Low cost drying and clean fuels company Coomtech has developed an innovative and patented Surface Moisture Removal (SMR) drying technology for pulverised fuel ash (PFA) with significant benefits for the cement and building products industries.
Demand for PFA is increasing due to a number of benefits that it brings to cement and concrete producers. These include the fact that PFA increases cement performance, allows for more resistant, durable concrete and reduces the embodied CO2 and cost of cement by lowering the clinker factor. Dry PFA can also be moved with ease pneumatically around a production site and blends easily with the other ingredients of the cement or concrete mixture.
However, there is currently an increasing shortage of PFA, particularly in the UK and elsewhere in northern Europe. This is due to the diminishing use of coal-fired power plants. Some companies dry PFA in-house using gas as an energy source. This is expensive and not ecologically sound. However, there is around 50Mt of wet pulverised fuel ash stockpiled in the UK alone, meaning that there is significant potential for this source to be used instead.
Coomtech SMR drying process
UK-based Coomtech Ltd has developed a low-cost drying technology for PFA which uses a gas combined heat and power plant to generate both power and increase the moisture carrying capacity of the air. This can save 20 - 25% of the cost of traditional thermal drying, assuming 80% thermal efficiency.
he Surface Moisture Removal (SMR) drying system is a simple, non-thermal, mechanical process that involves feeding PFA into a tubular device in a stream of high velocity, low pressure air. At the same time a supplementary, high-velocity air is introduced to create severe aerodynamic turbulence. This shears any surface moisture that is present on the PFA particles. While it is not heated using a fuel source, the air in the system has a temperature of around 80°C due to the generating process. This temperature is not enough to dry through the latent heat of vaporisation but is enough to ensure the air in the system can carry the maximum amount of moisture possible.
he SMR system comprises individual components that are familiar to materials handling engineers everywhere and includes hoppers, feed valves, pipes, tubes, cyclones, catchment bag filters and containers. Around 80% of the SMR system consists of tried-and-tested off-the-shelf equipment, greatly reducing the engineering risk for commercial users.
The SMR process can remove moisture from PFA from more than 20% to below 3% or to clients’ specific requirements. For some manufacturers’ processes the moisture may be reduced to as low as 0.5% if required.
Figures 1 & 2 show Coomtech’s test facility at the Wolfson Centre for Bulk Solids Handling at the University of Greenwich in Chatham, UK, at which the company constantly optimises the SMR system’s efficiency, both for drying efficiency and throughput. The suspension zone and turbulence zones are shown. In the turbulence zone, the moisture is sheared from the PFA particles. In the moisture extraction zone moisture is released from the particles to the airstream, mainly as vapour and aerosol particles. Tests have shown that smaller particles lose surface moisture more easily than larger ones. The best results have been achieved when processing material particles of <5mm.
Current stage of development
The upscaling and industrialisation of trials at the University of Greenwich over the past two years has led to a throughput of 1.5t/hr for PFA, using two drying zones. The promise of the system has led Coomtech to the final stages of contract negotiations for the building of commercial-scale PFA drying plants for construction products and cement manufacturing companies in the UK and Europe as well as power generators in the UK for the forward sale of dried PFA into the cement and construction industries. Figure 3 shows a conceptual system for commercial use as a PFA drying system.
Future steps for the SMR system
oomtech is working with Fairport Engineering (EPC contractor), Greenbank Engineering (pneumatic handling) and the University of Greenwich on a scaling-up programme for the SMR system. This has led to a modular design that will process from 50,000t/yr up to 0.5Mt/yr per modular unit. Coomtech and its partners are now at the final stage of discussions regarding commercial contracts for plants in the UK and Europe.
Other materials and technologies
While Coomtech has commercialised the SMR process for PFA, it also has a focus on drying coal and, in principle, any other bulk material that can be pneumatically conveyed, such as aggregates, waste streams, metal oxides and other organic and inorganic materials that have significant surface moisture. In the case of coal, the process allows the efficient combustion of lower grade coals.
Coomtech is also developing two other technologies that it expects to be available to the market in the next 12 - 18 months. The first is Inherent Moisture Removal (IMR). IMR is a globally-patented process that moves the inherent moisture within materials to the surface. SMR can then be used to remove the surface moisture. This has particular potential to upgrade use with low rank coals, lignite, wet biomass and other organic materials.
IMR uses a unique heating technology called Ohmic heating at around 275°C and low pressure. Ohmic heating uses the moisture in the material to provide the heat source and create thermo-chemical conditions for decarboxylation to occur. This avoids the efficiency losses of other processes and leaves the pores hydrophobic, preventing moisture from re-entering the pores.
For coal, the benefits of using IMR and SMR in combination are: Higher calorific value; Increased grindability; More efficient combustion due to lower moisture; Lower CO2 emissions.
The second other technology is Trapped Moisture Removal (TMR). It is designed to push tightly-bound surface moisture to the surface of more fibrous or open-particle materials such as some sub-bituminous coals and other organic materials in situations where the drying process must not alter the structure. The details of this technology are not currently available due to Intellectual Property restrictions that will apply during development.