3.3 Description Coal Drying Plant
Great River Energy (GRE) has developed and patented a unique coal drying and coal upgrading technology, termed DryFiningTM, which enables improvement in the performance efficiency of a coal fired power station and thereby reduce the emissions and the station’s carbon footprint. The DryFiningTM concept involves utilizing waste heat which may be available in a power station to partially dry the feed (raw) coal in a fluidized bed dryer (FBD). Using gravitational segregation feature of a fluidized bed and a special design of the coal dryer, denser materials present in the coal such as pyrites, small rocks, and sands are segregated and separated from coal, thereby improving the quality of the coal fed to the power station. DryFiningTM was developed in conjunction with the first round of the U.S. Department of Energy (DOE) Clean Coal Power Initiative (CCPI) and with the additional support of the National Energy Technology Laboratory (NETL) based in Morgantown, West Virginia. Over the last decade GRE has proved technical feasibility of the concept firstly in a proof-ofconcept 2 t/hr pilot-scale coal dryer, then a 75 t/hr prototype-scale dryer integrated in the GRE Coal Creek Station, located near Bismark, North Dakota, USA, and finally in the complete conversion of this 2 x 600 MW mine-mouth station to DryFiningTM the whole coal feed for the power station. The commercial coal drying system at Coal Creek includes four commercial size (125 t/hr) moving bed fluidized bed dryers per unit, crushers, a conveying system to handle raw lignite, segregated, and product streams, particulate control system, and control system. The system is fully instrumented for process monitoring and control. Systemwas completed in December 2009.
The resultant reduction in emissions, including NOx, SOx, mercury and carbon dioxide as well as the increase in power production per ton of coal have confirmed the technical and economic viability of this concept. Under the conditions of the DOE Initiative, GRE is able to promote the use of the DryFiningTM technology elsewhere in the world. Power stations which fire high-moisture coals will find particular advantage with the implementation of DryFiningTM technology.
3.3.1 Sources of Waste Heat
In as much as Loy Yang A Power Station units fire very high moisture fuel, Loy Yang Power commissionedin 2010, to perform an initial pre-feasibility study (Phase 1A) into the application of Coal Drying (DryFiningTM) at Loy Yang A Power Station. The objective of that evaluation was to perform a high level process analysis to allow Loy Yang Power to make an informed decision for a possible Phase 1 study.
The analysis of the Loy Yang “soft” lignite (brown coal) provided for that study showed that Loy Yang coal is geologically much younger than the Great River Energy’s “hard” lignite from North Dakota, USA. At 60 wt% (or 1.5 kg of water per dry kg of coal) Loy Yang’s brown coal is also significantly higher in moisture content compared with Great River Energy’s 38 wt% coal (0.61 kg water per kg).
Reducing the moisture in Loy Yang’s coal feed has therefore the potential to provide significant improvement in power station performance. In that study the size of the coal drying plant was based on drying coal for one complete 500 MW plant (Unit 3) at LYA.
The major source of waste heat for DryFiningTM on most units is from the flue gas system. This heat is extracted by locating flue gas coolers (FGCs) in the flue gas stream at a convenient location downstream of the air heaters and particulate removal equipment.
LYA operates flue gas temperatures at typically between 170°C and 190°C at the ID fan inlets. However, the operating philosophy limit is not less than 160°C, which is just 20°C to 25°C above the sulphur dew point of that of the stack gas.
Despite the fact that corrosion due to sulphuric acid condensation in the flue gas system has been a serious problem in the LYA operating history, that study assessed the potential available at a stack temperature on the estimated dew point with a margin of 5 ºC above that point. Allowing for a design margin of 5°C, a lower stack gas temperature of 140°C may be used. This equates to the exit temperature of the flue gas cooler. The resulting heat available for coal drying then suggested a fuel moisture reduction from 60% to 54% is possible.
Reduction of the moisture content to 54% results in a reduction of fuel flow to the boiler and through the majority of the coal handling system. The reduced moisture load on the boiler results in an increased boiler efficiency from 72% to 75.4%. Additional operational benefits expected are reduced flue gas flows through the back-end equipment, reduced loads on coal handling and processing equipment (especially the pulverisers), reduced loads on the ID fans and air handling equipment (air preheaters, ESP, ducts, etc.). Most importantly the carbon foot print of the LYA is reduced due to the resultant reduction in emissions.
A summary of the coal drying results from that study is presented in the Table 3.7.
|Fuel Moisture Content||Moisture Removal||Firing Rate (kg/s)||Boiler Efficiency||Gas Flow Reduction||HHV (MJ/kg)||CO2 Emission (kg/kWh) Gross|
The data provided by Loy Yang Power and collected from the previous 2010 coal drying study forms the design basis for the coal drying base model for this study.
The size of the coal drying plant for this study’s model is determined by
- the quantity of flue gas required by the 5000tpd PCC plant, and
- limiting the existing stack gas temperature to 140°C.
The boiler plant of LYA is assumed to be able to operate at the modelled design conditions without any adverse effect, as in the case of the 2010 study.
From the design basis described above, a two-stage FGC was considered in this study.
The first stage, cooling the flue gas stream from 181°C to 140°C, would operate above the acid dewpoint temperature.
The second stage will cool approximately 37% of the flue gas flow down to 70°C (conditions as required by the 5000tpd PCC plant) and, thus will operate below the acid dewpoint temperature, requiring corrosion-resistant alloys to be employed for its construction.
A higher-grade heat may also be obtained from the flue gas upstream of the air heater.
The circulating water system is also a potential source of waste heat for DryFiningTM, although it has to be used indirectly because of its relatively low temperature. This can be done by transferring the heat into the boiler system via fan inlet or outlet coils. The nature of this benefit requires more detailed heat integration modelling. As the units at LYA are equipped with cooling towers, using the hot water fed to these towers may not only benefit the DryFiningTM process but also lessen the heat rejection load on the cooling towers.
As coal moisture is reduced, the efficiency of the plant increases resulting in a decrease in coal feed rate and emissions. A trade-off study is recommended to compare the cost of equipment required to reduce plant CO2 emissions versus the capital and operating cost of thesystem and price of the CO2 emission allowances.
3.3.2 Fluidized Bed Coal Dryer
A coal dryer developed by a team led by GRE employs a moving bed fluidized bed dryer fluidized by air. The fluidization velocity is optimized to minimize the flow rate of fluidizing air and fan power. The dryer operates at low drying temperatures (110°C to 120deg;C) at near-atmospheric pressure. The heat to the dryer is supplied by the in-bed heat exchangers (typically 65 to 70% of the total heat input) and by the fluidizing air (30 to 35%). A fluidized bed coal dryer is designed to maintain the temperature of the coal bed in the 45°C to 50°C range, while the maximum bed temperature is restricted to 55 deg;C. The high heat and mass transfer characteristics and high throughput of a moving fluidized bed result in a compact design of the drying system. Due to a low operating temperature of the dryer, no exotic materials are needed, and the dryer is manufactured of carbon steel.
The fluidized bed coal dryer is designed for a coal crushed to a mean size of 6.4mm. Therefore, new coal crushing equipment would need to be installed at LYA since the existing coal crusher only prepares coal to 75mm.
Using gravitational segregation features of a fluidized bed and special features of the Mark II coal dryer design, denser materials present in the coal such as pyrites, small rocks, and sands are segregated and separated from coal, thereby improving the quality of the coal fed to the power station. For LYA, the brown coal has lower sulphur; ash and mercury content than the GRE lignite, but reportedly has excursions in sodium, iron, and total ash. Therefore, segregation feature of DryFiningTM may be desirable to Loy Yang for mitigation of furnace slagging and heat recovery area (HRA) fouling.