Are biomass pellet boilers environmentally friendly

biomass pellet boilers

Are biomass pellet boilers environmentally friendly?

Application practice of biomass pellet boiler

Abstract: A company’s original coal-water slurry boiler was converted to a biomass-fired boiler due to low thermal efficiency and incomplete solution of dust, SO2 and NOx emission pollution problems. However, the conversion was initially completed with a number of problems caused by alkali metal deposition, such as increased flue resistance, induced draft fan power consumption, and exhaust smoke temperature. For this reason, further measures were taken to install additional steel bead ash scavengers and air preheaters, and to introduce fan frequency conversion. So far the biomass pellet boiler has been operating stably for one year with good results. In addition, the biomass pellet boiler system has realized the recycling of waste water and slag.

At present, the air pollution in China is worrying, especially the problem of PM2.5 pollution has attracted more and more social attention. Since sulfur dioxide, nitrogen oxides, volatile organic compounds (VOCs) and other harmful gases are the main sources of PM2.5 pollution, all major cities in China have launched a series of policies to ban the use of coal-fired boilers in urban areas as one of the measures to control sulfur and nitrogen oxides.

The company originally used a certain type of coal-water slurry boiler to provide steam for the drying process, but since the coal-water slurry boiler did not completely solve the problem of sulfur and nitrogen pollutant control, and the concentration of its soot emissions was much higher than that of the chain furnace, coupled with the higher operating costs of the coal-water slurry boiler, the company finally decided to convert it into an 8t/h biomass-fired boiler, and to provide the company’s steam supply business in the form of a trustee operation. outsourcing.

The renovation work was started in August 2011, and the specific renovation measures mainly included the replacement of combustion equipment and the adjustment of the furnace structure. The adjustment of the furnace structure is mainly for the characteristics of high volatile content of biomass fuel, increasing the secondary combustion space and the proportion of secondary air distribution, and adding a furnace arch. In particular, the water-cooled short front arch was adopted to address the problem of burning of the feed baffle due to the flammability of biomass. The modified biomass pellet boiler basically met the requirements of continuous production of the enterprise.

However, some problems were found during the trial operation of the boiler, such as ash accumulation and high exhaust temperature, as shown in Table 1. For this reason, further modifications were made to the boiler, including the addition of a steel ball ash cleaner at the furnace outlet, an air preheater in the tail flue, waste water slag recycling, and fan frequency conversion. The retrofitting work was completed in September 2021, and has been in stable operation for more than 1 year since then, and has passed the clean production assessment.

2 Pollutant Emission Control
<2.1.> Carbon sulfur and nitrogen oxide emissions

The original water-coal slurry boiler still uses coal as fuel, and the actual thermal efficiency is only about 66%, with high CO2 emissions. In addition, the combustion temperature of water-coal slurry boiler is only 100-200℃ lower than that of coal-fired chain boiler, and there are still high SO2 and NOx emissions.

The change to biomass pellet boilers not only achieves zero CO2 emissions, but also further reduces the sulfur content in the flue gas because of the extremely low sulfur content of biomass fuel and the high alkali metal content in the fuel, thus the SO2 content in the flue gas emissions is extremely low, only 2.86 mg/m3 (standard state), which is hundreds of times lower than the sulfur content in the flue gas of coal-fired boilers, and can fully meet the most stringent pollutant emission standards without the use of any desulfurization facilities. The most stringent pollutant emission standards can be fully met without any desulfurization facilities.

In addition, the two-stage combustion method in biomass pellet boilers makes the combustion temperature low, so the thermal NOx emission is basically negligible, which also significantly reduces the NOx emission.

<2.2 > Ash treatment

Although coal-water slurry is usually pre-treated for ash removal, the slag discharge from coal-water slurry boilers is still high. For biomass pellet boilers, although theoretically there is little ash, the ash emissions are still higher due to the high amount of sand and soil in the collected feedstock, which is not ideal for direct use as potash fertilizer.

Therefore, before the boiler renovation, all ash residue was entrusted to external transportation, which was costly and wasted resources. After the renovation, a set of brick-making equipment was introduced to make hollow bricks from ash slag, wastewater, lime and cement in a certain ratio, which not only solved the problem of ash slag and wastewater treatment, but also increased the benefit and realized the waste recycling of “turning waste into treasure”.

<2.3>Waste water recycling

Wastewater is mainly composed of three parts. One part comes from the softening process of boiler feed water, the second part comes from boiler drainage water, and the third part comes from the high temperature condensate of the carton production plant, none of which has been considered for recycling before.

Among them, except for the third part of high temperature condensate which is very clean, the first two parts of the generated effluent are weakly alkaline water, mainly calcium and magnesium ion salts, which were only recycled to the neutralization tank for treatment and then discharged before the renovation. After the renovation, the brick-making process introduced recycled the first two parts of the effluent, achieving the ideal effect of both economic and environmental benefits. Table 2 shows the emissions of waste gas and slag before and after the renovation.

3 Alkali metal deposition control

One of the main characteristics of biomass fuels is the high content of alkali metals, which will enter the gas phase in the process of fuel combustion in the form of chloride salts, etc. When in contact with the heat exchanger surface, condensation will occur and eventually adhere to the surface of the heat exchanger tube to form ash accumulation.

Moreover, the reaction with sulfur oxides will also form hard deposits of sulfates that are difficult to remove, especially in the superheater area at the furnace exit. The presence and continuous generation of alkali metal deposits will not only seriously affect the heat transfer efficiency of the heat exchanger, but also increase the flue flow resistance or even block the tube bundle, resulting in increased energy consumption of the induced draft fan, in addition, the deposits will also cause corrosion of the heat exchanger tube .

<3.1> Steel bead ash cleaning system

In order to solve the problem of alkali metal deposition, a steel bead cleaning system was added at the heat exchanger bundle at the furnace outlet. This system uses the vacuum formed in the venturi throat to pump the steel beads from the lower part of the tail flue to the upper part and then from the top to the bottom to clean the tail heating surface, and is set to automatically remove ash three times a day, which well improves the thermal efficiency of the boiler, saves 300t of standard coal per year, and effectively extends the service life of the superheater.

<3.2>Inverter transformation of the induced draft fan

Boiler induced draft fan in the high pressure ac motor often exists “big horse-drawn car” phenomenon, in some occasions even if the margin is not very large, but because the boiler load fluctuations, the motor can not follow the adjustment in time, resulting in increased energy consumption of the motor. In particular, the serious deposition of alkali metal will also further increase the rise of tube bundle flow resistance.

For this reason, during the later renovation of the biomass pellet boiler, a frequency converter was set on the boiler induced draft fan to make the air volume of the induced draft fan change with the change of boiler load. After adopting the frequency conversion regulation, the annual energy saving was more than 80,000 kWh due to the energy saving achieved by adjusting the motor speed. When the load rate is low, the motor and fan speed is also reduced, the main equipment and the corresponding auxiliary equipment such as bearings are less worn than before, the maintenance cycle can be lengthened, and the operating life of the equipment is extended.

4 Waste heat recovery

<4.1>Add air preheater

Biomass pellet boiler just after the transformation, due to the original coal-water slurry boiler heat transfer area is not enough, the exhaust temperature is still as high as 240 ℃, so in the boiler tail flue to add a heat pipe air preheater to reduce the exhaust temperature to below 150 ℃, the boiler efficiency increased by 6%. Equivalent to the reduction of sulfur dioxide 20t, soot 12t, carbon dioxide 2200t.

<4.2>High temperature condensate recovery

Water vapor comes out of the boiler into the workshop production line, and condenses into high temperature clean water of 60℃ after pre-heating and drying the raw materials. Since the original system did not have a condensate recovery device, resulting in a waste of heat and water resources, this part of the condensate was recovered after the renovation in 2011, significantly reducing production costs.

Since the recovered condensate is very clean, it can be used as feed water directly into the boiler. The system can recover 35,000 tons of high-temperature condensate in 1 year of operation. In terms of condensate temperature 60℃, it is about 45℃ higher than direct feed water, which is equivalent to energy saving of 6.6×109kJ, equivalent to 220t of standard coal. in addition, it can directly save about 200,000 yuan of industrial tap water cost per year.

5 Conclusion and outlook

In the process of transforming water-coal slurry boiler into biomass pellet boiler, combustion equipment replacement and furnace structure transformation have been carried out successively, and additional systems such as steel ball ash cleaner, air preheater and high temperature condensate recovery have been installed, and fan frequency conversion technology has been introduced. So far, the renovated biomass pellet boiler system has been running successfully for more than one year, achieving a win-win situation in terms of economic and environmental benefits. The specific effects are as follows.

(1) Providing nearly 60,000 tons of steam continuously for the company.

(2) Biomass fuel instead of coal obviously reduces the emission of bottom slag, fly ash, SO2 and NOx, and achieves zero CO2 emission, and can meet the requirements of relevant air pollutant emission standards without setting any desulfurization facilities.

(3) The introduced brick making process enables the recycling of waste water and slag at the same time.

(4) The additional steel ball ash cleaning system and air preheater significantly improve the thermal efficiency of the boiler.

(5) The fan frequency conversion technology has played an obvious power saving effect.

(6) The mode of boiler hosting operation makes the company’s production line completely free from a series of problems brought about by its own boiler operation, which is worth promoting.

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