Whether a biomass boiler burns well or not is directly related to the plant's energy costs and environmental compliance. Burning reasonably, not only can use less fuel, reduce emissions, equipment can also be less problematic, and last longer. The condition of the fuel itself, whether the air is enough, whether the equipment design is reasonable, and whether the routine maintenance is in place or not, all of these will affect the combustion effect.
Simply put, combustion efficiency is the proportion of the total heat of the fuel that can actually be used after the fuel is burned. Want to make this ratio high, the core is to let the fuel burn through, do not waste heat.
To determine the efficiency, just look at a few key figures. Fuel burnout rate must be more than 95%, less than 90% means that it is not burned through, the energy lost for nothing. Smoke away from the heat accounted for 60% -80% of the total loss, exhaust temperature of every 10 ℃, heat loss of 0.5% -0.8% more. Excess air coefficient control in 1.2-1.5 between the best, too little to burn not enough, too much will let the smoke with more heat to run away. Carbon monoxide should be less than 100ppm, unburned carbon not more than 3%, these two values are high, the efficiency will certainly decline.
Biomass boilers and boilers burning fossil fuels, combustion is not the same. Biomass is easy to burn more volatile components, accounting for 60% -80%, low ignition temperature, 250-350 ℃ can be burned, that is, burned quickly, short duration. Fossil fuels have fewer volatile components and higher ignition temperatures, but they burn more stably. And biomass density, composition fluctuations, easy to make the combustion state is not stable, so you have to pay more attention to fuel pretreatment and combustion parameter adjustment.
Fuel pretreatment and air control are the key to make the boiler burn well. These two do adjust, have to combine specific data, calculate the input cost and can save money, but also to deal with a variety of changes in operation.
Too much water in the fuel will not only pull down the heat generation, combustion will also absorb heat, so that the flame temperature is lowered, the furnace is also easy to accumulate ash slag. Reduce the moisture commonly used in three ways: natural drying without spending money to buy equipment, but rely on the weather, the moisture can not be stabilized, suitable for small-scale use; hot air drying equipment to be installed to deal with 1 ton of fuel per hour to 20,000-50,000 tons of equipment, the operating cost is not low, but dry fast, can be stabilized moisture; waste heat drying exhaust waste heat, without additional energy consumption, is a slightly higher investment in equipment, 30,000-60,000 tons of equipment - when the drying effect will be affected by the exhaust smoke situation. The drying effect will be affected by the situation of exhaust smoke.
If you want to stabilize the moisture content for a long time, you have to install monitoring equipment, invest 0.5 million -20,000, and calibrate every year, the cost is 5% -10% of the cost of equipment. According to the monitoring data to adjust the drying process, can control the moisture fluctuations within ± 2%. Large-scale boiler is recommended to use waste heat to heat the wind drying combination of ways to save money and ensure the effect.
particles or uniformly sliced fuel, can be evenly distributed in the hearth, and air contact is sufficient, burn stable, burn through, burn rate can be more than 96%. To make pellet fuel to install crushing, molding equipment, 30,000-80,000 / ton - time, equipment wear and tear, the annual maintenance fee is 8-15% of the cost of the equipment; slicing equipment is cheaper, 10,000-30,000 / ton - time, maintenance is simple, but the uniformity of the particle size is slightly difficult to control.
Grain size inconsistency will lead to uneven distribution of fuel on the grate, thick places lack of air, burned insufficiently, thin places where the heat is easy to dissipate. This will not only reduce the efficiency of 5-10%, but also easy to slag, have to stop to clean up. Installation of a screening equipment (0.3 million -10,000), regular clearing of the screen, you can ensure that the particle size is basically uniform, industrial boilers are generally controlled within 20 millimeters, the deviation is not more than ± 5 millimeters.
combustion, air to how much to give and fuel combustion rhythm match. Volatile components of fuel release to give more air, otherwise it will produce carbon monoxide and carbon; late coke combustion, air don't give more, lest the smoke with heat to run away. Theoretical air-fuel ratio of wood fuel 10-12, straw 8-10, the actual operation should be adjusted according to the situation.
Insufficient air will make carbon monoxide over 200ppm, efficiency drop more than 2%; air excess 10%, exhaust heat loss will increase 0.8%-1.2%. Installing O₂ correction system (10,000-30,000, annual calibration fee of 0.2-0.5 million), variable frequency fan (50%-100% more expensive than ordinary fans, but energy consumption is reduced by 15-25%, and the cost can be recovered in 2-3 years) and real-time sensors (0.5-20,000, changed once in 1-2 years, annual maintenance fee of 0.1-0.3 million), you can accurately control the amount of air, to ensure the stability of combustion.
Biomass fuels come from mixed sources, with large differences in composition and density, and direct use will make combustion unstable. Therefore, it is necessary to do pretreatment before entering the furnace to reduce the impact of such fluctuations.
The core of the fuel blending, the different characteristics of the fuel mixed in proportion, such as wood and straw according to 3:1 to 2:1 mixing, so that the heat stabilized at 16,000-18,000 kilojoules / kg, volatile matter stabilized at 65% -75%. Mixing to install special equipment (10,000-40,000/ton-hour), but also real-time monitoring, the annual monitoring fee of 0.3-0.8 million.
Coupled with crushing, screening, drying and other links to remove impurities (controlled at less than 5%), to ensure that the particle size and moisture content is stable. The whole set of pretreatment equipment investment of 50,000-120,000 / ton - hour, the annual operation and maintenance fee is 10% -15% of the cost of equipment, but can make the fuel characteristics of the fluctuations in 60% -80%, the efficiency of the lift 2% -5%, equipment failure will be reduced.
Furnace design is very critical, volume and boiler load matching, to ensure that the fuel can stay in the furnace chamber 2-4 seconds, not enough time to burn through, too long and will increase equipment costs. The shape of the hearth should be reasonable, do not have dead ends, to avoid fuel buildup and uneven temperature.
Industrial boiler common combustion method has three kinds: grate furnace stable, suitable for all kinds of biomass, 800,000-1,200,000 / steam ton, combustion rate of 92% -95%, but the grate wear and tear fast, the annual maintenance fee is the cost of equipment, 3% -5%; Fluidized bed combustion is sufficient, combustion rate of 96% -98%, 1,000,000-1,500,000 / steam ton, the disadvantage of the fan is the energy consumption is high, the fuel particle size requirements are strict (≤ 10) mm); suspension combustion is suitable for fine particle fuel, fast heat transfer, but unstable, easy to slag, 1.2-1.8 million/steam ton, control and maintenance are troublesome.
The automatic control system can help the boiler to burn stably, collect data in real time, and adjust the fuel and air supply. Small and medium-sized boilers with PLC system (50,000-200,000), easy to operate, easy to maintain, that is, the control accuracy is general, oxygen content fluctuations ± 1%, furnace temperature fluctuations ± 50 ℃; large boilers are recommended to use DCS system (200,000-1 million), accurate control, fluctuations are small, but the operation is complex, the annual maintenance fee is 5%-8% of the cost of equipment.
The system will monitor the oxygen and carbon monoxide content in the flue gas and the exhaust temperature, and adjust the parameters according to the data, with a response time of 1-3 seconds, which can quickly respond to load changes. When the boiler load fluctuates between 30%-110%, the system will automatically switch the parameters to ensure that the efficiency drop is not more than 3% at low load (<50%) and stabilized at more than 90% at normal load (70%-100%).
Ash, scale or slag on the heating surface of the boiler will affect heat transfer. For every 1 mm of ash accumulation, the heat transfer efficiency drops by 5%-8%, and the combustion efficiency follows by 1%-2%.
Accumulation of ash and scaling occurs in the convection heating surface and economizer, mainly ash buildup and fuel in the alkali metals, chlorine and other elements caused by condensation; slagging in the furnace water-cooled wall, is the formation of molten ash cooling, serious boiler shutdown will result. Regular cleaning of the heating surface is an important part of ensuring efficiency.
If you want the boiler to burn well for a long time, technical optimization alone is not enough, routine maintenance must be in place. Fuel conveying and air distribution system is the key inspection object, can not be blocked, leakage and other problems.
Feeder accuracy should be controlled at ± 3%, conveyor belt 6-12 months to check once, wear and tear is serious to change in time (100-300 yuan / meter), storage silo should be cleaned up once a year (0.5 million -20,000 yuan). Fan bearing temperature can not exceed 80 ℃, lubricating oil 1-3 months to change (0.1-0.3 million yuan / time), adjusting the damper adjustment precision should be guaranteed at ± 5%, to avoid unstable air supply.
It is also critical to do regular preventive maintenance, for example, the grate is overhauled once in 6-12 months (10,000-30,000 yuan/time), and the furnace chamber is cleaned of slag once in 1-3 months (0.5-1.5 million yuan/time). The annual maintenance cost is about 2%-4% of the total investment of the equipment. Although it costs a little more, it can reduce the downtime and avoid a significant drop in efficiency.
Regular monitoring and evaluation is the only way to detect problems and adjust operations in time. The core is to keep an eye on a few key parameters to ensure that the efficiency is stabilized in a reasonable range.
Flame temperature (800-1000℃), oxygen content (3%-5%), carbon monoxide (<100ppm), exhaust temperature (120-180℃) and fuel consumption should be continuously monitored. The accuracy of fuel metering should be controlled at ±2%, and the heat generation per unit of fuel can be calculated from the consumption and boiler output, and it is no problem if it is stabilized at 14,000-16,000 kJ/kg.
Efficiency should be assessed once a day, looking at the burnout rate (unburned carbon <3%), exhaust heat loss (10%-15%) and boiler thermal efficiency (85%-90%). If the thermal efficiency drops more than 2%, check for problems with fuel, air supply, and cleanliness of the heating surface.
Combustion efficiency is the key to biomass boiler operation, fuel, air supply, equipment design and maintenance, no piece of it can be sloppy. Fuel pretreatment to do a good job of foundation, air supply and automatic control to ensure that the burn through, reasonable design and standardized maintenance can make the boiler long-term stable operation. In practice, according to the size of the boiler and fuel, calculate the cost and benefit, you can find the most suitable optimization scheme.
