Process Control

Process Control - Landfill Gas Blowers and Flares

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Processes surrounding landfill gas extraction is centered around maintaining consistent flow rate across all the individual wells and the proper destruction of landfill gas. The remainder of processes relate to either gas delivery, conditioning or limiting input energy to the flare. Pressure, temperature and flow are all process variables commonly involved in extraction, delivery and destruction of the gas.

Background

Landfills are large biological system covering hundreds of acres and involve anaerobic digestion. As the name suggests anaerobic digestion occurs in the absence of air. To create an environment conducive to this process all waste entering the landfill is eventually sealed inside a liner. Bacteria begin to multiply and break down the organic waste. The metabolization waste results in heat, water vapor, carbon dioxide and methane. To aid in the rapid decomposition of waste appropriate gas extraction rate must be found and maintained. .

Gas Collection

To collect gas from the expanse of the landfill wells are installed in a grid throughout the landfill. Individual wells are connected to header pipes. Each well has a manual valve that is adjusted to optimize the extraction rate from the individual well. As the landfill is constructed the waste is placed into cells and sealed the headers from the various cells commonly terminate at a single point creating a common collection system. Due to the large area and distance involved with the gas collection system active extraction is required to insure an even removal of gas across the entire area of the landfill.

Active Extraction Equipment

A complete landfill gas extraction system includes

The pipe and components can be field assembled on a pad or delivered on skid as a complete assembly fully tested with instruments and controls.

Prime Mover and Media

At the terminus of the landfill piping a pump, fan or blower is installed. Due to the flow and pressure for gas extraction centrifugal blowers or fans are most commonly employed. Landfill gas is typically characterized as ...

These conditions would be found at the exit of the landfill header (inlet to the blower) providing a medium BTU fuel offering the means to destroy the gas and the accompanying odors when gas is sent to a flare for destruction. As a practical consideration water vapor present in the gas reduces the total molecular weight. When compressing gas with centrifugal blowers, molecular weight has an impact on the final pressure and meaningful when exit pressure must be maintained at or near the limits of the equipment performance.

Given the characteristic of landfill gas it can replace / displace commercially available fuels used for engines, turbines, burners, or any place natural may be used. Some adjustment of the equipment may be needed and some conditioning of the gas prior to use is typical. When landfill gas is used in this manner it is being put to beneficial use where some of the energy in gas destruction is recovered.

Landfill Gas Appliances

Example of appliances and their categories are...
Passive Appliances
Continuously Variable Flow - no process dependency, flow can be adjusted at will
Flares

Demand Based Appliances

Process Dependent Flow - batch process, flow varies with process demand Burners

Constant Input Energy - Fixed output energy once started

Passive Appliances
Each appliance type has a preferred fuel delivery method based on its type. Passive appliances such as flares have a functional range as defined by regulatory emission requirements, burner design and gas quality. Although performance is commonly defined as a flow range it is more appropriate to look at input energy (mmBTU/HR). Fundamental design requirements for landfill utility flares can be found in 40CFR and the basis for practical open flare emission standards. To the process flare look like a open pipe to atmosphere. If the gas source is not inherently flow limited some form of flow limiting must be applied.

As the category name suggests passive appliances do not demand a specific input energy for operation. Operating anywhere along its performance curve affords the user great flexibility, an important feature when used in combination with gas to energy projects.

Demand Appliances
Demand based appliances have an operating range limited to the process they serve. Whatever the input energy requirement is it must be met to maintain the process. Demand based operation is not typically conducive to landfill operation where a constant vacuum is desired. For the same process output energy flow increase as methane concentration reduces. Increased gas flow leads to increased header pressure drop / vacuum on the landfill. This relationship typically puts landfill operations at odds with the beneficial use project when process demand outstrips available landfill energy.

When using landfill gas as fuel it is important to consider variability in gas quality and total volume when sizing a system. Minimum continuous total BTU output of the landfill should be determined. Also consider landfill energy output varies based on season, temperature, barometric pressure to name a few. This short list tells us available energy varies based on weather and time of day without considering human error.

Equipment for extraction, delivery and conditioning of landfill gas needs to compensate for the varying site conditions while meeting input energy requirements of the utilization equipment. Following are descriptions of several types of process controls employed to assist in these tasks.

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Typical Control Loop

Vacuum Control - Landfill Gas Extraction

Primary Control Loop
Landfill gas extraction is accomplished by applying a vacuum to the landfill. Several things are gained by this action when the vacuum is held constant.

Method
A pressure transmitter is connected to the landfill header, adjustable frequency drives are employed for the gas blower motor power control. Finally, a process controller or PLC is used to create a speed command for the adjustable frequency drive based on the operator set point and inlet pressure reported by the pressure transmitter.

Control loop tuning for gas extraction should be conservative as variation in the landfill happen over hours.

Flow Control / Limiting - Landfill Gas Extraction

Secondary Loop - Flares
Tertiary Loop - Gas Delivery

Flare flow Control is used to prevent exceeding flare performance limitation and to maintain regulatory compliance requirements for the burner. When the flare is pat of gas delivery system flow limiting is implemented using the flare inlet valve.

Gas delivery equipment such as compressors or blowers uses this type control when a downstream device flow control device at a flare or burner has failed.

Depending solely on flow meters in landfill gas control systems can be problematic when considering high moisture content (at or near saturation). Spiking of the flow signal or abnormally high flow due to water can cause errant operation and unwanted shutdowns. Careful consideration of the flow meter installation and proper drains on the blowers and flow run must be installed. When implementing flow limiting high flow should not be the only qualifying condition to trigger the control loop.

Implementing flow limiting

PV - Flow meter
Sp - Application dependent, maximum process flow, parallel device flow split, etc...
Cv - Blower speed command, valve position
Initiating Condition for Flow Limiting
Extraction System - Blowers, Gas Delivery
Primary Loop - Inlet Pressure
Secondary Loop - Exit Pressure
Tertiary Loop - Flow Limiting

In this scenario two other qualifying condition can be used to help validate if this is a real flow event, inlet and exit pressure

Inlet Pressure
Under vacuum control excess flow may occur when the inlet pressure from the gas source (landfill) is higher than typical. With landfills internal pressure increase during the time the gas extraction system is offline. Microorganism metabolization of waste does not stop while the blower is off. Pressure increases with time making it easier for gas to flow because gas is pushed toward the blower as opposed to extracted. Excess flow and blower motor overload are possible.

Qualifying condition enter flow limiting during this mode of operation is low vacuum and high flow. Indirect indicators low speed and high motor amps can be indicators as well.

Exit Pressure

Exit pressure control is a feature found in beneficial use projects. Pressure control is selectively applied to both flare inlet valve and extraction blower control. Some upstream processes may require a specific pressure to operate correctly (e.g. upstream booster blowers). When a specific pressure is not required exit pressure can be used to track changes in energy demand allowing blower and valves to follow process needs.

Exit Pressure Control - Flare Flow Splitting

To maintain landfill vacuum the extraction system remains under inlet pressure control providing a relatively constant gas extraction rate to the flare and utilization equipment. While the beneficial use project is not consuming all available energy, the flare takes up the remainder. In this mode the flare valve back pressures the line to track demand of the utilization equipment, excess gas goes to the flare stack.

Exit Pressure Control - Flare Standby, Exclusive Delivery to Demand Based Appliances

The flare enters a "standby" state once flow at the flare is below the minimum stable operating point. While the flare is on standby only the demand-based equipment remains in service. Flow previously sent to the flare remains in the landfill until the flare returns to service or the downstream process takes up the extra fuel. If the down stream process does not take the remaining gas vacuum cannot be controlled and will not meet the desired vacuum set point. as Since pressure and flow are not independent variables reduced flow result in lower pressure drop or vacuum. Reduced vacuum compels the control system to increase blower speed. Since the downstream process may not increase demand, flow and vacuum does not increase. To prevent run away speed the extraction blower process control should be switched from inlet pressure / vacuum control to exit pressure control to track the demand of the downstream system and landfill vacuum is left to float with that demand.

Demand Control - Exclusive Fuel Consumer

Keeping a consistent landfill gas extraction rate benefits the landfill and the beneficial use project by delivering consistent fuel quality. In some cases the demand based process can be adjusted to help regulate landfill vacuum while it is the exclusive consumer of landfill gas. When landfill gas is used as a supplemental fuel in combination with a commercial fuel more or less of the commercial can be used to throttle landfill gas demand. The change in landfill gas demand result in a change in the landfill vacuum.

When the process is capable of consuming more fuel than available from the landfill throttling the process output canbe used to set demand as in the case of utility cogeneration. In this scenario the landfill gas utilization is slowly adjusted until vacuum meets the specific set point desired. When the set point is attained further output adjustment is deferred until the vacuum drifts sufficiently to require adjustment then the control algorithm is activated to restore the vacuum to match the set point. Assuming a properly tuned algorithm and well maintained landfill this drift happen over hours and corrections occur occasionally over the course of the day.

Flow Limiting - Excess Inlet Flow

Extraction blower systems can experience higher than desired flow for couple of conditions, high internal pressure or loss of a vacuum source. When extraction stops landfill internal pressure rises.

Extended Downtime Rising landfill internal pressure is the result from extended downtime. The additional pressure on the landfill header pushes gas to the blower increasing flow through the blower. This is temporary condition often referred to a "bubble". After regular operation is restored the bubble is removed by burning the build up of gas, assuming there is sufficient capacity in the facility to take the gas.

Multiple Vacuum Sources Multiple vacuum sources are set up to cooperatively consume gas with one another. When a required vacuum source shuts down the additional gas becomes available to the remaining extraction equipment. If the gas is not consumed a pressure bubble can form.

Either type of event leads to increased landfill pressure (reduced vacuum) that results in addition gas. Added pressure may result in increase flow at the extraction blowers that has the potential

This scenario the qualifying conditions are low vacuum and high flow.

Flare Flow Limiting
Flares employed as part of a beneficial use project are present to help in this scenario. The flare role is to consume gas the beneficial process cannot use. Considering total capacity of the beneficial use and flare the combined flow offers at least double capacity when operated simultaneously. The additional capacity can be used to burn off additional gas supply.

When returning the extraction system back into service typically flare operation precedes starting the down stream process. While recovering from extend downtime typically additional gas is pushed to the extraction blowers. Flare only operation directs all gas to the flare burner. Additional flow from the extraction may exceed flare flow limits. Exceeding the flow limit activates the flare inlet valve flow limit control loop to prevent unstable flare burner operation and possible compliance violation. As the beneficial use system returns to service the flare continues operation until the pressure "bubble" on the landfill has sufficiently reduced returning the flare to standby.

Flow Limiting - Excess Extraction Blower Flow

During exclusive delivery of fuel to process dependent equipment (flare(s) on standby) exit or head pressure control is commonly used and fixed flow control is contrary to this type of operation. When flow beyond expected operation is detected it may be point to a potentially serious failure. Qualifying conditions for action are high than expected flow and low pressure at the extraction blower exit, an indication process demand is out stripping supply.

High flow to an inherently flow limited appliance can be an indication of a significant leak or rupture in the gas piping, failed valve at the flare inlet or control system failure. Limiting flow is recommended and accomplished with the extraction blowers. While limiting flow with the blowers low pressure is expected to force a beneficial use equipment shutdown. Flow limit during these conditions reduces the release of gas at the rupture and provides a chance to detect the event if pressure does not recover to normal limits in a specified time.

---IMPORTANT---

In practice this form of detection is limited to short pipe runs. Long pipelines pressure drop dominates the measured value at the pipeline inlet and masks leaks further out on the line but a full out rupture is still likely to be detected it albeit it will be a while to bleed down pressure. In this case significant gas volume is released before detection occurs and the release continues even after the compressor stops as pack in the line continues bleeding off.

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Flow Limiting / Control

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Enclosed Flare - Temperature Control

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