An in-house nitrogen gas generator employing high efficiency membrane bundles from Parker Hannifin Corp. is the heart of a system that provides the necessary gas, which also includes dehydration and filtration systems to remove water and particulate matter from air. A high efficiency membrane that uses an innovative polymeric design the desired gas, even at low temperatures. The generator does not require a heater, reducing operating expenses, can be installed underground or on the surface, and can be tailored to specific applications. The membrane bundles used within the coal mine industry have also been applied to severe oil and gas applications in harsh environments. Frequently, the most effective approach for a mine operator involves a service organization which has the experience in the area and can provide turn-key operation.
This article describes the basis of a system for on-site generation of nitrogen and provides an example of how the inerting gas is provided in a remote location. Basically, a coal mine in rural Colorado used a system mounted on a flat bed truck to generate a flow of 1,000 SCFM nitrogen at a pressure of 350 psig. This installation was coordinated by an independent organization which provides rapid turnkey site deployment, 24/7 support, appropriate training and is an economic solution to the problem. The system can be readily expanded if necessary.
Sealed Areas Have Become a Concern
Federal Mine Safety and Health Administration (MSHA) regulations require mine operators to test the atmosphere in sealed mine areas and to take corrective actions to prevent explosions or fire. The need for testing and corrective action has been exacerbated by recent tragedies. These incidents have prompted MSHA to focus increased attention on mine ventilation, rock dusting, methane monitoring and mine examinations. The Assistant Secretary of Labor for Mine Safety and Health Joe Main has recently stated the purpose of these inspections “is to provide assurance that no imminent dangers, explosions, hazards or other serious health or safety conditions are present at these mines.” To show he was serious, he ordered the agency to inspect 57 mines in a nation-wide blitz, examining bleeder systems, belt entries and seals.
Oftentimes the necessary corrective action regarding mine ventilation (and sealed areas) involves the mine operator injecting nitrogen gas into the sealed area to displace oxygen, carbon monoxide (CO), methane and other gases which might be present to levels below the lower limit of explosion (LEL). A continuous injection of nitrogen may be required for an extended period of time. MSHA has mandated the minimum seal pressure should be 50 psig and the seals must be designed to maintain a tight seal to a minimum of 120 psig.
Transporting liquid nitrogen to the mine site and then vaporizing it could provide the necessary gas. If, as is frequently the case, the coal mine is in a relatively remote area that is quite distant from a source of the liquid nitrogen, this can be an expensive approach. In addition, erecting a tank for the liquid nitrogen at site may be time consuming, and the operator must be concerned that continuous, reliable delivery of the liquid nitrogen meets the requirements of the inerting process. As an alternative, many mine operators employ a portable nitrogen generator to supply the requisite gas by removing oxygen, water vapor and particulate matter from the atmosphere to supply inerting gas.
How is High Purity Nitrogen for Mine Injection Generated?
The major components of air are nitrogen (78%), oxygen (21%) and argon (0.9%), with traces of CO2 and a broad range of other compounds including water vapor (the water vapor concentration depends on the local conditions). Nitrogen gas up to 99+% can be generated using a compressed air system that passes the air through a membrane that permits oxygen and water vapor to permeate the membrane and return to the atmosphere while nitrogen flows through the tube (See Figure 1) to the outlet. The purified nitrogen gas contains argon, which is not a problem as this gas is inert. Since a fiber membrane has a small internal diameter, a large number of fibers are bundled together to provide an extremely large surface area for the permeation of oxygen and water vapor. The general design of the mobile nitrogen generation system (See Figure 2) includes:
• An air compressor system that provides compressed air at the desired maximum pressure. The compressor system includes an after cooler and an air dryer to supply the air at a dew point of 40˚F or better;
• A particulate filter that removes larger dust, dirt and pipe scale from the incoming air;
• A filter to remove any particulate matter which may have come out of the air dryer;
• A filter to remove any oil vapors (e.g. from the air compressor);
• A coalescing filter for removal of any fine particulate matter;
• A series of membrane modules to separate the nitrogen from oxygen and water vapor; and
• An after filter to ensure a clean and dry nitrogen flow to the mine.
The mobile nitrogen generation system can be mounted on a trailer (See Figure 3) or on a skid (See Figure 4) which can be installed underground in the mine. These systems are essentially identical and each is specifically configured to meet the requirements of the end user. Typical specifications for the truck and skid systems are presented in Table 1.
An oxygen monitor is included in the system which will provide an external alarm if the oxygen level is above the user set limit. In addition, the generators include a system controller and allow for the monitoring of a broad range of system status parameters such as the flow rate and fault indicators (e.g. malfunction of the drier). A series of 4-20 mA contacts are provided which can be used for remote monitoring of parameters such as the nitrogen flow rate and the oxygen content of the gas.
The purity of the nitrogen generated is dependent on the flow rate of the incoming gas at the maximum incoming gas flow rate, up to 99+% nitrogen can be provided (at high flow rates, the concentration may be slightly lower).
A Typical Installation
Recently, nitrogen inerting was required for a longwall mine at a relatively high altitude (6,800 ft). One of the largest underground mines in the U.S. found it was necessary to develop a solution based on an on-site nitrogen generator. The mine operator, in conjunction with Red Dog Services determined a flatbed truck mounted unit that included 20 HiFLUXX membrane bundles with appropriate desiccant dryers with heat tracing to withstand -10˚F ambient temperatures and filters would provide the necessary nitrogen (See Figure 3). This system is capable of supplying 1,000 SCFM of nitrogen at very high purity (95%-99%) purity at 350 psi (the purity of the nitrogen is dependent on the flow rate; at lower flow rates, higher purity nitrogen is obtained). If desired, the nitrogen system can be readily expanded to supply 1,200 SCFM by the addition of four additional membrane modules.
The overall system was configured for low ambient conditions. Heaters are not needed (many systems that generate nitrogen via membrane separation require heaters), which reduces the overall power consumption. The system included an oxygen analyzer and an automatic block valve to stop delivery of nitrogen if the oxygen level was above the user preset level. The control panel, mounted inside the portable container unit, includes the standard operation indicators, a nitrogen flow display, inlet and outlet pressure gauges and a high oxygen and desiccant dryer fault lamps as well as 4-20 mA contacts for remote monitoring of the output flow and the oxygen content. The compressor system is powered by a 480 VAC/3 phase/60 Hz, other voltages are readily available.
The oxygen analyzer, powered by an independent fuel cell, is directly connected to the oxygen alarm system and a flow valve which diverts the output from delivery to the mine.
Benefits of an In-house Nitrogen Generator
An in-house nitrogen generator provides the mine operator with a reliable source of high purity nitrogen capable of providing a flow as high as 1,200 CFM continuously. Since the system is on-site, the mine operator is assured that the gas is always available, and supply is not compromised in the case of inclement weather, road closings, etc. The generation system requires little maintenance; the filtration cartridges should be inspected every three months and are typically replaced annually as is the carbon in the carbon towers. All filter housings are easily accessible for service personnel.
The heart of the portable nitrogen generator is a membrane system based on an innovative polyphenylene oxide polymer that separates the nitrogen from oxygen and other gases. This membrane is formulated from a very rugged plastic to minimize the possibility of rupture, is not sensitive to particle contamination and has a low pressure drop. Since the polymer is significantly more permeable than other membranes, its use leads to a higher nitrogen production and produces the gas at a significantly reduced air pressure than other polymers and does not require a special heater. This leads to a more reliable system, with lower energy usage and allows for the use of a standard low pressure air compressor.
Selecting and Employing a System
When a mine operator has determined that a nitrogen insertion system is required, a number of issues ranging from selection of the appropriate system (in mine or truck bed), the capacity of the system and the potential for upgrading must be considered. Similarly, once the system is in place, issues such as certification and validation, operation, maintenance and system support must be addressed.
There are a variety of general approaches to the selection and operation of a nitrogen inerting system; the mine operator can configure, purchase, rent and operate the system or can use a company to provide the most economical solution. A service company can provide a system and overall operation, or it can be used to simply operate and maintain the inerting system. Since the overall requirement of every nitrogen insertion system is unique, it is especially important for a mine operator to understand the tradeoffs involved in each of the approaches.
Many mine operators have selected Parker and one of its service providers to help develop a nitrogen inerting protocol; there are a number of benefits in this approach, including:
• Expertise in Designing and Acquiring the System—A nitrogen inerting system will include components from a number of manufacturers. The nitrogen gas generator manufacturer and the service company have the specialized knowledge and understanding of the various systems and manufacturers so the optimum solution is obtained. In addition, it has the experience and knowledge to prepare the Request for Quote (RFQ) and evaluate the proposals. In contrast, many mine operators have little experience in designing such a system and a significant start-up period may be detrimental to the solution of the problem.
• A Turnkey Approach is Provided—When a service company supplies a system, it provides an integrated solution to the problem. In contrast, if the mine operator develops and purchases a system, potential integration issues may arise. Eric Mehringer, president, Red Dog Services, has indicated that a turn-key approach to mine inerting is a superior approach to the issue, as a service company has faced all of the problems that underlie inerting. Red Dog has developed suitable solutions to them, minimizing the hazard, risks and challenges to the mine operator.
• Knowledgeable Service Technicians are Available—A service organization can provide continuous technical support for operation and maintenance. These technicians understand the needs of a mine operator and government requirements. As an example, the required flow of nitrogen is subject to change due to a variety of factors such as the change in atmospheric back pressure of the seal. An experienced technician will be able to anticipate these changes and ensure a safe mine environment. If the mine operator uses existing on-site technicians, these individuals may consider support of the nitrogen generator as an auxiliary task and may not adequately monitor the generator. While the membranes are manufactured from a sturdy polymer, they can be damaged if the system is not sufficiently monitored, leading to expensive repairs and hazardous system down time.
• A System is Available on an Emergency Basis—A service organization has a number of systems in inventory and the nitrogen generation system can be packaged for quick site deployment. As an example, a number of companies including CASL Inc. (Katy, Texas) and Red Dog Services (Plano, Texas) can provide nitrogen gas generator rental units.
• Service Contracts Frees Capital—A service contract allows the mine operator to use a nitrogen insertion system for the needed task, and it is not necessary to invest funds in a system that may be needed for a short term.
Many mine operators use a service company to operate, service and support the nitrogen generator as the service company has the experience and training to optimize the operation and minimize risks.
This article was supplied courtesy of Parker Hannafin Corp.
