Trends and limits in exhaust gas emissions

The emission legislation for combustion installations is subject to a continuous tightening. Dr Jacob Klimstra explores the issues involved in abating and legislating on different kinds of emissions.

Emissions legislation for combustion installations is continually tightening Credit: Christian Schràƒ¶der

Emissions have always accompanied human activities. The saying goes that there is no smoke without fire, but for many centuries there was no fire without smoke. Fire, combustion, was the driver of the Industrial Revolution and smoke-belching chimneys characterized innovation and growth. In children’s drawings, factories, ships and locomotives have funnels with black smoke emerging from them. For many years, emissions were seen as a consequence of progress. A housewife just had to live with the frequent black spots on her fresh laundry and the smell of sulphur was associated with income.

Initially, the negative effects of smoke were rather local. Soot-blackened buildings and poisoning by carbon monoxide were typical characteristics. Smog often deteriorated the air quality in cities. The extent of fuel use was not so high that large regions or continents were negatively affected. Next to that, the bad effects of ‘smoke’ on health were not fully understood. Many diseases only occurred after many years of continued exposure. With the growing use of fuels and concern about health, the drawbacks of emissions became apparent and measures were taken to clean up the exhaust.

Combustion of fuels inevitably leads to combustion end products. Complete combustion of hydrocarbons (HC) produces carbon dioxide (CO2) and water (H2O). Until a few decades ago, those species were considered harmless. Water vapour might only lead to local fog formation and blocking of sunshine by clouds emanating from the stack. That is relatively harmless. CO2 is now recognized as a major greenhouse gas responsible for global warming. Incomplete combustion results in poisonous carbon monoxide (CO) and in partially oxidized hydrocarbons such as aldehydes. Hydrocarbons can escape the combustion process and leave the chimney unaltered. High temperatures during combustion mean that nitrogen and oxygen present in the combustion air combine into nitrogen oxides (NOx). NOx can also result from oxidation of nitrogen present in the fuel. NOx is poisonous, causes acid rain and plays a role in smog formation. Many countries limit the concentration of NOx, CO, HC and aldehydes in the exhaust gas of combustion processes because of their toxicity. Sulphur is present in many fossil fuels and in some biofuels. Sulphur oxidizes in a combustion process to SO2 which is causing acid rain and smog. Solid fuels and heavy fuel oil often contain incombustible species (ash) resulting in dust emissions. Dust penetrates into the respiratory system which can lead to premature deaths. Trace elements in fuel such as mercury, chlorides and zinc are also undesirable from a health point of view.

Figure 1. The increase of primary energy use in the world in 45 years

Greenhouse gas emissions

CO2 is a major greenhouse gas that is produced by combustion of fossil fuels, biofuels and waste. Another greenhouse gases is methane. Methane is produced in nature by rotting processes in swamps and in the digestive tracts of cows. In addition, methane emissions occur during production and transportation of natural gas as well as from fuel escaping the combustion process. Nitrous oxide (N2O) is a very potent greenhouse gas which can be produced in improperly tuned exhaust gas cleaning catalysts. The CO2 emission per MJ of fuel energy depends very much on the fuel type. Table 1 gives some typical CO2 emissions expressed per unit of fuel energy. Natural gas clearly is a preferable fuel in this respect.

The CO2 emission to produce one kWh of electric energy with a generating installation depends on the fuel and the energy conversion efficiency of the process. For a machine running on natural gas with a fuel energy-to-electric energy conversion efficiency of 45%, the CO2 emission per kWh equals 56 g/MJ/0.45 ࢀ¢ 3.6 = 448 g/kWh. The factor 3.6 stems from the fact that 1 kWh equals 3.6 MJ. However, if the machine running on natural gas is a cogeneration installation with a combined fuel efficiency of 90%, each kWh of electric energy is accompanied by the supply of 1 kWh of heat. To produce this 1 kWh of heat with a separate boiler of 95% fuel efficiency, 56 ࢀ¢ 3.6/0.95 = 212 g/kWh of CO2 would have been emitted. This 212 g/kWh is avoided because of the utilization of the heat coming from the cogeneration installation. One might state that the net CO2 emission for electricity production with this cogeneration installation is therefore 448 g/kWh – 212 g/kWh = 236 g/kWh. A coal-fired central power plant without utilization of the released heat has a specific CO2 emission of 98/0.40 ࢀ¢ 3.6 = 882 g/kWh in case of a fuel conversion efficiency of 40%. The central coal-fired power plant that rejects the available heat therefore produces a factor 882/236 = 3.7 more CO2 per kWh than the cogeneration installation of this example. This is a substantial difference.

Many policy papers and government plans mention the option of carbon capture and storage (CCS). One might argue that CCS is less easy to establish for decentralized installations than for a 1 GW coal-fired power plant. However, CCS for such a big plants is estimated to cost at least between €60 ($68) and €80 per tonne of CO2 removed. This turns to between 5 and 6 €cents per kWh of CO2 removal costs, which will close to double the electricity production costs of central coal-fired generation. It is also estimated that the extra energy required for capturing, transporting and storing CO2 reduces the fuel efficiency of the power plant by at least a quarter. That means that the basic CO2 production of such a coal-fired plant will exceed 1 kg/kWh. If CCS can reduce the CO2 emissions by 90%, the remaining specific CO2 emission of the coal-fired plant with CCS is still at least 100 g/kWh. The carbon capture process requires a chemical process plant that will have optimum performance at a steady load. A steady load of fuel-based power plants is, however, something of the past. Renewable energy based on wind and solar radiation is very intermittent and the fuel-based generators have to compensate for that. Typical baseload, which was so characteristic for large power plants in the past, will fully disappear with an increasing fraction of renewable energy. Decentralized generators have the flexibility to compensate for intermittency, and are therefore the best solution for the integration of renewable energy.

NOx emissions

NOx in the exhaust gas of combustion installations originates primarily from nitrogen and oxygen present in the combustion air under the influence of high temperatures. This is the so-called Zeldovitch mechanism. The production of NOx can be minimized by sub-stoichiometric combustion where the bulk of the oxygen is consumed by the fuel so that hardly any oxygen is left for NOx formation. Stoichiometric combustion means that exactly enough air is mixed with the fuel for complete combustion. Sub-stoichiometric combustion results in high carbon monoxide (CO) and hydrogen (H2) production. The poisonous CO has to be removed in a three-way catalyst or in a subsequent oxidation step with additional air. In petrol-fuelled automotive engines, a three-way catalyst reduces the emissions of CO, HC and NOx. The catalytic process makes that CO and HC are oxidized with oxygen from the NOx so that H2O and CO2 result. The engines have to run on a close to stoichiometric mixture, which is ensured with a so-called lambda sensor in the exhaust. Stationary gas turbines and most spark-ignited reciprocating gas engines use fuel-lean premixed mixtures. Fuel-lean mixtures burn much colder than close to stoichiometric mixtures, which is beneficial for the life of the components of the machine. In fuel-lean mixtures, the low combustion temperatures mean that the NOx production level is often sufficiently low to meet the legal emission limits. In countries with a poor ambient air quality and excessive acidifying emissions, selective catalysts (selective catalytic reduction or SCR) are applied to further decrease the NOx production. SCR is also used for diesel engines where the diffusion type combustion makes it impossible to create internal engine conditions to decrease the NOx production as low as that of lean-burn spark-ignited engines. In case of an SCR, ammonia (NH3) reacts with NOx to produce nitrogen and water. Ammonia is very poisonous and therefore difficult to store and handle safely. That is why urea is preferred as the reducing agent. Urea decomposes in the hot exhaust gas upstream of the catalyst and produces ammonia via so-called pyrolysis or thermolysis.

Nijmegen power plant, the Netherlands Credit: Jacob Klimstra


Figure 2. The species concentration depending on the reference percentage oxygen in dry exhaust gas

A multiple of different units is used to express the NOx emission of combustion installations. Sometimes it is expressed as mass units per energy unit of the fuel (g/GJ), where for the fuel energy the lower calorific value is used. Another way is to use g/kWh, where the kWh refers to the net electric energy produced. In legislation, often the mg/m3 of dry exhaust gas is used for a fixed oxygen percentage of 3%, 5% or 15% in the dry exhaust gas. The measurement equipment applied gives the volumetric concentration in parts per million (ppm) in the dry exhaust gas. These ppm have to be converted into mass-based units by using the density of the species. For a cogeneration installation running on natural gas with an electrical efficiency of 45%, 100 g/GJ (fuel) of NOx equals 8 g/kWh (electric), 326 mg/m3 at 5% O2 and 121 mg/m3 at 15% O2 of dry exhaust gas. It equals 59 ppm at 15% O2 in dry exhaust gas. Figure 2 can help to convert the different concentrations depending on the reference value for oxygen.

Emissions monitoring Credit: Siemens

CO, aldehydes and ash emissions

Carbon monoxide and aldehyde (R-CHO) emissions originate from incomplete combustion, which can occur due to a bulk lack of air or a local lack of air, or by quenching of a flame against relatively cold surfaces. CO is poisonous but its toxicity is a factor of 150 lower than that of NO2. The maximum allowed concentration in air for an exposure of one hour is 0.2 mg/m3 for NO2 and 30 mg/m3 for CO. Yet legislators generally limit the emissions of both species to close to the same values. Aldehydes are known to be carcinogenic. An example for the maximum exposure level of formaldehyde (H-CHO) is 0.15 mg/m3 during eight hours. Oxidation catalysts can remove CO and aldehydes from the exhaust gas. The minimum flue gas temperature required for this oxidation is around 300à‚°C.

During the combustion of solid fossil fuels, wood, HFO and waste, particulate matter and undesired species such as chlorides and mercury can be present in the end products. Chlorides can be emitted as hydrochloric acid (HCl) and dioxins which are very toxic. Waste, especially, can contain up to one mass per cent of chlorine (Cl). Coal and waste also contain mercury (Hg), a toxic element. Special filters have to remove these elements.

SO2 emissions

Sulphur oxide emissions originate from combustion of sulphur (S) containing components in a fuel. Heavy fuel oil and coal, lignite and peat can have high sulphur concentrations. For coal, one mass per cent of sulphur is quite common, while lignite can have up to 3% of S. Heavy fuel oil can even have up to 4.5 mass per cent of sulphur. The molar mass of SO2 is twice as high as that of S. For a lower calorific value of 40 MJ/kg of the HFO and one percent of S, an SO2 emission of 500 g/GJ (fuel) results. In case of a fuel efficiency of 45%, this turns into 40 g/kWh. The acidifying effect of SO2 is a factor of 1.44 higher than that of NO2. In most industrialized countries, legislation has restricted the emission of SO2 to such low values that power plants using sulphur-containing fuels always need exhaust gas cleaning equipment. Official standards for natural gas generally limit the total sulphur content to 30 mg/m3 à¢â€°Ë† 0.004 mass percent. In some countries the limit is 10 mg/m3. Even a sulphur content of 30 mg/m3 is too much to ensure an economic life for fuel cells and automotive catalysts.


Emissions legislation for combustion installations is subject to a continuous tightening. Legislators are constantly searching for the best available technologies (BAT) and best references (BREFS). As long as negative health effects of emissions are present and better abatement solutions become available, narrower limits will be used. Local authorities are often allowed to tighten the maximum emission levels if the local air quality requires this. It is therefore not expected that worldwide standardized limits will be made for the emissions by decentralized generators of species such as NOx, SO2, CO and aldehydes. Manufacturers and users will have to live with benchmarking by policymakers, and with tightening local legislation.

Dr Jacob Klimstra is Managing Editor of Decentralized Energy This article is available on-line.

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