This article discusses the quality and quantities of limestone requirements for Flue Gas Desulfurisation (FGD) with expected quality of synthetic gypsum (SG) generated from power plants. As this development is a win-win situation for power and cement, various possibilities of collaboration between these two major industries are discussed in this article.

The Ministry of Environment & Forests (MOEF) vide their notification dated December 7, 2015 has prescribed limits for particulate and gaseous emissions from all the thermal power plants in India. As per the notifications, the SOx emissions are to be brought down to level of <600 mg/NM3 for plants (installed before 2016) and all the plants have to complied with this limit within 2 years from the date of notification. As on date, the coal-based thermal power generation capacity in India is about 1.96 lakh MW and it is projected by the Ministry Of Power (MOP), following implementation schedule of FGDs by thermal power plants.

A few of the thermal power plants have already initiated projects for meeting the above schedules and over a period of next seven years, all the FGD projects of thermal power plants will come on stream, producing synthetic gypsum. Majority of the power plants are considering wet process FGD technology, using limestone slurry.

FGD in power plants
Globally, Flue Gas Desulfurisation (FGD) systems have been installed in many thermal power plants in developed countries and FGD plants have been in operation in US since 40 years. Proven technologies are available for FGD in power plants. The technologies are primarily based on limestone and lime products such as quick lime or hydrated lime. FGD plants-based wet process technology, which uses limestone slurry is the most prevalent technologies in the US and China. In this process, the gaseous SOx is absorbed in limestone slurry and forms synthetic gypsum (SG).

SG as a substitute for mineral gypsum by cement industry is well established in the US and the following pattern of usage of SG reflects the usage in cement. The entire demand of gypsum of cement industry in the US is met by SG generated in FGDs of power plants in the country.

It can be expected, SG from FGDs of thermal power plants can become a major source of gypsum for the Indian cement industry. It can become a substitute for imported gypsum consumption by industry and also make up shortfalls in supply of mineral gypsum by indigenous gypsum mines.

Limestone requirements for FGDs
The quality requirements of Limestone suitable for FGDs are defined in FGD specification documents of the Central Electricity Authority (CEA). It is to be noted that the quality requirements are similar to the limestone quality requirement for clinker production. Higher the lime content in limestone, the purity of SG produced by FGDs will be better.

Typically, the limestone requirement for FGD plant with sulfur content of 0.5-0.7 per cent in coal fired in power plants is about 0.23-0.30 TPD of limestone/MW installed capacity. It is estimated that limestone demand by FGD systems will be about 15 million tonnes per annum (MTPA) by 2022.

Synthetic gypsum (SG)
FGD plants of wet process technology generate a solid product, which has similar chemical composition of gypsum i.e. CaSO4.2H20. This product, in the form of Slurry is collected at the bottom of the SOx absorption tower. The slurry passes through belt filters to generate filter cake of about 10-12 per cent moisture. This cake is stored in open spaces for sun drying and also for loading into trucks/wagons.

The expected quality of per cent from wet FGDs is as follows:
Gypsum Purity (as CaSO4.2H20, dry) – > 80-90 per cent (depends on LS quality)
Free moisture: 10-12 per cent
Particle size : 100 per cent passing 200 microns
Though above parameters are similar to Phospho gypsum, SG is not having phosphoric compounds and hence the setting times achieved with usage of SG is as good as that for mineral gypsum.

Gypsum Availability for Cement Industry
At present, in India, out of about 13.5 MT of gypsum (all types) consumed, 99 per cent of gypsum is used by cement industry. Due to shortage of gypsum production in India, about 4.0 million tons of mineral gypsum is imported over last few years from countries like Oman, Pakisthan and the UAE.

Gypsum consumption pattern
It is noted that consumption in cement industry is about 99 per cent of total gypsum consumption in India. During the year 2015-16, apart from mineral, marine and by-product gypsum sourced from India, about 4.6 MT of mineral gypsum was imported from Oman, UAE, Pakistan and Thailand. The purity of imported gypsum is about 85 per cent whereas mineral gypsum from Rajasthan have a quality of 60-75 per cent. The purity of imported gypsum is about 85 per cent whereas mineral gypsum from Rajasthan have a quality of 65-75 per cent. Because of low purity of Mineral Gypsum produced by Mines in India, a majority of the consumers are either moving to imported gypsum or a blend of mineral gypsum with high purity Phospho or chemical gypsum.

SG – Expected capacity and quality
The Product of FGD is Gypsum and as it is not a natural gypsum, it is called SG. SG of FGD is going to be available in large quantities in India in future as all Power plants have to installs FGDs for Control of SO2, With present power generation capacity of coal based power plants in India, it is estimated that the SG generation by Power sector is going to approximately 23 MT per year. This SG capacity can entirely meet the gypsum demand of the country in future, considering the growth projections of cement and wall board industries. At present these two industries, consume about 99 per cent of all types of gypsum consumption in India.

Expected quality of SG
The purity of SG generated by FGD is expected to be 82-88 per cent depending the purity of Limestone used. SG will have a moisture of < 12 per cent with fineness of 100 per cent passing 200 microns and the temperature at absorption tower outlet is about 50 deg C. The expected quality of SG by FGD as compared to other types of Gypsum given below. It can be seen that the expected quality of SG from FGD plants is as good as by product gypsum and unlike Phospho-gypsum, SG will not have the problem of delayed setting, due to absence phosphoric oxides.

In the US, apart from cement industries, wallboard industry is a major customer for SG utilisation. Though this industry is in nascent stages in India, boards made out of gypsum are becoming more and more popular in India for partition walls in high rise building, which results in substantial reduction of construction costs and time. All the major multinationals like Saint Gobain (Gyproc), Boral & Gyp Elite, India have already started manufacturing gypsum boards in India and have plans for expansion of existing plants and also to establish new manufacturing facilities.

In the next two years, the total wall/plaster board industry is expected reach a capacity of >1.5 MTPA and the demand for gypsum is expected to be > 1.2 MTPA. Various data from developed nations indicate that, the quality of SG from FGD plants can replace completely the traditional source of gypsum i.e. Mineral gypsum, thus conserving the natural gypsum in the country and reduced mining activities of gypsum. In India, by year 2022, with the projected availability of SG> 15 MTPA, it can completely eliminate the imports of gypsum, meeting the projected demands of cement and wallboard industry.

Recommendation for processing of SG
One of the major impediments for selling SG to cement and wall board industry is the high moisture content of SG (10-12 per cent), when it is generated at FGD plant. The cement industry (especially grinding units within a distance of 300 km) and paperboard industry traditionally use mineral gypsum, which contains a moisture content of <3 per cent. Hence handling, storage and processing of mineral gypsum is easy as compared to SG at these locations. This is one of the reasons for giving first priotiry for Mineral Gypsum by cement plants. Because of higher moisture content in SG, the logistics costs are higher for SGsts and also additional costs for drying by customers.

Action plan
It is expected that, over next five years, the availability of SG from power plants can become a major source of gypsum for cement industry. It is necessary for cement industry to initiate actions, the merits and demerits for its usage are elaborated below.

Merits: As the purity of SG is better than indigenous mineral gypsum and also it is in powder form, the usage factors such % consumption and specific energy consumption are favourable for it usage. SG is expected to have a more consistent quality as compared to Mineral gypsum. SG can partially or fully replace usage of Phospho-gypsum. As SG is a disposal problem for power plants, the landed costs of SG at cement plants will be attractive.

Demerits: As SG contains a moisture of 10-12 per cent, unloading and handling, storage and dosing systems of existing plants may require a revamp.

Suggested initiatives for SG utilisation
As utilisation by SG by cement industry is a win-win situation for both power and cement sectors, following initiatives may be considered:
Utilisation of SG for cement production is a new concept for Indian Cement Industry. Hence, as a first step, cement producers to carryout laboratory testing of SG from FGD plants, which are in operation and access its technical feasibility for its usage.
Once it is technical is feasible, Cement companies should carry out plant trials for arriving at the optimum levels of SG utilisation in their plants.
The Plant specific quality requirements of SG can be intimated to the Identified power plants from where SG can be supplied.
Short and long term contracts can be worked out between Cement & Power plants for supply of SG.

Common initiative
As limestone is required for FGDs, Integrated cement units can have an understanding with power producers about supply of Limestone to FGDs and in return and receive SG from Power producers. This will benefit both the sectors as it optimises the landed costs at both ends.

This initiative may require revision of conditions of ML for Limestone by cement companies and hence, a joint initiative to be taken up by CMA and CEA for representation with concerned departments of State/Central governments for relaxation of ML conditions and also for incentives for cement industry for reducing the problem of disposal of SG. Cement and power producers can also jointly explore the possibility of installing SG dryers at Power plants to reduce the handling, transport and dosing problems associated with wet SG.

Conclusion
For next five years, problem of disposal of SG generated from FGDs of power plants is going to be a major issue for power plants. It is an opportunity for cement industry for utilisation of SG in cement production and usage of SG in cement, not only reduces/eliminates the disposal problem but also substitute the mineral gypsum (imported/indigenous), used by the industry. Considering an availability of 15 MTPA of SG in the next five years, it can sustain a cement production capacity of about 350 MTPA. SG can act as import substitute and also conserve the scare resources of mineral gypsum in the country.

About the author:
Satyamurthy V. Yenamandra (Y. V. Satyamurthy) is a consultant for cement and mineral industries with over 35 years working experience in ACC and Reliance Cement in functional areas of process and project engineering, project management and training.