Combustion Emissions, Health and Energy Transition Aspects: The Biomass Gas Case

This paper is about the consequences of combustion and in particular of biomass gasification products, as an example, for CO and NOx emissions. The resulting fuel gas is typical a mixture of CH4 and CO2. The burning velocity is important for design of combustion equipment, either domestic or industrial. The particular choice of fuel mixture and the management of the provision of fuels is important for the resulting emissions with associated human health impacts. Combustion emissions are determined by experiments and numerical simulation. In the latter case the kinetic mechanism use impacts the result. A short overview of associated health aspects, as well as combustion research methods are given as well.

The usage of biomass, can be sustainable and is short cycle CO2 emitting and can be regarded as green energy. Therefore, it will become more and more widely in the future. Biogas is one of the renewable fuels that is produced in many different sources, such as sewage sludge, landfills, and organic material [1]. Methane (CH4) is the main component of biogas. It is valuable but also harmful to the environment. Biogas can be used for heat, electricity, vehicles, etc., to reduce environmental emissions and the use of fossil fuels. CH4 and carbon dioxide (CO2) are the two main components in biogas. CH4 accounts for 55 to 65% (volume fraction) and CO2 accounts for 35 to 45% in the biogas from sewage digesters. CH4 accounts for 60 to 70% and CO2 accounts for 30 to 40% of the biogas from organic waste. CH4 accounts for 45 to 55% and CO2 accounts for 30 to 40% in the biogas from landfills [2].

CO emissions are mainly produced by incomplete combustion, so by providing too few air or too much (carbon containing) fuel. NOx is produced at high temperatures, by means of thermal decomposition and recombination of N2 and other radicals in flames. An alternative route is by means of decomposition of fuel containing N. Thus, NOx will be produced at conditions that are able to combust the total fuel exactly with the amount of air provided. Too much air will reduce the temperature again. These emissions cause serious impact for human health and environment.

Potential health consequences

Emission of soot and particles have their health consequences, see for overviews e.g. Thangavel P, et al. [2] and Dries DJ and Endorf FW. [3]

Carbon Monoxide (CO) is a toxic gas, and its inhalation can have serious consequences for human health. Carbon monoxide binds to hemoglobin with much higher affinity than oxygen, which means that even low levels of CO exposure can displace oxygen in the bloodstream. This leads to reduced oxygen delivery to vital organs and tissues, potentially causing oxygen deprivation (hypoxia) in the body. CO poisoning develops at high-level exposure to carbon monoxide can lead to carbon monoxide poisoning, which can be life-threatening. Symptoms of severe carbon monoxide poisoning can include loss of consciousness, seizures, and in severe cases, death. Chronic exposure to low levels of carbon monoxide can lead to long-term health problems, see e.g. Hess DR [4]. These may include cognitive impairment, cardiovascular issues, and an increased risk of chronic conditions like heart disease and neurological disorders.

Carbon monoxide is a major air pollutant, particularly in urban areas with high traffic and industrial activities. It contributes to poor air quality and can lead to the formation of ground-level ozone. While carbon monoxide is not a potent greenhouse gas like CO2, it can still contribute to global warming. In the atmosphere, carbon monoxide can react with hydroxyl radicals and contribute to the production of CH4, which is a very potent greenhouse gas. High levels of carbon monoxide can harm vegetation by interfering with the plants' ability to photosynthesize. This can result in reduced crop yields and damage to forests and ecosystems. CO can dissolve in water and affect aquatic ecosystems. When it reacts with other compounds in water, it can form harmful byproducts. In addition, it can deplete oxygen in water bodies, leading to fish kills and other ecological disruptions. Wildlife can also be affected by elevated levels of carbon monoxide, especially in areas with poor air quality. This can lead to reduced populations of sensitive species and disruptions in ecosystems.

Exposure to NOx can have detrimental consequences for respiratory health as well, for a short overview, César AC, et al. [5] and see de Vries W [6] for N containing emissions in general. Inhalation of NO2 can irritate the respiratory tract, leading to symptoms like coughing, wheezing, and shortness of breath. Prolonged exposure to NO2 can reduce lung function, particularly in individuals with preexisting respiratory conditions like asthma or Chronic Obstructive Pulmonary Disease (COPD). Furthermore, exposure to nitrogen oxides can make individuals more susceptible to respiratory infections, as it can weaken the immune system's ability to fight off pathogens. Additionally, NO and NO2 can promote inflammation in the respiratory system, which can contribute to various health issues.

Looking at our environment, nitrogen oxides can react with water vapor in the atmosphere to form nitric acid, contributing to the formation of acid rain. Acid rain can harm aquatic ecosystems, damage vegetation, and erode buildings and infrastructure. When nitrogen oxides are deposited into water bodies through rain or atmospheric deposition, they can lead to nutrient loading and eutrophication. Excessive nitrogen can stimulate the growth of algae and other aquatic plants, which can disrupt aquatic ecosystems, deplete oxygen levels, and harm fish and other aquatic organisms. Last but not least, nitrogen oxides also contribute to climate change. They are greenhouse gases, which means they can trap heat in the atmosphere and contribute to global warming. While they are not as potent as carbon dioxide (CO2), they can have a significant impact on climate when their emissions are large.

Not only are particulate matter, soot, and chemical emissions concerning, but the thermal health effects stemming from climate change, brought about by rising temperatures, also raise both physical and mental health concerns. An overview of this issue and possible mitigation strategies in the built environment are given in Sharifi A, et al. [7].

Combustion research

Combustion research is key to mitigate NOx, CO emissions as well as CO2 release the latter two being inseparable connected to the use of fossil fuels. Nowadays most combustion research is connected to non-fossil fuels though the potential of reducing the amount of CO2 release from fossil fuels is still important. Experimental research is expensive and should be limited to representative cases and supplemented by numerical research which is cheap and can be applied to any design study. But for the numerical counterpart the chemical kinetics that is unavoidably involved remains quite uncertain.

For example Wang Y, et al. [8] caried out research on combusting fuel gases resulting from biomass gasification, with respect to the kinetic mechanisms that are quite standard. The gases considered were all mixtures of CH4 and CO2. They found that for a laminar burning velocity simulation case, four chemical kinetic mechanisms, GRI- Mech 3.0, [9] San Diego [10] Konnov A [11], and USC Mech II [12], all showed the same tendency compared with experimental results. The simulation results were all lower than the experimental results. Consistent with the conclusion of Nonaka HOB and Pereira FM [13], GRI Mech 3.0 showed the best agreement when the CO2 content was 20%. USC Mech II showed the best consistency when the CO2 content was between 40% and 60%. Agreement was limited when there were CO2 additions. The burning velocity showed a liner decrease while adding CO2. For NO emission, when the CO2 content was 40%, compared with the experimental results, the NO emission interval range could be predicted. The NO emissions showed a linear relationship with equivalence ratio with the addition of CO2, see figure 1. In (fuel) rich conditions, no matter how much CO2 accounted for, the NO emissions were all below 0.0001 mass fraction.

Figure 1: NO emission as function of equivalence ratio at 20 cm downstream of a flat adiabatic flame for experiments vs. different kinetic mechanisms of a CH4 / CO2 composition of 60 /40% [8].

Here turbulent combustion was not considered but this is by far the most common situation in industrial systems. The laminar case is more important for domestic cases and fundamental to the turbulent cases as well. The turbulent cases involve a further step in complexity though also much research is attended to scrutinize this issue.

Combustion of fuels not containing atomic C in their molecular structure is a valid green option to create power and heat. Electricity is often wrongly considered as green energy, but it might stem from combustion of C containing fuels. Therefore, generation of green electricity requires decarbonization. Additionally, electricity is most suitable for power generation but not for production of heat and energy storage and transport. It is important to look at the energy demand, mechanical (torque in general), electical power or heat; production of each energy form should be adjusted to this to minimize conversion inefficiencies. Furthermore, legislation to forbid e.g., internal combustion engines in vehicles and promoting electricity use is just political and not goal oriented, a clear mark of ignorance.

Biomass, containing C, is an alternative form of green energy under considerations, as it both emits and stores carbon in a relatively short cycle. These considerations include renewable management of biomass harvest and replanting. Especially for underdeveloped countries energy use of biomass would be a valid green option, also because of local availability, independence and price. Additionally, regularly harvesting yields a relative higher bio-diversity as overwhelming species get lesser overruling freedom. Possible associated decentralization leads to small enterprise opportunities, independence and frequently preferred responsibility.

It is still important to reduce harmful emissions. CO and NOx emissions of burning CH4 / CO2 mixtures, a typical product of biomass gasification, are considered in this paper. This to emphasize combustion research as a very important factor considering prediction of these harmful emissions. This also holds for reducing CO2 emissions.

Democratic political processes, legislation, release of permits and long-term planning and goal setting, mitigate the direct need to reduce CO2 and pollutant emissions. This is caused by respectively inherently associated to long term democratic processes (government terms, voting, consultation), frequently looking for best means (either practical or technological) while a choice for a good direction becomes more and more urgent., decreased trust in human entities in general, even up to tiny details, leading to a need for a large amount of associated work we cannot handle. Besides all of these long term planning leads to a tendency to get lost in delays on delays. Short term goals and planning should occur by rule. Furthermore, to my personal opinion “different size potentially fit all” holds and thus multiple energy mix might be used for applications for multiple applications and even amongst multiple equal applications.

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