Selecting Boiler Technology for Multi-Fuel Firing
Fuel availability stands out as one of the biggest challenges for an energy intensive industry. With weak or expensive grid, most of the energy intensive industries had to resort to captive power generation. However, with recent volatility in fuel supply and costs, industrial investors had to look at multi-fuel options.
As an investor, who is looking at investment in a mid sized power project, he has to look at the risks he carries, safeguards to put in place to mitigate them. The investor is stumped with the plethora of options at each stage, be it:
Development risks, including:
- Statutory clearances
- Financial closure
- Land and rehabilitation
Construction risks, like:
- Cash flow
Technical risks, including:
- Developer/contractor´s competence and experience
- Project schedule
- Contractor´s financial strength
Operations and maintenance related risk
- Heat rate guarantees
- Manpower cost
- Plant performance
- And last but not the least, marketing and revenue related risk.
For a power project to succeed, an investor looks at the financial viability of the project. Two foremost factors on the investor´s mind are the project cost and the operating cost. Project cost comprise of capital cost, interest cost and the development cost. The second most important parameter being the operating cost of the power plant, which will enable him to forecast the cash flow. In a power plant the main operating costs being station heat rate, manpower cost and the cost of consumables.
The investor is concerned about the return on his investments, which come from the basic technical feasibility of the project and the technology being utilised. His return on investment also depends on the guarantees that he can get on the project cost and how well he can estimate and mitigate the variations. The performance guarantees are far more important than the project cost guarantees. Performance variations can bleed income from the project for its lifetime, which is typically about 20-25 years. The IDC and the returns starting to accrue come from the guarantee of the schedule he sets for the project and how it is adhered to. Generally, based on all these parameters and risk taking abilities of the investor and his bankers, the decision is taken whether to go ahead with the project on a packaged route of to pass the risk to a reputed EPC contractor.
An EPC contractor takes the entire risk of construction upon himself. If the EPC contractor is also a technology provider like a boiler manufacturer in the case of power plant, then even the technological risk is totally on to him. If the EPC contractor is ready to undertake long term operations and maintenance of the power project then the O&M risks is also passed on to him, leaving only the development risk and part of commercial risk in the developer and banker´s scope. The commercial risk can be further diluted with a financially sound EPC contractor and having watertight contract in place, leaving only the development risk in investor´s scope.
Role of Technology
In today´s context of fuel uncertainty, technology plays a vital role especially regarding boiler choice. One has to look at aspects like:
Suitability of various kinds of fuels
Boiler pressure and temperature
Fuel firing limitations
Boiler efficiency and availability
Physical characteristics of the fuel should also be accounted for in the designing process. This is extremely important, in case biomass is being considered as a main or supplementary fuel. Physical characters include size, bulk density, flowability.
Chemical constituents such as chlorine (elemental chlorine and not chlorides in ash) as chlorine in biomass can cause corrosion problems. So these factors must also be considered while designing the system. Alkali content (Na2O+K2O) in fuel leads to problems like slagging and fouling.
Boiler efficiency depends on moisture content in the fuel. Combustion efficiency depends on ash content and excess air. High excess air increases combustion efficiency however it also increases dry flue gas losses. NOx generation is a function of temperature, staging of air and excess air percentage.
If moisture content in fuel is high, in bed tubes can be avoided. In case most fuels being considered are solid fuels like mix of different types of coal, lignite or petcoke the options on technology can be a little easier.
Circulating Fluidised Bed Combustion Technology
Uncertainty regarding availability and reliability of single fuel type, stringent emission norms, constraints of firing multiple type of fuels in pulverised coal fired boilers and need of additional capital intensive accessories like coal mill, FGD, etc. led to development of Circulating Fluidised Bed Combustion Technology (CFBC) design. CFBC technology in today´s time of high fuel uncertainty and volatility can be considered as a boon to power and process industry requiring power and process steam.
CFBC is a fuel flexible technology, which can handle variation in GCV from 1800- 8000 kcal/kg, ash 5-65 per cent and moisture from 1-45 per cent. The turbulent bed, which is operating at 4-5.5 m/s, is able to enhance the fuel burn ability by rapid mixing of fuel with hot bed material resulting in efficient carbon burnout.
The CFBC technology has versions that have wider multi-fuel firing capability including:
Anthracite, bituminous, sub-bituminous, lignite (Neyveli/Kutch/Barmer) and high-sulphur coal.
Washery rejects, char.
Petroleum coke (petcoke):
Other renewable fuels:
Sludge, oil pitches, biomass, agro-wastes and refuse derived fuel.
The new generation IRCFBC technology can easily cater to fuel with:
- Moisture content up to 60 per cent, e.g., in lignite, peat, sludge
- Ash up to 76 per cent, e.g., in washery rejects, char.
- Sulphur up to 8 per cent, e.g., in lignite, petcoke.
- Volatiles, as low as 1 per cent as in petcoke, washery rejects, char, etc.
- HHV as low as 1500 Kcal/kg as found in washery rejects, char, etc.
Factors to be considered while choosing boiler technology
Here is a list of few important factors that must be considered while choosing boiler technology.
Compact, economical design and construction
If the boiler technology design has lower furnace exit gas velocity and requires significantly less building volume, say by relying on internal recirculation, the design can eliminate J-valves, loop seals, high-pressure blowers, and soot blowers, which makes the boiler compact and economical on lifetime costs.
Separation in stages for better bed inventory control
If the design has optimal stage wise particle separation system, it will help to provide high-solids loading and a uniform furnace temperature profile. The benefits of this include superior combustion efficiency, high operational thermal efficiency, low emissions, low maintenance, low pressure drop, and high turndown, resulting in improved overall plant performance and particle collection efficiency as high as 99.8 per cent for better inventory control. The separation technology must be of fit and forget type.
Performance in varying and low load conditions
With effective bed inventory and temperature control through controlled solid recycle rate from MDC to furnace you get better performance and operation of boiler. Turn down ratios as high as 1:5 can easily be achieved in some designs.
Start up and shut down time
Some designs have much lower refractory heat retention as compared to other CFBC designs. This allows quick start and shut down of the boiler.
Boiler designs with higher velocity of gasses leaving furnace to achieve solid separation like using centrifugal action generally have higher pressure drops thus higher auxiliary consumption. Boiler designs with lower velocity of gases have comparatively negligible pressure drop and much lower auxiliary consumption.
Availability and lower maintenance
Maintenance of boiler is directly related to the quantum of refractory the boiler design carries. Boiler design with least level of thick, uncooled refractory and no hot expansion joints, reduces the expenses and the lost time associated with refractory maintenance. If the particle separators and super heater enclosures are constructed entirely of top-supported, gas-tight, all welded membrane tube walls. These systems do not require hot expansion joints, the maintenance over the lifetime of the boiler can be minimised substantially.
Some boiler designs ensure that there is no soot formation and uniform furnace temperature profile is maintained. Erosion is a major cause of maintenance problems in CFBC boilers due to high solid load in the flue gas. The severity of this erosion is exponentially related to the velocity of the flue gas through the system. While some CFBC designs have the particle separator based on an extremely high flue gas velocity. The high velocity provides the energy needed to efficiently disengage the particles from the flue gas. Other designs have particle separator designed to operate efficiently with much lower flue gas velocity (5 to 6 m/s) at full-load operating conditions. By operating at such a low gas velocity, the potential for erosion in these designs is reduced significantly.
Considerations in multi fuel firing
The lowest calorific value like washery will call for higher amount of fuel feeding into bed. The feeders need to be sized for 1:10 turndown.
The furnace cross section is decided by the maximum flue gas volume generated by respective fuel. In case of lignite or biomass with high moisture, low calorific value fuel, the flue gas generated will decide the cross-section dimension of furnace. In addition to this the ESP, ID fans need to be sized for handling higher gas volumes.
Higher ash content in fuels enhances the heat transfer rate in furnace. To maintain solids mass flux in furnace, the excess solids are taken out of system through bed ash cooler, located beneath the boiler. Hence, for high ash fuels like Indian coal, washery rejects, the number of ash cooler is to be decided based on the high ash fuel. The ESP will see higher dust loading in Indian coal; hence higher collection area will be required comparative to when firing petcoke or imported coal.
Imported, Indian coal, lignite, petcoke possess sulfur in the order of 0.7, 0.5, 2, 8 per cent in the fuel. In CFB the sulfur capture is done by adding limestone along with fuel. Limestone reacts with sulphate forming sulfur tri oxide that is removed through bed drains.
Hence, high sulfur in petcoke will require higher limestone content and hence the limestone RAVs will be sized to deliver the required quantity. These parameters must be given serious consideration before investing in a specific combustion technology.
Head-Business Development, Thermax, Power Divison.