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Correct use of lime is key to efficiency in keeping EfW clean

Lime helps energy-from-waste (EfW) plant operators to comply with air emissions legislation by neutralising the flue gases.

It is widely accepted as the most cost-effective alkali that can be used, since it requires a smaller dosage and creates fewer waste by-products compared with other reagents.

Injected directly into the flue gas stream, lime creates a byproduct known as air pollution control residue (APCr). This consists of a mixture of carbon, ash and spent lime. It is classified as a hazardous waste and disposed of via treatment with water or waste acid or directly into long-term storage/landfill at a cost of more than £100 a tonne.

During the mass burn process, a typical EfW plant will produce around 10,000 tonnes of APCr a year, which is approximately 3% of the waste input by weight. For every tonne of lime used in a typical EfW plant, it is likely to result in between 1.5-2.5 tonnes of APCr being produced.

With a new generation of plants scheduled to come on-stream in the UK by 2020, the amount of lime used to neutralise acidic gases and remove sulphur dioxide from plants is set to double to around 140,000 tonnes a year – and the volume of APCr will also increase.

It is clear that there is a real benefit to plant operators and their technology providers in working with lime providers to gain a deeper understanding of how correct application and specification can optimise plant efficiency and minimise APCr volumes for both existing plants and those in the pipeline.

The reality is that, if an operator is using excessive volumes of lime, their costs to safely dispose of the resultant APCr will be excessively high too. I have seen instances where EfW plants use up to 25% more lime than is necessary.

So what steps can the industry take to reduce unnecessary lime consumption and cut APCr levels? The crucial first step is to examine a plant’s processes and performance. There are broadly two types of lime used in EfW plants and these require slightly different approaches.

EfW facilities more than 10 years old are likely to use calcium oxide (quicklime), which reacts with water to create a slurry or milk of lime that is injected into the system. Plants built more recently, and those planned to be commissioned during the next five years, are likely to use calcium hydroxide (hydrated lime).

For quicklime-using plants there a number of areas to examine. First, it is important that they are using the correct grade of lime because this will ensure that the slaking and handling systems are working efficiently. An incorrect grade could create handling difficulties and inconsistent dosing which will affect the quality of the milk of lime and, ultimately, lead to the use of too much lime.

Plant efficiency 586

For quicklime operations it is also important to understand reactivity and optimise the quicklime/ water reaction (known as slaking). This will give the best available quality milk of lime for efficient operation of the gas cleaning system.

We can examine the slurry by conducting a range of tests including solids content and settling rates, which indicate consistency and particle size and are crucial to efficiency in gas treatment.

EfW plants using hydrated lime typically add lime into the gas stream with a small amount of moisture. Most modern plants are designed to ensure good distribution of the lime into the gas stream and reactivation of any unreacted lime in the recycled APCr.

These plants operate well with Lafarge Tarmac’s flagship standard grade of Limbux, which was confirmed via a series of detailed tests and trials with leading EfW equipment and process manufacturers. However, some of the plants of older design require a more reactive hydrated lime, for which the company supplies Limbux Extra, which has a higher surface area and increased pore volume.

Correct handling of the lime product is critical, otherwise it can lead to failures in the gas cleansing system. To assess whether the selected lime is being handled correctly, it is important to look carefully at the silo system and ducting. It is crucial to achieve a constant flow from the silo, so the ducting and parts of the handling system should be optimised to minimise blockages.

Plant operational issues like this can affect lime consumption significantly. Whether plants are using quicklime or hydrated lime, it is important to test and analyse APCr because this can give a clear indication of the balance of reaction and plant efficiency.

The use of lime and the production of APCr are inherently linked. To avoid using too much lime and cut APCr disposal costs, there is a golden opportunity for EfW operators to engage more closely with lime providers. These companies have the skills to interrogate a plant and its processes to ensure its capabilities and efficiencies are maximised, and that APCr levels and associated costs are kept to a minimum.

By engaging early, from the commissioning of plants through the lifecycle of the asset, Lafarge Tarmac is able to advise on optimising operational aspects of the system – not only in terms of specification of the lime itself but also on filtration systems, APCr and bag analysis.    

Lime management checklist

Quicklime plants

  • Select appropriate grade of lime to ensure the slaking and handling systems are working efficiently
  • Understand lime reactivity and optimise the quicklime/water reaction to give the best available quality milk of lime for efficient operation of the gas cleaning system
  • Examine the slurry by conducting a range of physical tests to check solids content and settling rates to test efficiency in gas treatment

Hydrated lime plants

  • Assess whether lime is being handled correctly by reviewing the silo system and ducting
  • Ensure there is a constant flow from the silo system

All plants

  • Test and analyse APCr to give indication of the balance of reaction and plant efficiency

Viv Russell is lime and powders director at Lafarge Tarmac

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