Evolving boiler technology for multi-fuel firing
The power sector, which includes captive power generation capacity, has lately seen many changes in policies, in options for sale and purchase of power, in technology, business models and above all, in fuel access structures. Selection of a boiler that is flexible enough to handle these changes without compromising on efficiency is a must.
An investor planning to invest in a mid-sized power project may well be stumped by the plethora of options. Some of the inherent risk factors are as follows:
- Statutory clearances
- Financial closure
- Land and rehabilitation
- Cash flow
- Marketing risks
- Risk of insufficient revenue
- Insufficiently proven technology
- Developer / contractor's competence and experience in question
- Project schedule
- Contractor's financial strength
Operations and maintenance risks
- Heat rate guarantees
- Manpower costs
- Plant performance
For a power project to succeed, an investor invariably looks at the financial viability of the project. Two factors foremost on his minds are the project cost (comprising the capital cost, interest cost and development expenses) and the operating cost. Based on these, the investor will be able to forecast the cash flow. In a power plant, the operating costs comprise mainly of station heat rate, manpower cost and the cost of consumables.
The investor is of course, concerned about the return on the investments, which is connected with the technical feasibility of the project and the technology being utilised. The return on investment also depends on the guarantees that can be obtained on the project costs and how well one is able to estimate and mitigate the variations. Then again, performance guarantees are far more important than the project cost guarantees, since performance variations can bleed the project financials for the lifetime of the project, which is typically about 20 - 25 years. The IDC and returns accumulate from the guarantee of the schedule that is set for the project, provided it is closely followed.
Based on the risk appetite of the investor and the insight of the funding banks, the project is reviewed for its feasibility and if seen as a profitable venture, further steps are then taken.
In the next phase, the project is awarded to the EPC contracting firm that takes up the entire construction risk. If the EPC contractor is also a technology provider, like a boiler manufacturer in the case of a power plant, then even the technological risk is passed on to the contractor. If the EPC contractor is ready to take up long- term operations and maintenance of the power project, then the O&M risks are also passed on to the contractor. That leaves only the development risk and part of commercial risk. The commercial risk can be further diluted with a financially sound EPC contractor. A legally strong, watertight contract put in place will leave only the development risk. In today's context of fuel uncertainty, technology plays a vital role, especially when choosing the right boiler.
Following factors are important:
1. Boiler technology for various kinds of biomass fuels.
2. Boiler pressure and temperature.
3. Fuel firing limitations.
4. Boiler efficiency and availability.
Factors affecting boiler design:
1. Physical characteristics of the fuel. This is extremely important if biomass is to be combusted at any time as main or supplementary fuel.
Characteristics that matter are:
b. Bulk density.
c. Flow ability.
2. Chemical constituents are extremely important in case of biomass.
a. Chlorine (elemental chlorine and not chlorides in ash) as, in biomass, it can cause corrosion problems.
b. Alkali content (Na2O+K2O) in ash. This content in fuel can lead to problems like slagging and fouling.
3. Boiler efficiency depends on the moisture content in the fuel. Combustion efficiency depends on the ash content and excess of air. Excess air increases combustion efficiency; however, it also increases dry flue gas losses.
4. NOx generation is a function of temperature, staging of air and excess air percentage.
5. If moisture content in fuel is high, in-bed tubes can be avoided.
Uncertainty regarding availability and reliability of any one type of fuel, stringent emission norms, constraints on firing multiple types of fuels in pulverised coal fired boilers, and the need of additional capital- intensive accessories like coal mill, FGD, etc, has led to the evolving of the Circulating Fluidised Bed Combustion Technology (CFBC).
CFBC is a fuel- flexible technology, which can handle variations in GCV from 1,800- 8,000 kcal/kg, ash 5 per cent-65 per cent, and moisture from 1 per cent-45 per cent. The turbulent bed which is operating at 4-5.5m/s is able to enhance the fuel burn ability by rapid mixing of fuel with hot bed material, resulting in efficient carbon burnout.
Within the CFBC technology itself, there are several options of evolved CFBC technologies with wider multi-fuel firing capability like the following:
- Lignite (Neyveli / Kutch / Barmer)
- High-sulphur coal
- Washery rejects
Petroleum coke (petcoke)
Other renewable fuels
- Oil pitches
- Refused derived fuel
These technologies can easily cater to fuel property ranges of:
- Moisture upto 60 per cent, e.g. in lignite, peat, sludge.
- Ash upto 76 per cent, e.g. in washery rejects, char.
- Sulphur upto 8 per cent, e.g. in lignite, petcoke.
- Volatiles as low as 1 per cent as in petcoke, washery rejects, char.
- HHV as low as 1500 Kcal/Kg as found in washery rejects, char, etc.
- Some factors need to be considered while choosing the right technology.
Compact, economical design and construction
If the boiler design has a 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. This makes the boiler compact and economical regarding lifetime costs.
Separation in stages for better bed inventory control
If the design hasan optimal stagewise 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, as well as a particle collection efficiency as high as 99.8 per cent for better inventory control. If the separation technology is of the fit-and-forget type, then it will not require any kind of maintenance like the U-beam type technology.
Performance in varying and low load conditions
With an effective bed inventory and temperature controls through controlled solid recycle rate from MDC to furnace, one can get better performance and from the boiler during varying and low load conditions. This is achieved without affecting the steam parameters and gives a much better turndown ratio without any auxiliary fuel support. Turndown ratios as high as 1:5 can be easily achieved in some designs.
Start -up and shutdown time
Some designs have much lower refractory heat retention as compared to other CFBC designs. This allows for quick start and shutdown of the boiler.
Boiler designs with gasses leaving the furnace at a high velocity to achieve solid separation, using centrifugal action, generally have higher pressure drops, and thus, a higher auxiliary consumption. Boiler designs with a lower velocity of gases have a comparatively negligible pressure drop and much lower auxiliary consumption.
Availability and lower maintenance level
Maintenance of the boiler is directly related to the quantum of refractory of the boiler design. The boiler design with the least level of thick, uncooled refractory and no hot expansion joints reduces the expense and 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 which do not require hot expansion joints, the lifetime maintenance of the boiler can be minimised substantially.
Some boiler designs ensure that there is no soot formation and a uniform furnace temperature profile is maintained, thereby further reducing maintenance time and improving the performance.
Erosion is a major contributor to maintenance problems in CFBC boilers, usually resulting from high solids loading 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 a particle separator based on an extremely high flue gas velocity to provide the energy needed to efficiently disengage the particles from the flue gas, other designs have particle separators 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 significantly reduced.
CFBC boiler design considerations
The lowest calorific value like washery will call for higher amount of fuel feeding into the bed. The feeders need to be sized for 1:10 turndown to feed low calorific value fuels as well as high calorific value fuels like say petcoke.
The furnace cross- section is decided by the maximum flue gas volume generated by respective fuels. In the case of lignite or biomass with high moisture and low calorific value fuel, the flue gas generated will decide the furnace cross-section. In addition to this, the ESP and ID of the fan needs to be sized for handling higher gas volumes.
Higher ash content in fuels enhances the heat transfer in the furnace and in convection pass. To maintain solids` mass flux in furnace, the excess solids are taken out of the system through a bed ash cooler located beneath the boiler. Hence, for high ash fuels like Indian coal, washery rejects, the number of ash coolers is to be decided based on the high ash fuel. The ESP will see a higher dust loading in Indian coal, hence a higher collection area will be required than when required while firing petcoke or imported coal.
Imported or Indian coal, lignite, petcoke, all possess sulfur in the order of 0.7, 0.5, 2, 8 per cent in the fuel. In CFB, the sulfur capture is a simple process where adding limestone along with the fuel allows it to calcine and sulphate with sulfur trioxide, which is removed through bed drains. Hence, to capture the maximum amount of sulfur in petcoke, a higher limestone requirement is needed and hence the limestone RAVs will be sized to deliver the required quantity.
With such advantages and flexibility, the CFBC technology in the current climate of fuel volatility, can only be considered a boon.
Vivek Taneja, Head of Business Development, Thermax Limited