Biogas, which is a mixture of several gases, may be produced by the breakdown of organic matter in low oxygen conditions. In particular, it may be produced by the anaerobic digestion or fermentation of organic matter (e.g., manure, sewage sludge, municipal solid waste, biodegradable waste, biodegradable feedstock, etc.).
The composition of biogas, which varies with the type of organic matter (e.g., biomass) from which it is derived, is predominately methane (CH4) and carbon dioxide (CO2), with relatively small amounts of hydrogen sulfide (H2S), ammonia (NH3), hydrogen (H2), nitrogen (N2), carbon monoxide (CO), oxygen (O2), water vapour, and/or siloxanes. For example, the composition of biogas may include about 60% CH4 (e.g., between about 45% and about 70%) and about 35% CO2 (e.g., between about 25% and about 55%).
Biogas is considered a renewable fuel. In its raw form, biogas may be combusted (e.g., in an engine, turbine, or boiler) in order to generate heat (e.g., steam) and/or electricity. For example, biogas has been used in combined heat and power (CHP) plants in order to generate electricity and heat. Optionally, raw biogas is cleaned to remove H2S and/or water vapour prior to combustion in order to reduce equipment corrosion.
In general, the methane content of raw biogas and/or cleaned biogas is low (e.g., about 40% to about 60%) relative to that of fossil natural gas (e.g., greater than about 96%). While biogas CHP plants are often designed and/or configured to combust this lower quality gas, the quality of biogas (e.g., heating value) is generally too low for various other applications (e.g., direct injection into a distribution system carrying natural gas and/or for use as a transportation fuel). In addition to reducing the heating value of biogas, the large amount of CO2 in biogas may increase compression costs and/or increase transportation costs. For example, with regard to the latter, since biogas does not typically meet natural gas pipeline standards a separate dedicated biogas pipeline or pipeline system may be required to transport biogas to end users (e.g., where a natural gas pipeline cannot be used).
Accordingly, biogas use traditionally has been limited to near the point of production. In fact, one important biogas application is the cogeneration of heat and electricity for use in the plant and/or process that produces the biogas (e.g., a waste treatment plant and/or cellulosic ethanol plant that require heating the digester, space heating, water heating, and/or process heating). For example, cogeneration may reduce the amount of electricity and/or heating fuel imported into the plant/process, thereby reducing costs. In addition, it may enhance power reliability for the process and/or provide the opportunity to reduce greenhouse gas (GHG) emissions (e.g., since a renewable fuel may replace an imported fossil fuel).
In order to use biogas as a transportation fuel, or for unlimited injection into a distribution system, where it may be used to produce electricity, used for residential heating, or used as a transportation fuel, biogas may be upgraded (e.g., to renewable natural gas (RNG)). The terms “renewable natural gas” and “RNG”, as used herein, refers to biogas that has been upgraded to a quality similar to fossil natural gas. For example, RNG may refer to biogas that has been upgraded to natural gas pipeline quality (e.g. >95% CH4) and thus is substantially interchangeable with natural gas. Pipeline standards or specifications may vary by region. For example, Canadian pipelines standards may require a H2S content that does not exceed 6 mg/m3, a CO2 level that is less than about 2%, and/or a CH4 level that is greater than 95%. In addition, or alternatively, the natural gas pipeline standards may refer to the purity of the gas expressed as a heating value (e.g., in British Thermal Units (BTU)/cubic foot). In general, the higher the heating value, the cleaner the gas (e.g., fewer and/or lower amounts of impurities). Pipeline standards may require, for example, that the heating value of RNG be greater than about 950 BTU/cubic foot, greater than about 960 BTU/cubic foot, or greater than about 967 BTU/cubic foot. In one embodiment, the pipeline standards are dictated by the Federal Energy Regulatory Commission (FERC). In one embodiment, the pipeline standards are defined by the standards developed and adopted by the Wholesale Gas Quadrant (WGQ) of the North American Energy Standards Board (NAESB). Optionally, propane or another fuel is added to the upgraded biogas to improve the heating value and/or meet pipeline standards. Optionally, the upgraded biogas is mixed with fossil natural gas prior to injection in order to provide the required heating value and/or meet pipeline standards.
Upgraded biogas (e.g., RNG) may be injected and distributed through a distribution system (DS) after it has been compressed to the local pipeline pressure. The terms “distribution system” and “DS”, as used herein with reference to natural gas, refers to a pipeline and/or pipeline system that carries natural gas and serves an end user (e.g., consumer). The term “pipeline system”, as used herein, refers to an interconnected network of pipes (e.g., physically connected). For example, a distribution system for natural gas may include large transmission pipelines and smaller distribution pipelines, as well as the connections to them, including, for example, auxiliary equipment such as control and metering equipment. Transmission pipelines are medium to high-pressure pipelines used for transporting natural gas to one or more distribution companies (e.g. including across provincial, state, or country borders). In order to propagate the natural gas along transmission pipelines, the natural gas may be compressed periodically at compressor stations. Distribution pipelines are relatively low-pressure pipelines typically used to distribute natural gas sourced from the transmission pipeline to the consumers. Natural gas storage facilities may be provided in a pipeline system for instances when the natural gas being transported is not immediately required. One example of a distribution system (DS) is the US natural gas grid (i.e., highly integrated network of transmission and distribution pipelines that can receive natural gas and/or biogas and serve millions of locations).
Advantageously, injecting upgraded biogas (e.g., RNG) into a distribution system (DS) takes full advantage of the existing pipelines and/or pipeline system(s) and existing demand. In addition, it enables the biogas to reach new customers. Further advantageously, injecting upgraded biogas into a DS may also displace fossil fuels used for transportation. For example, RNG used for transportation could displace on-road diesel fuel. Since the basis of commerce in distribution systems is displacement (e.g., matching injections into the DSs with withdrawals out of the DS without tracking specific molecules), and since RNG is essentially interchangeable with fossil natural gas (e.g., is fungible), a renewable transportation fuel may be withdrawn far from the point of injection of the RNG. Unfortunately, the purification processes used for upgrading biogas (e.g., to RNG) are relatively expensive. In addition, the capital investment may be high.
Although the cost of upgrading a given volume of biogas (e.g., to RNG) may be lower for larger systems, the diversion of all of the biogas produced by a process to an upgrading system may result in additional electricity and/or natural gas having to be imported into the process. For example, if the energy costs and/or GHG emissions of the process could be reduced by using some heat and/or electricity provided by a biogas CHP located at or near the biogas producing plant/process, the diversion of the biogas used in the CHP to a biogas upgrading system that produces RNG for injection into the DS and/or for transportation use would result in the process having to import additional natural gas, thus further increasing costs.
In addition, additional cost and/or complexity may arise from the nature of the DS and/or the location of the RNG injection point (e.g., which may be in a rural or low demand area far from high demand industrial areas). For example, in a DS, the natural gas fed from transmission pipelines will generally find its way to the customers through a network of pipes from high pressures and large diameters down to low pressures and smaller diameters. Although the flow of the gas is driven largely by the demand of the customers, if a large influx of RNG is introduced into the system, the pressure and/or flows may change. Accordingly, the existing DS may have to be modified to make it possible to distribute the RNG to a sufficient number of customers. This modification takes time and money.
It would be advantageous to reduce the cost of upgrading biogas and injecting the upgraded biogas into a DS and/or to incentivize the utilization of RNG as a transportation fuel.