane.

[0009] The objective of the inventor is to optimize the economics of scrubbing gas when considering water to be a valuable resource. Many of the biogas waste streams that could potentially be purified are in areas that water is a valuable commodity. Thus, the driving factor of water preservation has caused the inventor to create a system for recycling scrubbing water that can be employed under most environmental conditions.

[0010] As described earlier the keys to successful scrubbing are contact time, water volume and gas pressure. When considering 100% recycling of water additional factors play a role in scrubbing efficiency and effectiveness. They are water temperature (as noted by Henry's Law) and carbon dioxide saturation levels in the recycled water stream.

[0011] In order to minimize water flows and gas pressures the scrubbing process emphasis controlling scrubbing water temperature and the

carbon dioxide saturation level of the scrubbing water. Through employing the simple measures of controlling water temperature and carbon dioxide saturation levels the scrubbing process can effectively clean biogas to pipeline grade methane gas with 100% recycled water. The affect of controlling these two critical factors of scrubbing efficiency and effectiveness also allow the system to conserve energy through pumping lower water volumes and pressurizing gas less than has generally been employed by other technologies that scrub gas in this general fashion.

Brief Summary of the Invention

[0012] The invention provides a method of purifying low pressure biogas streams to pipeline grade methane or D.O.T. spec. gas. The scrubbing methodology employed is unique due to the systems ability to recycle the scrubbing water and be deployed in areas where extreme weather conditions and restricted water availability do not alter the systems ability to produce pipeline grade methane or D.O.T. spec gas. Through controlling the scrubbing water temperature the systems is able to minimize biogas pressures and water flows generally employed by other scrubbing systems.

[0013] All purification and moisture parameters required by the pipeline specifications and/or D.O.T. specifications are programmed into the gas chromatograph which controls the valve that allows the gas to be injected into the pipeline or tanker truck. If gas does not meet spec, it is return to the front of the scrubbing process for further purification. In the final step the system compresses the gas to meet a preprogrammed pressure that will allow the gas to be injected into a pipeline or tanker.

In one embodiment of the present invention, an automated process of removing contaminants from a low pressure biogas stream and converting the gas to pipeline grade methane or D.O.T. spec gas comprises the following steps:

i. Remove substantially all of the hydrogen sulfate from the biogas stream under low pressure by allowing the hydrogen sulfate to be oxidized in an iron sponge reaction vessel.

ii. Elevate the pressure of the gas to the point were it can be compressed to an adequate pressure where it can be effectively scrubbed of other impurities.

iii. Allow pressurized gas to enter into a series of scrubbing towers, scrubbing tower size is predicated by the actual quantity of gas being scrubbed, containing packing that allows for the uniform dispersion of the gas as it migrates vertically from the bottom of the scrubbing vessel to the top of the scrubbing vessel.

iv. Force water through an opening at the top of the scrubbing towers in adequate quantity to allow for uniform dispersion through the packing, thus contacting the biogas as it is forced from the bottom of the vessels to the top.

v. Through controlling the quantity of water in gallons per minute and the pressure of the biogas the gas is allowed adequate contact time with the water to cause the carbon dioxide to become absorbed in the water. This scrubbing process can be accomplished at different volumes of water and different pressures. They key to economic optimization is derived at the lowest possible pressures and the lowest possible volume of water needed to remove the impurities from the gas. The only

ways to accomplish these economies when recycling water are to de-carbonate the water and control the water temperature. Henry's Law dictates that the solubility of gases is decreasing with increasing water temperatures. Thus, without controlling the water temperature economic optimization cannot be achieved.

vi. As the water is recycled it first goes into an in ground de- carbonization tank. The water is forced through a spray bar that is located above the packing. The spray bar helps achieve de-carbonization by reducing water droplet size and evenly distributing the water over the packing while air is being forced up through the packing to help release the absorbed carbon dioxide.

vii. The scrubbing water then enters a holding tank where the temperature is monitored. As the water leaves the holding tank the temperature is controlled with a chiller and plate heat exchanger. Through control of the water temperature we are able to minimize the volume of water and the pressure level of the gas entering the scrubbing tanks. Thus, optimizing the economics of cleaning gas under the principles of Henry's Law.

viii. As the cleaned methane leaves the top of the last scrubbing tower it is dried and then its quality is monitored by an in line gas chromatograph. The gas chromatograph controls a two way valve that diverts gas that meets pipeline specifications to the final compression stage and diverts out of spec, gas back to the beginning of the scrubbing process.

ix. Gas that meets pipeline specifications is then dried and compressed to the pressure necessary for introduction into a tanker or a natural gas pipeline.

Brief Description of the Drawing

[0015] Figure 1 is an process flow diagram of an embodiment of the overall process of the present invention.

[0016] Figure 2 is an enlarged process flow diagram of a hydrogen sulfide removal portion of the process shown in Figure 1.

[0017] Figure 3 is an enlarged process flow diagram of a biogas compression recirculation header portion of the process shown in Figure 1 .

[0018] Figure 4 is an enlarged process flow diagram of a scrubbing towers portion of the process shown in Figure 1.

[0019] Figure 5 is an enlarged process flow diagram of a scrubbing water de- carbonation system and flash tank portion of the process shown in Figure 1.

[0020] Figure 6 is an enlarged process flow diagram of a scrubbing water storage and cooling portion of the process shown in Figure 1.

[0021] Figure 7 is an enlarged process flow diagram of a drying and final compression portion of the process shown in Figure 1.

Detailed Description of an Embodiment of the Invention

[0022] An embodiment of the present invention, depicted in Fig. 1 , will be described below in further detail by reviewing each of the individual portions of that embodiment, depicted in Figs. 2-7.

[0023] Diagram A: Dual Iron Sponge reaction vessels

[0024] As shown in Fig. 2, the dual iron sponge vessels (1) are used to remove the hydrogen sulfide prior to the gas entering the absorption scrubbing process. Duplicity is not necessary but allows for the recharging of one unit while the other continues to remove hydrogen sulfide. Only one unit is active at all times. The low pressure gas stream from the anaerobic digestion source is pulled through the Iron Sponge Vessel by fan (10). As the gas migrates down through the Iron Sponge vessel the hydrogen sulfide comes into contact with iron oxide impregnated wood chips that make up the Iron Sponge packing (5). As the hydrogen sulfide comes into contact with the iron oxide the reaction produces iron sulfide and water which remains in the Iron Sponge vessel (1 ). The gas is now free of Hydrogen sulfide and is pulled to Diagram B: by fan (10).

[0025] Diagram B: Biogas Compression and Recirculation Header

[0026] As shown in Fig. 3, fan (10) which is driven by a motor that is controlled by a variable frequency drive (VFD) is used to elevate the pressure of the gas from the source to allow for the proper feeding of compressor (20). The VFD motor on fan (10) allows for optimization based upon the availability of gas from the source. Recirculation header (15) allows for the reintroduction of gas that did not meet spec, at the gas chromatograph (1 15) and the reintroduction of gas that was recovered in the flash tank (50). The motor on compressor (20) is also controlled by a VFD which allows for optimization based up available gas and desired pressure of the gas being delivered to Diagram: C scrubbing tower (25).

[0027] Diagram C: Scrubbing Towers - Water & Gas Flow / Control

[0028] As depicted in Fig. 4, the scrubbing process takes place in dual scrubbing towers (25) & (30). The scrubbing towers (25) & (30) are identical with the exception of specific control functions. Each tower

has a demister pad (40) and a set amount of packing (45) that causes even distribution of gas and water as they flow in a countercurrent fashion through the towers (25) & (30). The gas first enters at the bottom of tower (25) and flows out the top of tower (25) to enter in the bottom of tower (30) and then out the top of tower (30). The water is first introduced into tower (30) and exits the bottom of tower (30). Centrifugal pump (35), which is controlled by a VFD motor for optimization, assures that water is delivered to the top of tower (25) at a predetermined flow rate in gallons per minute (GPM). The water then exits at the bottom of tower (25). The countercurrent flow of clean water and raw gas is optimized through the dual tower set up. The raw gas enters tower (25) and is scrubbed with water from tower (30). As the partially cleaned gas leaves tower (25) and enters tower (30) it is scrubbed by water that has been de-carbonated and chilled for optimum scrubbing. Tower (30) is the finishing tower in the purification process. As the gas exits the top of tower (30) it is sent to Diagram F: for final drying and compression. As the water exits the bottom of tower (25) it flows to Diagram D: and enters flash tank (50).

[0029] This scrubbing process can be accomplished at different volumes of water and different pressures. It is preferable for economic optimization to use the lowest possible pressures and the lowest possible volume of water needed to remove the impurities from the gas. These economies can be achieved when recycling water by decarbonating the water and/or by controlling the water temperature. Henry's Law dictates that the solubility of gases decreases with increasing water temperature. Therefore, it is preferable to control the water temperature for economic operation of the system.

[0030] Diagram D: Scrubbing Water De-carbonation System & Flash

Tank

[0031] As shown in Fig. 5, the water first enters flash tank (50) where pressure is reduced rapidly to allow the residual methane to flash separate and return to the recirculation header (15) in Diagram B:. The carbon dioxide rich water is then directed to the in-ground de- carbonization tank (55) where the water is evenly sprayed over the packing (65) by a spray header (60). The spray header (60) minimizes the droplet size to maximize the de-carbonation process. The spray bar helps achieve de-carbonization by reducing water droplet size and evenly distributing the water over the packing while air is being forced up through the packing to help release the absorbed carbon dioxide. As the small droplets migrate through the packing air is forced in a countercurrent fashion from the bottom of the packing by blower (70), which is controlled by a VFD motor for optimization. The large quantity of air that is forced through the packing ensures the removal of the carbon dioxide from the recycle scrubbing water. The carbon dioxide is then vented to atmosphere through vent (75). When economical the carbon dioxide will be captured and process for commercial use. As the water exits the de-carbonization tank it flows to Diagram E: and enters the in-ground water storage tank (80).

[0032] Diagram E: Scrubbing Water Storage & Cooling

[0033] As shown in Fig. 6, the de-carbonated water flows into the in-ground storage tank (80) where it is picked up by submersible pump (85). Pump (85) circulates water to plate heat exchanger (90) where the water is cooled to a temperature that allows for the most efficient scrubbing (absorption) in towers (25) & (30). Through control of the water temperature the volume of water and the pressure level of the gas entering the scrubbing tanks is minimized, thus, optimizing the economics of cleaning gas under the principles of Henry's Law.

[0034] On the control side of plate heat exchanger (90) centrifugal pump

(100), which is controlled by a VFD motor for optimization, circulates

water through chiller (95). Chiller (95) is controlled by the temperature sensor on the water line as the water exits flash tank (50). The temperature controlled water leaving the plate heat exchanger (90) is picked up by centrifugal pump (105), which is controlled by a VFD motor for optimization, and elevated to the desired flow rate for entering tower (30).

[0035] Diagram F: Drying and Final Compression

[0036] As shown in Fig. 7, as the gas leaves tower (30) it flows to dryer (1 10).

Dryer (1 10) dries the gas to a pre-specified moisture content determined by the pipeline or D.O.T. specification. As the gas leaves dryer (1 10) it flows to the gas chromatograph (1 15). Gas chromatograph ( 1 15) analyzes the gas quality to determine if the gas meets the preprogrammed pipeline or D.O.T. specifications. If the gas meets the preprogrammed specifications valve (120) directs the gas to compressor (125), which is controlled by a VFD motor for optimization, for pressurizing the gas to a preprogrammed pressure which allows the gas to either flow into a pipeline or tanker for delivery to the end user. If the gas does not meet the preprogrammed pipeline or D.O.T. specifications valve ( 120) opens the recycle line and allows the gas to return the recirculation header ( 15) for further scrubbing.