LIMESTONE IS A USEFUL AND diverse material. It is used to construct roads and buildings, to help make steel, treat water and help crops to grow. Its high alkalinity means that it is great for neutralising the acidic gases produced by combustion. For this reason, historically, it is the main reagent used in flue gas treatment.
It is also abundant – at Lhoist we believe there is at least another 100 years’ supply in our quarry at Hindlow near Buxton, even though up to 500,000 tonnes a year are being extracted.
Some of the rock is used in its natural form but, when burnt, it becomes calcium oxide or quicklime. Add water and you produce hydrated lime.
This is the main product, in some cases mixed with other reagents, which we supply to energy-from-waste (EfW) plants, not just in the UK but all over the world – Lhoist currently operates in 25 countries across four continents.
Historically, milled limestone suspended in water was the most common reagent used to scrub the sulphur that came from the chimneys of coal-fired power stations. However, in the UK, the last coal-fired giant will have shut down by 2025.
Today’s metal-clad, computer-designed EfW and biomass plants are a far cry from the brick-built factories and power plants that used to emit acidic, particulate-laden smoke into the sky.
But hydrated lime, used in dry rather than wet form, is still performing the same function: reducing hydrogen chloride (HCl), sulphur dioxide (SO2) and hydrogen fluoride (HF) from flue gases.
To do this, it is pneumatically delivered in precise, measured quantities as a powder into the gas stream, after it has left the combustion chamber and boiler, where steam is made but before the gasses are filtered.
Most EfW and biomass plants have two silo-fed reagent lines, one for activated carbon, which reduces heavy metals such as mercury, and another for hydrated lime, which deals with the acidic pollutants.
For those plants which have only one line, Lhoist supplies a range of blended products, from the Sor-bacal® family, which combine the functions of both reagents. It is these reagents that help to keep the emissions of EfW and biomass plants within the strict limits which are set by the Environment Agency. Contrary to common belief, what comes out of the plant’s chimneys is not smoke – it is steam.
During the past few decades, Lhoist has refined and adapted its hydrated lime-based reagents to make them more efficient. We have helped power stations and EfW plants to stay within the ever-reducing pollution limits that were set from the early 1970s by the EU.
The quantities of reagent that are needed have gone down and power outputs have increased. The UK’s largest EfW facility, operated by Viridor in Runcorn, generates up to 70MW a year of electricity. It also produces 51MW of heat for use on a nearby industrial site.
Grate firing is the technology used for more than 90% of the industry, but Sorbacal® reagents are also used by the latest generation of plants designed for gasification technology. This is a process in which thermal decomposition of waste at elevated temperatures occurs in a low/zero oxygen atmosphere to produce syngas.
The heat generated is still used to generate steam pressure that drive turbines, but it still needs to be scrubbed afterwards.
Many other technologies are now becoming available to convert waste streams into useful chemicals or fuels and this trend is likely to continue. It is possible in the future, as we tackle new and legacy waste issues, that chemicals and fuels will consume more of the waste stream than the direct energy sector.
As demanding EU targets have been imposed to reduce the UK’s use of landfill, incineration has become an important policy tool. It is a vital technology for dealing with residual, unrecyclable waste and reducing the climate-damaging methane that would otherwise have come from burying waste in the ground.
At the end of 2018, there were 43 EfW facilities operating, or in late commissioning, and 15 being built, representing 42% of the UK residual waste market. Significant new plant capacity is planned for the next decade.
Recent unforeseen development, such as waste bans in China and other parts of Asia and the proposed Dutch refuse-derived fuel (RDF) incineration tax, have ex-posed a growing capacity gap in the EfW sector, and highlighted the difficulty of getting new plants up and running quickly enough.
Damaging misconceptions abound; most people are unaware that EfW and biomass plants work to tough emission limits and that one bonfire night produces more dioxins than the entire sector in a year.
Most will not know either that feedstocks are changing. None of the latest EfW facilities are designed purely to incinerate the mixed paper, card, plastic, metal and food that comes from black bags. Instead, they run on processed waste or RDF, which is produced by a large and growing number of MRFs.
“Sorbacal reagents are also used by the latest generation of plants designed for gasification technology.”
For operators, RDF is far superior to unsorted municipal solid waste (MSW). The varying calorific value of MSW leads to fluctuating process conditions as a result of changing fuel composition, which is not helpful to plant operators. When RDF is used, these peaks and troughs are evened out.
There is something else that most people don’t know: thanks to the efficiency of MRFs in removing recyclable materials, the amount of plastics being burnt is reducing significantly.
The anticipated raw gas data for EfWs in the planning phase is showing increasing ratios of SO2 to HCl. HCl is the acidic gas that comes from burning plastic, so that provides indisputable evidence that ever-improving recycling infrastructure means that EfW plants are burning less plastic – something that everybody will welcome. This trend will continue as RDF becomes the standard material used for incineration.
In the next two to five years, even stricter emissions limits are set to be applied, when changes are introduced to the EU’s best available technique reference document (BREF) for EfW.
They are going to impose tough demands across the industry. The BREF is likely to extend continuous monitoring, which at present only applies to the easier-to-measure acidic pollutants, to heavy metals including mercury and also micropollutants such as dioxins and furans.
Continuous monitoring will require the installation of more complex analytical equipment and possibly more focus on specialty reagents to ensure that continuous monitoring is always compliant.
The industry will rise to the challenge but, with the new emission limits for heavy metals and micropollutants on the horizon, the challenge could be to find suitable technology to accurately measure in values as low as nanograms.
While Lhoist can join trade groups in lobbying on new legislation, they cannot prevent other countries from closing the door on the UK’s exported waste or taxing incineration – we can only deal with what is in front of us.
However, as an innovative UK provider of hydrated lime-base flue gas treatment reagents, we can help to give the sector the tools that it needs to do its job.
We welcome tougher emission limits, whether for acidic gases, heavy metals or micropollutants. They are great for the environment and will provide a growing market for the specialised and blended reagents that we are known for, produced by a constant pipeline of innovation. For us, hydrated lime is not just a commodity – it is a resource that we can build on for the benefit of our customers.
The ability to innovate is partly a function of our size. Lhoist has a global turnover of £2.2bn and approximately 6,500 employees, but it is also part of the company’s overriding philosophy. In Belgium, where the company originated, we benefit from an innovation team which includes a core group of market specialists and the latest technology testing facilities.
Part of their role is to develop new products for the EfW, gasification and biomass markets. They do this by carrying out research, market studies and gaining insights from our commercial teams.
The collaborative sharing of knowledge of market trends, technological developments and scientific excellence allows the innovation pipeline to review the main issues facing the industry and find resolutions to existing and anticipated problems. This process has been proved historically and will be beneficial to EfW operators in the future also.
A multi-year research and development programme has recently concluded with another addition to our Sorbacal® range of specialty hydrated lime products. We believe this will again set new performance benchmarks for the industry, tackling current and future issues relating to emissions challenges and operational difficulties.
The roll-out of this new product is gathering speed, and we are excited to see it develop into another market-leading hydrated lime product. There area dynamic times ahead in a fast-developing sector, so watch this space.