Kiln dust’s second life
28 July 2016
Aluminium cans, rubber tyres and cement kiln dust are three products that, at one time, almost exclusively ended up in landfills – and in staggering amounts. In the case of kiln dust, an unavoidable byproduct of cement production, Siemens process instrumentation helps manage dust in the cement plant, and makes reuse and repurposing of the material possible.
By Graham McGregor, Siemens AG, Canada
Recycling has enabled the reduction of a considerable volume of waste sent to landfill in recent years. In 2011 the US recycled more than 737,000t of aluminium cans – enough to make nearly 50bn new ones. In 2009 Canadians recycled more than nine out of every 10 vehicle tyres.
Turning to cement kiln dust (CKD), the story is similar: between 1990 and 2006, the US cement industry slashed the amount of CKD sent to landfills nearly in half. In addition to being put to valuable use in the cement plant itself, CKD can be found in commercial applications such as wastewater treatment, as a stabilising base for pavement and as a soil enhancer. However, the dust must be captured first.
Effective dust capture
Recovering as much dust as possible is crucial if the cement plant is to comply with government regulations on
particulate matter. In the US violations of the Environmental Protection Agency (EPA)’s Clean Air Act have seen cement companies pay heavy fines.
A vital part in limiting such airborne particles into the environment is played by baghouses. Dusty air enters the baghouse, where filter bags collect the dust particles into hoppers. A screw conveyor moves them to process and storage vessels.
At one particular North American cement plant the effectiveness of the baghouse was helped by the installation of Siemens Sitrans P DSIII differential pressure transmitters. These measure the differential pressure across filter media inside the baghouse and hence, indicate the dust build-up and efficiency of the filter media. A high differential pressure signals the need for maintenance or cleaning.
In addition, a Sitrans AS100 acoustic sensor, connected to a Sitrans CU02 control unit immediately tells technicians if a filter has been damaged. In this case, the sensor detects the impact noise of the dust particles and alerts operators.
Preventing filter damage
CKD starts to build up in the hopper if the dust is no longer being transported from the baghouse hopper to the screw conveyor. If the dust build-up reaches a very high level, it can severely damage filter bags, requiring costly repairs and production downtime.
To avoid such damage, technicians at the North American plant installed Siemens Pointek CLS300 capacitance level
switches. The point level device, mounted at the top of each hopper, provides high-level alarming if the dust reaches a certain point in the hopper, before it hits a critical level.
The screw conveyors are also equipped with cost-saving instrumentation. If a screw conveyor stops, a Siemens Milltronics MSP-12 motion sensor probe, mounted on the outside of the screw conveyor housing, relays information to a Milltronics MFA 4p motion failure alarm.
The system alerts technicians of any changes in the motion and speed of screw rotation, so remedial action can be taken.
Each process instrument is a building block of safety and efficiency. Without alarming, CKD could damage equipment or cause paralysing production downtime – expensive for a plant that runs 24/7.
Reliable continuous CKD feed to the kiln
Once dust moves from baghouses to vessels, level monitoring helps safeguard a plant’s efficiency but also ensures production quality.
While CKD has many uses outside the cement plant, it also plays an important part in the plant’s own output. CKD can be reintroduced as raw meal to the kiln and as such is a valuable source of material that costs the plant only the infrastructure required to transport and store it. A medium-sized process vessel stores the dust and instrumentation measures the amount introduced into the kiln as needed. The amount reintroduced saves process energy, but excessive quantities will have a negative impact on the specifications of the cement being produced.
In the case of the North American plant, nuclear technology was used to measure this CKD flow from the process vessel into the kiln. However, this nuclear device had become unreliable and ultimately failed.
The company consulted Siemens to find a replacement technology and a solids flowmeter was recommended. A Siemens Sitrans WF330 solids flowmeter quickly became the plant’s preferred choice, as it has two per cent accuracy and a simple design requiring no nuclear permits.
As a result, the plant can rely on a repeatable device that provides a consistent signal to indicate the amount of CKD reintroduced into the kiln. The device’s sensing plate is placed in the path of the material flow. The material strikes the sensing plate and the horizontal force component causes a mechanical deflection, which is then converted into an electrical signal in the sensing head.
Verified vessel levels
Sitrans LR560 radar transmitters measure levels in the process and storage vessels for CKD that will be used for commercial applications.
The transmitter’s high-frequency antenna does not have any issues with the extremely dusty environment inside either vessel. The device’s narrow beam allows technicians to install Sitrans LR560 almost anywhere on top of the vessel.
For back-up level alarming, Siemens point level devices alert technicians if CKD levels in the vessel are too high or too low. If levels become too high, CKD can damage equipment or overflow out of the vessel. Siemens point level switches alarm if high or low level situations occur so that technicians can quickly resolve the situation.
CKD not used as raw meal supplement is kept in storage vessels, also monitored by Sitrans LR560 and point level devices. Precise level measurements are also crucial here, as technicians need to ensure that it is shipped out as efficiently as possible. If a vessel contains insufficient CKD, trucks scheduled to pick up materials may make wasted trips to the cement plant.
Kiln dust: the bottom line
This dusty by product of cement production has become a valuable resource but requires the benefits of process instrumentation to help fully realise its value.
From reducing the waste stream of materials heading to landfills to ensuring environmental regulations are being closely followed, Siemens’ wide portfolio of instruments are the eyes and ears of the cement plant.
Kiln dust’s second life is guaranteed by the precise monitoring of devices that reliably operate in this rugged environment.