Dr outying

If recent media attention is anything to go by, the level of interest in low rank coal drying is gaining momentum: and it should.

Coal has made up almost half the increase in energy use over the last decade.1 The global demand for coal is forecast to increase by more than 1 billion tpa by 2035, despite concern over its role in global warming.2 Demand pressure, driven primarily by the growth in electricity demand in emerging nations, is consequently driving up the price of thermal coal. This makes lower rank coals

increasingly attractive, both in terms of price and as an energy security option for existing and new power plants.

While low rank coal is worthy of this increased attention, it presents the following challenges:

l High moisture content.l Significant risk of spontaneous combustion

compared to bituminous coal.l Inefficient transportation cost due to high water

content.

Adam Giles, Environmental Clean Technologies Ltd, Australia, explains the fundamentals underpinning brown coal densification and how a new low rank coal drying process aims to deliver the 

first large-scale demonstration project in Australia.

April 2012 | Reprinted from World Coal |

Further, power plants built to handle bituminous coal typically cannot accept a straight feed of low rank coal. Blending becomes necessary to homogenise the feedstock.

This combination of factors means that low rank coal struggles to trade in the export market. Those that do trade are discounted relative to their lower net calorific value.

Where low rank coal is used to generate electricity, it emits more CO2/MWh than bituminous coal, attracting the ire of those keen to mitigate emissions, while setting off alarm bells in the minds of financiers

fearful of exposure to future CO2 pricing.

The practical impact to low rank coal is that it needs to be dried if it is going to substitute bituminous coal in power plants. To achieve this, low rank coal needs to be dried cost-effectively, while also dealing with the risk of spontaneous combustion, to avoid costly transport measures. Environmental Clean Technologies Ltd (ECT) has developed a low rank coal drying process named Coldry, that offers a solution for drying low rank coal. The process is also a low-cost solution, due to its use of “free” low-grade waste heat.

Techno-economic evaluationIt is a simple equation: raw coal cost + beneficiation cost = cost of production. There are two distinct application subsets:

l Export. If the dry coal is headed for export, the margin over cost of production typically needs to provide a return on investment of 15% or more, and a payback period of between 4 – 7 years on the capital cost of the plant itself. For the end user, the delivered price then needs to be equal to or less than the cost of alternatives.

l Mine-mouth. If the dry coal is heading straight into mine-mouth power plants, then the cost of production needs to be less than the delivered cost of alternative coals or other fuels.

Drying brown coal in a manner that also reduces the risk of spontaneous combustion is key to export opportunities. Brown coal power plants typically sit on large, captive brown coal reserves, though are unable to sell them to the open market due to the issues highlighted above.

Applying Coldry, brown coal mines are able to produce and sell more than their mine-mouth power plant consumes, thus generating additional revenues from otherwise stranded reserves.

The following section outlines the technical and economic aspects of the Coldry process.

Technical overviewColdry is an evaporative drying process based on brown coal densification.

BackgroundBrown coal densification research gathered considerable pace during the 1980s with a collaboration between Melbourne University’s Department of Organic Chemistry and Conzinc Riotinto of Australia’s (CRA) Advanced Technical Development group.

The research identified and explained the physical and chemical

Figure 1. Loy Yang “A” and “B” power plants consume around 30 million tpa of brown coal. Coldry aims to export an initial 2 million tpa to Asian markets.

Figure 2. Newly extruded Coldry pellets undergoing conditioning before delivery to the packed bed dryer.

| Reprinted from World Coal | April 2012

transformation of brown coal to a dense, dry, hard material when subjected to mechanical shear and evaporative drying at or near ambient conditions.

How it worksEssentially brown coal is sheared and attritioned, reducing the mean particle size and releasing water naturally held in the porous coal

microstructure forming a plastic mass. This dispersal of surface and physically trapped moisture lends itself to evaporative removal at or near ambient temperatures. However, the real benefit to drying using brown coal densification lies in its liberation of some of the chemically-bound moisture without the need for high temperatures or high pressures.

Shearing also opens fresh coal surfaces exposing reactive molecular species, which participate in new bond-forming reactions. This chemical cross-linking often results in a significant temperature rise (exothermic reaction) during attritioning.

Examples, in the case of Victorian brown coals, are the phenol groups: reactive molecular sites are involved in

1. Raw coal feed: Raw coal is screened to remove oversize and contaminants, and sized to ensure a uniform feed into the next process step.2. Attritioning and extruding: A small amount of water is added to the mill attritioner, where the coal is sheared to form a coal paste. This intensive mixing initiates a natural chemical reaction within the coal that ejects both chemically‑trapped water, as well as physically‑absorbed water within the coal pore structure. The coal paste is then further masticated, finally being extruded into pellets.3. Conditioning: The coal paste pellets are surface dried on the conditioning belt to provide sufficient green strength to withstand the transition to the next step: the packed bed dryer.4. Heat exchange: Waste heat from the co‑located power plant is recovered using heat exchange. This low‑grade energy stream is used to provide the warm air streams required to evaporate surface water from the coal pellets.5. Packed bed dryer: Incoming moist coal pellets from the conditioning belt are further dried to their ultimate moisture level within the packed bed dryer. Warm air from the heat exchangers removes the moisture rejected from within the coal pellets. The cross‑linking reactions come to completion within the dryer, increasing the coal pellets’ strength to levels sufficient to withstand bulk transport.6. Coldry pellets: This incoming brown coal has now been converted into a black coal equivalent (BCE) through the permanent elimination of structural and physical trapped water. These high energy pellets are available for thermal applications, as well as other uses.7. Pulveriser: The pulveriser reduces the pellets into finely ground coal dust, suitable for injection into a pulverised coal combustion boiler.8. Boiler: The coal is burned in excess air, producing a high temperature gas stream. The high temperature heats the water in the boiler, generating the steam needed for power generation.9. Turbine: High temperature, high pressure steam is injected into the steam turbine, which is connected to the generator. High voltage electricity is the finished product from this operation.10. Condenser: Steam exhausted from the turbine is passed into the condenser, where it is cooled to form liquid water. This liquid water is pumped back into the boiler to start the steam cycle once again. The cooling water from the condenser is now at elevated temperatures, and needs to be cooled. It is pumped to the Coldry plant for heat exchange (Step 4).11. Cooling tower: Return water from the Coldry heat exchange is now at a lower temperature, but still requires further cooling. This water is now pumped into the cooling tower, where a portion evaporates, cooling the remainder down to suitable temperatures for the condenser operation. Make up water is added to replace that which was lost to evaporation.

Figure 3. Schematic layout of integrated Coldry-fired power plant.

April 2012 | Reprinted from World Coal |

bonding reactions that release chemically trapped water. As the pellets densify, the newly formed structure shrinks, resulting in a significantly more compact microstructure compared to the original coal (Figure 2). This new structure reduces the propensity to self-heat to that of a typical bituminous coal.

Developing the processThis is where the Coldry process builds upon the early work by the CRA and Melbourne University.

The original research proposed air-drying the extruded pellets, but this approach had issues:

l Evaporative drying based on ambient conditions is highly variable. It is therefore almost impossible to integrate within a commercial supply chain.

l A 75 ha./million tpa of production lay-down area would be needed for evaporation.3

l Using high-grade energy sources such as gas or steam to facilitate drying would not be cost-effective.

The development of the Coldry process to its current state therefore focused on:

l Primary processing: shear, attrition and extrusion of the coal to form pellets with the necessary microstructure destruction and cross-linking reactions that produce a dense, dry, hard pellet.

l Controlled drying: low temperature, cost-effective evaporative drying to overcome the variability of atmospheric drying.

The result is the Coldry process (Figure 3).

OutcomeWhen applied to lignite and some sub-bituminous coals, the Coldry process produces a feedstock in the form of densified pellets that are of similar calorific value to many bituminous coals, while significantly reducing CO2 emissions/MWh compared to its original brown coal form.

Changes in the coal result in a pellet with a similar self-heating profile to commonly traded bituminous coals out of Australia, making it ideal for:

l Export to the thermal coal market. l Downstream chemical processes. l Feedstock for coal-to-liquid (CTL)

and coal-to-gas (CTG) processes, since the process retains the volatile matter that lends itself to coal conversion processes.

The Coldry process has been tested successfully on a wide range of low rank coal samples from Australia, China, Greece, India, Indonesia, Mongolia and Poland.

The single most important distinguishing factor between Coldry and other technologies is its use of low temperature or “cold” drying. The temperature range for drying is between 35 – 45˚C (95 – 113˚F). This forms the basis for the synergy with existing mine-mouth power plants and avoids the need to incur OPEX by generating process heat or by calling on high-grade heat, from other processes, that may have higher value in other applications.

Figure 4. The Coldry pilot plant in Bacchus Marsh, Victoria, Australia, has formed the basis for developing the process around brown coal densification.

Figure 5. The Coldry process uses equipment from the brick making industry, adapted to the characteristics of the brown coal, to achieve the primary processing steps of attrioning and shearing.

| Reprinted from World Coal | April 2012

Integration with supporting power plantAs mentioned, key to the economic performance of the Coldry process is its integration with a co-located power plant, providing the following benefits:

l Virtually free, low-grade waste heat to facilitate evaporative drying and minimise purchased energy input.

l Reduction of evaporative loss of water through power plant cooling towers.

l Ability to optionally capture water extracted from brown coal as a result of the Coldry process.

l Improvements in turbine efficiency for the power plant courtesy of the condenser cooling return water following heat exchange.

Net energy footprintThis is extremely important in evaluating the economic performance of any coal drying technology. The “uplift” in net calorific value has to be greater than the purchased energy consumed to dry the coal; otherwise it becomes a negative-sum game.

Depending on operational modes, the Coldry process uses between 100 – 150 kWh of electrical energy to run the plant to process 2.25 t of raw coal producing 1 t of Coldry pellets.4,5 That electrical energy is derived from around 1.2 – 1.8 GJ of raw coal energy.6 Therefore, the Coldry process has a net energy footprint, in the case of Victoria brown coal, of 4 – 5 GJ/t.

Net CO2 footprintThe above can be extended to net CO2 footprint. If a coal drying technology is being considered for adoption in a CO2 priced market, then its exposure from process emissions needs to be understood.

Depending on the mode of operation, and assuming combustion of the Coldry pellets in a new coal-fired plant with 43% efficiency, ECT has calculated net savings of

between 3 – 6 t of CO2/t of CO2 associated with electricity consumed by Coldry production. This provides a net beneficial CO2 footprint compared to business-as-usual; i.e. burning the wet coal in an inefficient brown coal power plant.

Economic overview

Indicative case study: Victoria Coldry projectIn November 2011, ECT commenced the Design for Tender (DFT) for its first commercial-scale Coldry plant (Table 2 and Figure 6):

l Location: Loy Yang mine and power plant, Victoria, Australia.

l Capacity: 2 million t finished product.

l Application: export to the Asian market.

Table 1. Typical Coldry specification

Feature Brown coal1 Coldry pellets

Black coal2 (higher moisture)

Black coal2 (lower moisture)

Moisture (%) 60 12 15.5 3.3

Net calorific value (MJ/kg) 8 24 20 24

Notes1. Coal source: Latrobe Valley, Victoria, Australia.2. Source: CSIRO.

Figure 6. The Coldry plant has a modular basis for design. This example module would be integrated into the host power plants condenser return water to access the necessary low-grade heat.Coal input: 765,000 tpa at 60% moisture.Output: 340,000 tpa at 12% moisture.

Key:1. Coal feed hopper.2. Primary processing and pelletisation.3. Conditioning.4. Drying.

April 2012 | Reprinted from World Coal |

ConclusionColdry is ideally suited to drying low rank coals situated adjacent to a mine-mouth power plant. The plant can be deployed as a retrofit to existing power plants or tailored around the deployment of a new power plant, providing increased mutual efficiencies, decreased CO2 emissions compared to ROM lignite and increased revenue streams.

Key to achieving cost effective drying is the low energy input courtesy of low-grade waste heat

recovery coupled with the process of brown coal densification initiated through the finely tuned yet mechanically robust primary processing train.

Rounding off the process is the unique, patented packed bed dryer that provides control over the drying conditions to allow programmed production delivering into a commercial supply chain in addition to fuelling the host power plant.

Economically, the process aims to deliver a cost of production that is competitive with bituminous coal, providing owners of typically stranded brown coal assets the opportunity to sell into the growing thermal coal market.

And last, but increasingly more important, Coldry provides emerging nations with an energy security option that strikes a balance between affordable base load power and CO2 mitigation compared to the alternative of simply burning the wet, CO2 intensive low rank coal.

Notes and references1. International Energy Agency,

World Energy Outlook 2011.2. International Energy Agency,

World Energy Outlook 2011; New Policies Scenario.

3. Based on stockpile of 60,000 t cleared every 16 days, produced from 60% moisture ROM coal.

4. Refers to typical ROM Latrobe valley coal containing 60% moisture.

5. Finished product moisture range 10 – 14%.

6. Assumes brown coal power plant with 30% efficiency.

SourcesJOHNS, R.B.; CHAFFEE, A.L; HARVEY, K.F.; BUCHANAN, A.S. and THIELE, G.A., “The Conversion of Brown Coal to a Dense, Dry, Hard Material”, Fuel Processing Technology 21 (Elsevier Science Publishers; 1989), pp. 209 – 221.

Table 2. Indicative project economics

Expense Cost (AU$/t)

Operational expense 13

Feedstock (approximate) 18

Freight to port 20

Total cost FOB Australia 51

Value (approximate) 110

Potential margin 59

Simple payback 3 – 5 years

| Reprinted from World Coal | April 2012