The efficient alternative Fuel. Petroleum coke‐ for Steam Boilers
The efficient alternative fuel -Petroleum coke
Abstract:
Petcoke is not just a refinery’s by-product any more, but a real alternative cost effective fuel that can be successfully fired in AFBC boilers either alone or in blends with coal. With its specific ash content, which is typically lower than 1 percent and very high sulfur,petcoke is an effective source of energy increasing plant economy.
MAGO THERMAL New Steam Boiler and Retrofitting Kits make your existing steam boiler suitable for efficient combustion of petroleum coke.
Mago Thermal delivered efficient and fuel-flexible petcoke-fired boilers / Retrofitting kits to its many Customers in Paper, Texitle, Milk Processing, Tyres industries. All these customer Were using earlier Indian / Imported Coals and their fuel cost reduced 25 %- 33 % based on their steam boiler capacity and location of plant.
Mago Thermal designed AFBC furnace on Unbrunt Carbon / Ash Recirculation Principal and showed very good results whereas the fuel efficiency is concerned.
1. Fuel Origin and Characteristics
Petroleum coke (Petcoke) is a carbonaceous solid derived from oil refinery coker units or other cracking processes. In spite of the fact that it is not considered to be a “fossil fuel” like bituminous coal or lignite, the chemical composition of petroleum coal is mostly composed of elementary Carbon ( usually over the 85% C dry ). The most attractive feature of this fuel is its high heating value and very little ash content.
Petcoke has become a competitive fuel, which has made it very popular in Process / Power boilers
Petroleum coke basically comes in two types named the fluid coke and delayed coke.
Fluid coke is typically left with / contains about 5 % volatiles and because of its small particle size is not generally very suitable for combustion in AFBC boilers.
Delayed coke contains 8 – 15 % volatiles and sulfur in the range of 3 – 8 %, although some petroleum cokes are available with less than 1 % sulfur. That’s why petcoke is an ideal partner for AFBC/ FBCC Combustion technology, since the emission control concept and the sulfur capture are the key issues when calculating a plant’s feasibility.
Mago Thermal is offering Flue Gas Desulphuriser to control Sulfur.
The ash content of petroleum coke is typically very low, usually less than 1-2 %.Because of the low ash content the additional bed material is required. The heavy metals content of the ash is generally high with Vanadium and Nickel contents ranging at 500 – 3000 ppm each, although petcokes with >10,000 ppm Vanadium also exist. Because of the fuel particle characteristics, inherent moisture is very low in petcoke. Most of the water is present surface moisture. If allowed to drain for several days, the moisture content will typically stabilize less than 5 %
PETCOKE PARTICLE SIZE DISTRIBUTION IS AS UNDER-
+ 50 mm : 03.22%
25-50 mm : 10.73%
12.5-25 mm : 11.50%
6.4-12.5 mm : 11.17%
less than 6.3 mm : 63.68%
PETCOKE SPECIFICATIONS-
Calorific value on air dried basis : 8200 kcal/kg
Fixed Carbon : 87-89%
Volatile Matter : 8-9%
Ash : 1% maximum
Moisture : 4-8%
Sulphur : 4-7%
There are a few important aspects which should be taken into consideration when petroleum coke is to be fired in FBC boilers.
Vanadium and Nickel
Vanadium content in petroleum coke is very important because of the possibility of generating low-temperature-melting compounds that can cause fluidization problems. Low melting compounds can also deposit on backpass heat transfer surfaces and result in under-deposit corrosion.The Vanadium content may have a relatively small effect on the sintering process in CBF technology. The presence of a large content of CaO (a calcination product of limestone) in the bed material causes Vanadium capture and ties it up in higher melting compounds, minimizing or eliminating potential deposition problems.
Limestone
Because of the extremely low ash ( <2% ) and high sulfur contents of petcoke, limestone is used in the considerable majority of bed materials of the AFBC. Limestone sizing is very important for desulphurization, not only for efficient sulfur capture and effective fluidization,but also fuel mixing, uniform temperatures and heat transfer.Limestone can be considered an attractive addition when firing high Vanadium petcoke,because of its Magnesium content which has vanadium-absorbing properties.
Bed material quality
Inert bed material is typically fed as a separate material such as Silica sand / Crushed Refractory. In cases where calcium based deposition is a problem, inert bed material is very helpful because it breaks up and weakens the deposit. The sorbent and sulfur capture products typically become the main constituents of the bed material in case of boilers firing 100% petcoke. In case of coal and petcoke co-firing the positive coal ash characteristics may prevent Recirculation system deposit problems with the same efficiency as fresh sand addition.
Alkalis
The alkali content of petroleum coke is very similar to coals unless it is increased by the process or storage. In this case the same precautions as with other fossil fuels should be considered for alkalis in petcoke.
Low NOx emission and sulfur capture
Because of their very low volatile content the circulating solids have a high fixed carbon content, resulting in inherently low NOx emissions. Due to a relatively low combustion temperature in the furnace (approx 850-950 °C) the NOx emission limit can often be achieved Due to the effective fuel burnt-out process and utilization of limestone, desulfurization process is very efficient and allows to achieve low SOx emissions.
The high retentions were achieved with petcoke and petcoke-coal blends in the AFBC combustion. By the principle desulfurization takes place in the furnace with the optimum burning temperature which is suitable for that process. It is possible to achieve more than 98% of sulfur capture. Moreover, the fuel burn-out process affects the low CO emission levels, which causes a reduction in UBS losses in ash. The primary reactions involved in the generation and reduction in SO2 emissions with calcium-based sorbents are:
S + O2 → SO2 formation of SO2
CaCO3 + heat → CaO + CO2 calcination of limestone
SO2 + CaO + 2O2 → CaSO4 sulfur capture
2. Market situation
The utilization of petroleum coke has already made it a good substitute for the traditional fossil fuels such as coals. The main application of petroleum coal is an energy source and as well as a carbon source. The energy sources are power plants and cement industry, others are used in the steel industry. The increasing demand for petroleum coke of heavy crude oil has constantly been growing. In 2000, the global production of petroleum coke was 80.815 MMTPA, which increased to 123.058 MMTPA in 2010. In 2015, the production is expected to 161.271 MMTPA.
The main sources of Petroleum Coke are located in Brazil, Middle East, China, India, Spain, Taiwan, Canada, USA, Venezuela and South Africa.
3. Design requirements
Petroleum coke is a fuel that has long been considered an ideal fuel for the circulating fluidized bed combustion technology.Despite the low volatile content of petcoke, combustion efficiency has been quite good in a FBC, thus there is no need to design for higher combustion temperatures to improve combustion efficiency. Lower temperatures improve the emissions performance as well as reduce the potential for agglomeration and deposition. The recommended furnace design temperature should be typically in the range of 850 – 900°. However, all process-related
parameters are analyzed case by case. The desulphurization takes place inside the furnace of a FBC boiler. The combustion process is conducted with 20 % excess air. The amount of primary air which is supplied to the furnace through the grid nozzles as fluidizing air, is typically higher than total combustion air for coal. Secondary air is supplied through two levels to the furnace.
To stabilize / maintain a boiler’s operating process stable the minimum technical load in a FBC boiler is even less than 40% MCR. That value can be safety guaranteed when firing petroleum coke. Because of the low volatility of petroleum coke, combustion tends to take place lower in the furnace. This means that lower furnace temperatures will tend to be higher at low load, than at a comparable load when firing more volatile fuel. Higher low load combustion temperatures provide a stable operation over a wider load range. The safe boiler operating load range for maintaining constant flue gas emissions, is dependent on the particular emissions requirements.
4. MAGO- RFBC Technology
Fuel is burned as it mixes with other materials, mostly limestone in case of petcoke firing and ash if coal is present, in an upward flow of combustion air. Because of high turbulent and excellent mixing sorbents, air and fluidized material in the furnace it is possible to achieve a uniform temperature throughout the furnace chamber and long residue time of fuel particles.
Combustion efficiency is increased by Re-circulation of fines arrested by solid separators. Fluidized material and unburnt fuel go out of the furnace to Ash collector and return to the lower part of furnace. In this part of combustion chamber the limestone is fed as a sulfur sorbent.
Furnace temperature is kept in a suitable range of 850 – 950°C.
The main features showing advantages for utilities to use RFBC technology:
1. High combustion efficiency:
- long solids residue time in the furnace
- Furnace temperatures are below the ash softening temperature
- Due to excellent fluidization and recirculation of fuel with bed material the
- opportunity of burning up particles of fuel is remarkable
2. Excellent combustion stability over a wide boiler load range
3. Relative low concern of low-melting temperature Vanadium and Nickel compounds
4. Fuel flexibility
The opportunity to fire a mixture of fuels (a good ash analysis of fuels is required )
5. Emissions well under stipulated Norms.
6. RE-Firing of Fly Unbrunt Carbon
5. Utility Scale Boilers Coal and Petroleum coke fuel flexibility
The design of AFBC boilers in the range of 50 -100 TPH is supported by MAGO THERMAL extensive experience with under bed / over bed utility boilers and by MAGO THERMAL’s extensive Thermal Engineering experience base.
MAGO THERMAL RFBC is proven system best to meet the challenge of Power /utility AFBC boilers.
In some of MAGO Projects, Mago has achieved removal over 95% of the sulfur dioxide and still improving the overall economics and environmental performance.
It is equipped with a polishing scrubber employed to minimize reagent consumption while firing petcoke containing up to 8.0% sulfur. The relatively low furnace operating temperature of about 950 ºC results in lower nitrogen oxide emissions compared to conventional coal-fired power plants.
6. Petcoke Projects in India
Over the past few years MAGO THERMAL has delivered Power / Utility boilers Petcoke firng solutions to Tyre, Paper, Textile, Food and many more Industries in INDIA.
MAGO THERMAL focus was in both cases the same. To utilize local sources and to deliver units tailored to the very precise customer expectancy. RFBC technology selection allowed the investors to utilize local fuel or sorbent sources significantly lowering the operational cost of the plants. Petcoke-fired boilers typically have a multi-fuel design where a blend of coal and petroleum coke may vary in different proportions.
CASE STUDIES ( attached herewith)
- MAGNUM PAPER
- GARG DUPLEX
- NAHAR INDUSTRIAL
7. Conclusion
Petroleum coke is an opportunity fuel due to its high carbon and energy content it appeared to be an ideal fuel for RFBC technology. The multi-fuel AFBC boilers can be designed / Retrofitted for both coal and petcoke are an interesting alternative allowing a plant to operate at high efficiency and economy. The RFBC technology provides customer-tailored solutions for utility scale and industrial size boilers.