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. Metabolization of waste results in heat, water vapor, carbon dioxide and methane. To aid in the rapid decomposition of waste an 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. 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 fully assembled on a skid, 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 using a flare for destruction.

As a practical consideration landfills are biological systems subject to the influence of weather, landfill contents, human error in operations to name a few factors. This makes these characterizations typical but not assured. Variation in quality and volume are realized at a given landfill. Changes in total volume over the course of a day can be seen including seasonal variations in quality should be expected.

An other factor for delivering gas to the desination process 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 is 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 gas may be used. Adjustment of the utilization equipment and 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 is recovered for commercial use during gas destruction of the landfill gas.

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 a flare looks like an open pipe to atmosphere. If the gas source is not inherently flow limited some form of flow limiting must be applied to keep the flare inside it's operational limit.

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 /extraction rate is desired.

As total landfill gas volume and quality variation are exepected flow to the process will vary. To maintain process output energy flow increases as methane concentration reduces. In turn increased gas flow leads to increased vacuum on the landfill. This relationship typically puts landfill operations at odds with the beneficial use project where 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

When used with ... Flares flow control is a secondary loop
Gas delivery it is a tertiary loop

Flare flow Control is used to prevent exceeding flare performance limitation and to maintain regulatory compliance for the burner. When the flare is part of a 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 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) of the gas. 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 technology and installation is needed. Make sure proper drains on the blowers and flow run are present and maintained.

When implementing flow limiting high flow should not be the only qualifying condition to trigger the control loop.

Implementing flow limiting

Extraction System - Blowers, Gas Delivery

PV - Flow meter
Sp - Application dependent, maximum process flow, parallel device flow split, etc...
Cv - Blower speed command, valve position

  Primary Loop - Inlet Pressure
  Secondary Loop - Exit Pressure
  Tertiary Loop - Flow Limiting

Initiating Condition for 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 while the gas extraction system is offline. Microorganism metabolization of waste does not stop while the extraction equipment is off. Landfill internal pressure increases with time making it easier for gas to flow. Excess flow including blower motor overload are possible.

Qualifying conditions to use flow limiting is low vacuum and high flow. Indirect indicators low blower speed command and high motor amps can be indicators as well.

Exit Pressure
If using exit pressure control with the gas extraction equipment and the down stream process is not limiting flow the upstream pressure drops. The reduced resistance allows for greater flow through the blower.

In this case low exit pressure and high flow are the qualifying condition to use flow control. Secondary indicators would be high motor amps and high blower speed.

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 blowers and valves to follow process needs.

Exit Pressure Control - Flare Valve Flow Splitting, Demand Tracking

To maintain landfill vacuum the extraction system remains under inlet pressure control providing a relatively constant gas extraction rate from the landfill when a flare is part of the landfill control equipment. When 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. Flow and pressure are not independent variables, reduced flow result in lower pressure drop or vacuum. Reduced vacuum compels the control system to increase blower speed. If the downstream process does not increase demand, flow and vacuum does not change and the blower speeds runs away to maximum.

To prevent run away speed control 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 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 can be used to set demand as is the case of utility cogeneration. In this scenario the generator output to the utility is slowly adjusted until vacuum meets the specific set point desired. When the vacuum set point is attained further output adjustment is deferred until the vacuum drifts sufficiently to require correction. The control algorithm is activated and restores the vacuum 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. These conditions are descibed next.

Extended Downtime
Rising landfill 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 temporary condition is often referred to as 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 results in increase flow at the extraction blowers and has the potential to...

Qualifying conditions for this scenario 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's 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 to service typically flare operation precedes starting the beneficial use ewuipment. While recovering from extended downtime any additional gas is pushed to the extraction blowers.

During flare only operation all gas is directed to the flare burner. Additional flow from extraction may exceed flare flow limits. Exceeding the flare flow limit and low vacuum activates the flare inlet valve flow limit control loop. Limiting flare flow prevents 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 and vacuum has been restored flow limiting is released. As the benefical use equipment returs to full load the flare may return 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 pointing to a potentially serious failure. Qualifying conditions for action are higher 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 pipeline ceate a large reservoir. When the line break further out on the line the stored volume masks leak. However a full out rupture is still likely to be detected albeit it will be a while to bleed down volume and 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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Enclosed Flare - Temperature Control

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Enclosed flares quotes at a later a date

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Beneficial Use

Introduction

Engines operating on landfills have a limited fuel source. The availability and quality of fuel for engines is based on many factors that change over the life of the landfill and can change over the course of the day. Changes in gas volume are due to weather (temperature, barometric pressure), landfill maintenance and equipment issues. Long term gas availability is based on materials in the landfill, landfill construction, moisture distribution, collection system maintenance and consistency of gas extraction to name a few.

Landfills should not be thought of as an industrial process where fuel is produced with consistent quality and volume, but as a biological system. Production of energy producing gas (methane) is due to microorganisms in the biomass of the landfill, breaking down cellulous, food waste etc. These microorganisms thrive over a limited temperature range, moisture, and gas concentration. As waste in the landfill is consumed in an anaerobic process, the microorganisms release Carbon Dioxide, Methane and water vapor. If the methane and carbon dioxide are not removed the bacteria will suffocate.

Industry experience has characterized the optimum extraction rate based on gas quality. Extraction is considered optimized when the gas composition is 50% Methane (CH4), 42% Carbon Dioxide (CO2), 4% Nitrogen (N2), Oxygen (1%) and 3% water (H20).

Collection of landfill gas is accomplished by the installation of wells in the landfills biomass. Each well has an influence over predetermined area. The wells are arranged so the areas slightly overlap with proper vacuum applied to gas collection system. Each well is connected to a header forming a common collection system.

To encourage energetic decomposition of landfill waste and maximize total energy output, consistent gas extraction must be maintained at each well. The well field technician works to maintain specific conditions that work best for the bacteria in the landfill. Adjustment of the gas well vacuum is based temperature, methane and oxygen concentration measured. Vacuum at the well is related to an extraction / flow rate.

To generate suction extraction blower(s) are connected to the collection system header. Vacuum is distributed across the wells via the collection system header. Each well then contributes flow to the total volume extracted by the blower.

If the gas cannot be put to beneficial use it is flared or burned to destroy the methane for safety, environmental, and odor control. Maintaining consistent extraction rate over the entire landfill is required to maximize total energy output for use by the energy.

Beneficial Use - Engine Operation

Engine generator sets used in landfill gas application are operated at fixed output power. The operator chooses the output power set point based on current landfill generation rate. As quality and quantity of landfill gas varies the engine’s energy requirements does not.

When the operator recognizes the landfill pressure is not maintained adjustment to engine loading is used to correct the pressure. If the landfill gas supply is insufficient to maintain engine load and left uncorrected, line pressure drops and the engine trips for low pressure resulting on no export and lost revenue. In addition to lost revenue sudden abrupt stops under load puts the engine at risk of damage.

To maintain landfill pressure the operator is required to continuously monitor and adjust the engine output power to correct for the changes in gas production. Manning facilities 24 hours a day is economically impractical and the human resource is better used to maintain the engine(s) and facility.

Continuous adjustment of engine output optimizes energy export (referenced to available fuel) and maximizes revenue. The additional revenue can easily justify the cost of automation to take advantage of any available fuel and prevents low gas pressure trips. Effect on Operations Optimum suction pressure is determined by landfill technicians for best extraction volume and quality. Constant landfill suction pressure is maintained while extracted volume varies with gas production and collection system health.

Reduced gas production results in a lower internal landfill pressure. Extraction equipment reduces speed to maintain the vacuum. Speed reduction at the extraction blower results in a lower delivered volume to the downstream utilization equipment. If demand from the engine is not adjusted delivery pressure drops in response. When the flare is operating the flow at the flare is reduced in response to the change in available volume. The change in flare flow negates the need change engine output. Further reduction in available gas takes the flare offline. Any reduction in fuel requires adjustment of engine output power to maintain landfill pressure.

As internal landfill pressure rises available gas volume is increasing. Extraction equipment increases speed to maintain the suction pressure. The down stream pressure to the engines increase as the engine will not respond to the additional volume, due to the engines fixed output.

At this point gas flaring equipment should be started to destroy the unused portion of the gas and maintain pressure. If the flare is out of service (process or equipment problem), and a pressure limiting algorithm is not employed, extraction blower speed runs away. Such a condition results in wasted energy, increased equipment wear and in some instances equipment damage (dependent on extraction equipment design).

Compounding the problem is the extraction blower and flare has minimum operating conditions (turndown) that prevents operation below a predetermined flow. When remaining gas volume is below the minimum operating volume of the flare it must remain off until sufficient volume to reliably operate is present. The cyclic operation prevents controlling the pressure on the landfill to the desired level.

Ideally a mechanism to continuously adjust engine output power to match available gas volume is employed. Demand control allows the landfill pressure to be maintained at the desired set point while the all available gas goes to beneficial use. Control Strategy The concept is simple, less gas reduce power, more gas increase power.

As described previously it is desired to maintain a constant pressure on the landfill. Practical control will set a window around the ideal set point. The landfill pressure is allowed to float between the limits with no output power adjustment. Once the limits are breeched engine output is incrementally changed up or down in response to the change in pressure.

Each step in output power is followed by a waiting period to determine if the pressure is back inside the window. To prevent the control from riding on the window limits (high or low) the control algorithm continues to adjust the engine output until the landfill pressure is restored to the operator set point.

This method can be digital or analog. The digital method use up and down outputs, each transition changes the engine power by a fixed step value. Analog signal works in a similar way except the analog output changes by a predefined step. The output steps at a rate defined by a timed period. The timed steps continue until the landfill pressure nominal set point is crossed through Logic Rules

It should be noted when the gas conditioning system is integrated with a flare the pressure monitoring algorithm should not be activated until the flare is offline. While the flare is operational the flow to the flare should be used to determine the available energy for the engines. This is used to determine a base KW output. After the engine load is at the base value the KW step value should be reduced to prevent over pulling the landfill gas blowers. After the flare is no longer in service adjustment of the engine output KW can be used to set landfill pressure. If multiple engines are employed they should be staged in one at a time.

Summary Control of the landfill pressure using engine loading requires the following Engines are staged in one at a time Engine KW is stepped up until the flare is offline The demand algorithm is enabled after the flare is offline Flare blowers transfer to exit pressure control while the flare is offline The engine(s) have the capacity to take all available landfill gas The step value chosen should not cause large pressure sags at the landfill blower exit

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