Welcome to discuss the paradox of low-nitrogen retrofitting for furnaces

Can reducing 1800 to 1300 meet the process requirements? If it is used for heating water to 100 degrees, such a modification will definitely improve thermal efficiency. If there is a significant difference in the value generated by heat at 1800 degrees and 1300 degrees, then it would be putting the cart before the horse

1800 and 1300 are just examples; I don’t know the exact figures. I just want to know if there’s anything wrong with my logical thinking. Thank you for your reply!

This logic is based on a furnace-centered approach and is unreliable; what really matters is the amount of standard coal consumed per ton of product, which represents the most fundamental measure of energy consumption. If your modifications alter the process and reduce production efficiency, wasting more energy will not be worth it

The last edit to this post was made by Time Pioneer on 2019-2-13 at 14:33. Heat transfer in a heating furnace occurs as a result of the transfer of thermal energy due to temperature differences; the physical meaning of Fourier’s law of heat conduction is that the temperature gradient serves as the driving force. Therefore, since low-nitrogen combustion in the heating furnace burner requires a reduction in the combustion temperature, the flame temperature of the heating furnace decreases compared to before low-nitrogen combustion. This reduction in the \"driving force\" inevitably affects the transfer of thermal energy, thereby directly lowering the thermal efficiency of the heating furnace. The main factors affecting the thermal efficiency of a heating furnace are flue gas temperature, fuel combustion rate, flue gas energy utilization rate, and combustion temperature. It is generally agreed in the industry that reducing the flue gas temperature has the greatest impact on heating furnaces; for every 17–20°C decrease in flue gas temperature. The thermal efficiency of the heating furnace increases by one percentage point ； The higher the fuel burn rate, the higher the thermal efficiency of the furnace ； The energy utilization rate of smoke flames is a forgotten factor ； If low-nitrogen combustion is not used, the combustion temperature is also an overlooked factor. The order of influence of the four factors on the thermal efficiency of a heating furnace is likely to be flue gas temperature, flue gas and flame energy utilization rate, combustion temperature, and fuel burnout rate. “The two methods of fuel grading and air grading to achieve low-nitrogen combustion fall within the scope of \"fuel burnout rate\"; moreover, research on fuel burnout rates is still ongoing both domestically and internationally, and there are no authoritative tests to determine the differences in fuel burnout rates among various burners. A certain type of low-nitrogen burner has a relatively high fuel burn rate, which results in an insufficient positive impact on the thermal efficiency of the heating furnace; this is not enough to compensate for the negative effect of lower combustion temperatures on the furnace’s thermal efficiency, as the effects of these two factors are different. It should be particularly noted that conditions for low-nitrogen combustion also include oxygen-deficient combustion, and it is necessary to control the excess air coefficient. If control deviates, it will exacerbate incomplete combustion, reduce the fuel burn rate, and cause the carbon monoxide levels to exceed the allowable limits. Conventional low-nitrogen burners reduce the thermal efficiency of heating furnaces, leading to issues such as reduced operational flexibility of the furnaces, unstable flames, and elevated levels of carbon monoxide ； Only high-efficiency, low-nitrogen burners can improve the thermal efficiency of heating furnaces, enhance their operational flexibility, prevent the flames from drifting, and ensure that carbon monoxide levels remain within acceptable limits.

Oxygen-deficient combustion is already achieved through the stratification of fuel and air; although the air is stratified, the total amount of air remains constant (for example, when burning natural gas, the air excess factor is 1.15, and the same total amount of air is used with low-nitrogen burners). I’m not quite sure why high-efficiency low-nitrogen burners can also improve the thermal efficiency of a furnace – what’s the reason behind that?

The beneficiaries of the low-nitrogen combustion modification are foreign imported burners, which shows that our policymakers have been bought off by Europe and the United States!

1. The curves for NOx and COx in combustion products are inversely related; reducing nitrogen emissions and saving energy are, from a combustion perspective, inverse processes. To achieve levels of 9 PPM at 3% O2 in Southern California, which corresponds to 18 mg/Nm3, or 5 PPM in San Francisco Bay corresponding to around 10 mg, it is necessary to sacrifice energy consumption. Since reducing nitrogen emissions requires either certain combustion solutions or exhaust gas treatment methods, the costs for operators inevitably increase. Reducing nitrogen emissions is done to contribute to the human environment. 2. Whether it is a fire-tube, water-tube, or other type of boiler, simplifying things shows that it is essentially a heat exchanger. If the amount of natural gas required to produce a unit volume of hot water or steam remains unchanged, then the thermal efficiency will not decrease – but this is practically impossible. Regarding nitrogen oxides generated during combustion, thermal NOx accounts for over 80%, so most techniques focus on controlling the combustion rate in order to reduce the temperature of local high-temperature flames. For the same heat exchanger, a decrease in flame temperature leads to a reduction in thermal radiation efficiency; heat convection also plays a role here. If the relatively cool flue gas stays inside the heat exchanger for a longer period of time, then the loss due to reduced thermal radiation can be partially compensated for by heat convection. But the heat exchanger remains the same; at most, an automatically adjustable baffle can be installed on the chimney to control it, but its effect is minimal. So, the design inside the heat exchanger, the distribution of the hot spots, and the overall heat balance calculations are not very useful even when using CFD. Beyond the concept of the system, when heating water or steam per unit volume or mass, no matter which burner is used, the amount of natural gas required remains unchanged; therefore, the thermal efficiency of the boiler stays the same. The oxygen content in the flue gas is higher than before; it can be said that this machine has a high excess air coefficient, which certainly means it is not energy-efficient, as the N2 in the combustion air gets heated unintentionally and inevitably carries away heat through the flue gas. They keep talking about reducing nitrogen, but has anyone paid attention to COx in the flue gas? If nitrogen levels are not reduced, it is the burnout rate of the burner that plays a key role; the COx level in the flue gases can be as low as 5PPM. In the case of low-nitrogen systems, the nitrogen level is usually between 30-50mg. With ultra-low nitrogen levels, it becomes even more difficult to control COx levels. As for gas staging and air staging, they involve controlled combustion with lean conditions first followed by rich conditions, in order to avoid the situation where, in designs without low-nitrogen features, intense combustion occurs as soon as the gas is injected, resulting in a high-temperature flame. However, it is difficult to control the air stratification at the far end of the flame; you can refer to the pressure of the air used for combustion – it is much lower than the gas pressure in any type of burner. At this time, it is essential to ensure complete combustion; some manufacturers increase the excess air coefficient slightly. In any case, as long as the requirements regarding NOx are met after making the necessary adjustments, everything will be fine. For fully premixed conditions, the oxygen content in the flue gas is around 6-8%, right? The water and other inert gas by-products in the flue gas drawn back after FGR is applied get heated unintentionally, don’t they? Although FGR increases the thermal convection efficiency, by only a small amount. Moving forward, in China, there might be a shift toward using Hybermix without FGR 30mg, with water or steam being sprayed onto the flame to help reduce its intensity; however, heating these substances unnecessarily also increases energy consumption. I once asked foreign boiler manufacturers about the issue of thermal efficiency after low-nitrogen modifications, and they asked me in return: if thermal efficiency is a concern, then what is the purpose of using high-efficiency energy savers in the flue gas system? The amount of flue gas varies, and the temperature does as well; it’s simply a matter of raising or lowering the temperature of the medium that is being heated in the cycle. There’s no need to get entangled in complex combustion and heat exchange processes – after all, this is just a modification.

The answer is very professional; excellent!

The answer was very professional; I’ve learned something. The reality is, if the cost of burner modification plus the increased cost associated with fuel consumption > the operating cost of the off-gas treatment system, does that mean there is no need to modify the burner?

If we use a 10-year period as a benchmark for comparing operating costs, the cost of burners plus gas cannot be higher than the operating costs associated with post-treatment processes. We need to take into account the costs of ammonia or urea, as well as the corrosion issues they cause. Urea is safer but more expensive, while ammonia is cheaper; however, it is easier to control ammonia leakage. The one-time investment required to achieve levels of 2-5 PPM, as required in the United States, is also substantial. In China, the local manufacturing capabilities help to reduce the price of Low NOx or Ultra Low NOx burners.