Hydrogen – a future fuel gas for brick plants? (part 1)

1 Introduction

To start off, currently available data on the future possibilities and limitations of hydrogen supply were analysed and the necessary requirements for the brick plants elaborated. In addition, selected physical properties of natural gas and hydrogen and their mixes as well as empirical data on heat transfer and energy balance were considered. The exhaust gas composition was calculated both for stoichiometric combustion and for the conditions in the tunnel kiln. These results served as the basis for laboratory investigations conducted on heavy clay ceramic raw materials, the results of which are presented here and serve as an example.

2 Hydrogen supply in brick plants

In the first sub-project, the possibilities for the production, storage and transport of hydrogen (H₂) as well as for the required safety equipment in brick plants were investigated. On the one hand, local electrolyzers or tank storage (delivery by truck/rail) for brick plants are feasible. On the other hand, supply via existing natural gas pipelines has been considered. The European natural gas network is to be developed in future to enable transport of up to 100 % H₂. A near-nationwide gas supply as is currently the case for natural gas, however, is considered unrealistic, at least for the forthcoming decade [1], [2], [3], [4].

Currently, up to a maximum of 10 % H₂ can be mixed into European natural gas [1], [5]. However, the irregular feeding of alternatively generated fuel gases into the gas network leads increasingly to a fluctuating gas composition. Industrial consumers like brick plants are therefore advised to conduct a continuous gas analysis on site and to adapt the firing gas and combustion air quantities to the real heating value [6], [7].

From a planned concentration of 20 % H₂ in the natural gas, testing and, if required, modification of the existing pipeline and firing technology at the consumer facility is necessary [7]. As shown in Section 3.2, a H₂ content < 80 % in the natural gas, however, has a comparatively limited impact with regard to the actual goal of using H₂, that is, the reduction of CO₂ emissions.

The mixing of H₂ into the natural gases currently used in Central Europe is also dependent on the calorific value and the density of the respective natural gas (cf. Equation (4)). The possible limits of the mixing in of H₂ are described in [7] und [8]. Maximum tolerable Wobbe indices are specified for natural gas H at 3.6 – 15.7 kWh/m_N³ and relative densities (gas density relative to the air density) of 0.55 – 0.75 [2], [9]. The resulting maximum amount of H₂ that can be mixed into the natural gas is therefore dependent on the composition of the specific natural gas. In [10], depending on the change in the Wobbe Index, the following different effects on the production process were established: cf. »Table 1.

Another project focus was assessment of the cost efficiency of H₂ utilization. For this purpose, the production data of tunnel kilns from five brick plants in Thuringia were used. Drying of the bricks upstream of the firing, which has a similarly high energy requirement, was not analysed. As a nationwide supply of 100 % H₂ via the natural gas network cannot be expected over the next decade, to begin with, delivery of the H₂ on a truck trailer was considered. The basis for calculation is listed in »Table 2.

For the specified production capacities, around 2 – 10 trucks H₂ per day and brick plant are required. In addition, an interim storage tank for the H₂ is recommended to enable continuous further operation of the kilns in the event of supply bottlenecks. For a tank capacity of 350 kg [11], the brick plants need – based on reserves for ten days – 37 - 197 tanks per plant. For a spherical tank shape, this tank volume corresponds, to diameters of 18 – 32 m.

For the five Thuringian brick plants, a total H₂ requirement of 24.5 t/d results. To minimize transport costs, this quantity should be available within a certain radius of the plants and is currently not realizable. The biggest PEM electrolyser built in 2022 at Linde AG in Leuna, with a capacity of 24 MW, produces, for example, around 9 t/d H₂, all of which is needed for the installations on site.

For this reason, in the following, the variant of production of H₂ in a plant-owned electrolyser on site is analysed. The operation of the electrolyser is possible both with electricity from the grid and with electricity from renewably generated energies, for instance with an inhouse PV or wind power plant, from hydropower or biogas plants. The basis for calculation is shown in »Table 3.

The price for the PEM electrolyser (Proton Exchange Membrane) was specified by Linde AG in 2022 and applies for a capacity up to a maximum of 20 MW_el. The operation of a PEM electrolyser also requires treated, deionized water [12]. The capacity of onshore wind turbines is currently 3 – 6 MW per wind turbine [13]. In Germany, around 7 300 hydropower plants exist, which have a combined installed capacity of around 5 600 MW_el. 94 % of the plants have an installed capacity of under 1 MW and are therefore classed as small hydropower plants [14].

For the calculation of the necessary capacity of the wind turbines as well as PV, biomass and hydropower plants, the specified full-load hours specified in »Table 4 were used [12]. The less the full-load hours of a plant are, the larger this must be designed to be so that during production hours, sufficient H₂ can be produced without sun, wind, hydropower or biomass. These downtimes require additionally for the time without H₂-production much larger gas interim storage tanks that shown above (for PV e.g. for 326 days in the year). In addition, it should be mentioned that while power generation from biomass does demonstrate the highest number of full-load hours, however, with 52.6 ha/GWh/a requires a much larger area than, for example, photovoltaic plants with 1.0 – 1.2 ha/GWh/a [15].

The results, like the number of plants and the installation areas needed as well as the H₂ and water requirement per brick plant is shown in »Table 5.

The results show by way of example how many plants and what areas are needed to generate renewable power to operate an inhouse electrolyser for H₂ production and fully supply the kilns with renewably produced H₂.

The calculations on cost efficiency reveal that, based on current prices, the production and supply of the hydrogen alone is five to eight times more expensive than natural gas. On top come other costs for water treatment, the building costs for the PV plant, wind turbines, buffer storage, burner modification, adaption of the gas pipelines and seals as well as measurement, control and safety systems. Moreover, a similar amount of energy as well as other investments can be expected for brick drying, which is not addressed here.

According to the current situation, enormous expenses are incurred for in-plant generation of H_2, which cannot be borne by the brick plants alone. In view of this, policy makers are called upon to create suitable general conditions for the provision of renewably produced hydrogen in the available power networks. In response to the increased natural gas prices, the operators of brick plants are currently continuing to focus on energy-saving measures. With new high-efficiency kilns, ­further potential is available to reduce energy consumption by up to 70 % [16].

3 Basic principles for natural gas and hydrogen

3.1 Properties of pure gases

Natural gas is composed primarily of methane (CH₄). The complete oxidation of CH₄ takes place according to Equation (1). Besides the heat released, which corresponds to the enthalpy of formation (dH_B), the gases water vapour (H₂O(g)) (g - gaseous) and carbon dioxide (CO₂) are formed in the specified quantities. The enthalpy of formation has a negative value and is exothermic. It corresponds to the heating value H_i,n (i from inferior, n = relative to the standard state).

CH₄ + 2O₂ 2H₂O (g) + CO₂ dH_B = -802 kJ/mol (1)

1,00 kg + 4,00 kg 2,25 kg + 2,75 kg

1,00 m³ + 2,00 m³ 2,00 m³ + 1,00 m³

The combustion of H₂ takes place according to Equation (2). Besides heat, only water vapour is formed.

H₂ + 1/2 O₂ H₂ O (g) dH_B = -242 kJ/mol (2)

1,00 kg + 7,94 kg 8,94 kg

1,00 m³ + 0,50 m³ 1,00 m³

The quality of heat relative to the mass or volume that fuels release on complete combustion as well as the use of the condensation heat of the water is termed the gross calorific value (H_s,n) (s from superior, n = relative to the standard state). The calorific value of gaseous fuels is calculated based on their composition, which is determined by means of gas analysis [3], [7], [17], [18].

The heating value (H_i,n) is calculated from the calorific value less the condensation heat of the water in the exhaust gas in accordance with Equation (3), in a simplified calculation that does not take the volume work into account. The water comes from both the fuel moisture content and the combustion of the hydrogen present in the gas as well as of the hydrocarbons. The calorific value as the maximum usable heat quantity of a fuel is, because of the addition of the condensation heat of the water, always greater than the heating value (»Table 6) [3], [7], [17], [18].

H_i,n = H_s,n - V_H2O * r_n (3)

H_i,n Heating value, relative to the standard volume [kJ/m_N³]

H_s,n Calorific value, relative to the standard volume [kJ/m_N³]

V_H2O Water content from the elemental analysis, relative to the standard volume of the dry fuel gas [-]

r_n Evaporation enthalpy of water at 25 °C of 1 990 kJ/m_N³, relative to the standard volume

The Wobbe index (W_s,n) is a measure of the energy delivered by a burner and a gas pipeline. It is used to assess the exchangeability of a firing gas and calculated according to Equation (4) from the calorific value and density ratio of the firing gas relative to air [3], [7], [18].

W_s,n =  H_s,n/√(ρ_Gas/ρ_Air ) (4)

W_s,n Wobbe index, relative to the standard volume [kJ/m_N³ ]

H_s,n Calorific value, relative to the standard volume [kJ/m_N³]

ρ_Gas Density of the gas in standard conditions [kg/m_N³]

ρ_Air Density of the air in standard conditions [kg/m_N³]

If gases with different compositions have the same Wobbe index, then theoretically at the same pressure and with one and the same burner, the same flow of heat can be realized. This also applies for the flow of heat transported in a gas pipeline [19].

The content of CH₄ in natural gases worldwide ranges between 62 % (USA, Cunningham) and 100 % (Italy, Corregio, Ravenna) [18]. Other constituents in natural gas can be higher-order hydrocarbons like ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀) as well as nitrogen (N₂) and hydrogen sulphide (H₂S) [18], [19], [20].

»Table 6 lists selected properties of CH₄ (representative of natural gas H) and H₂[18], [19], [21], [22], [23]. Additional ­literature sources are given in the table. Depending of the specific composition of natural gases, their properties can deviate from the CH₄ properties. [Table 6]

The properties of gaseous fuels are generally specified in standard cubic metres relative to the volume of the gas. The design of a firing installation requires – besides the gas volume that has to be transported and supplied – a certain amount of energy for material heating and material conversion. The specific energy consumption for firing heavy clay ceramics in the tunnel kiln is 400 – 3 500 kJ per kg fired material [26]. For a comparison of the fuel gas properties for one and the same amount of energy, the amounts of air and gas, the exhaust volume as well as the water and CO₂ emissions were therefore related to the specific heating value (»Table 6, second column from the right). H₂ has in comparison with CH₄ the following properties:

The heating value per unit of mass is 2.40 times that of CH₄

The heating value per unit of volume on the other hand is only 30 % and the density 13 %, as a result, for the same energy input, 3.33 times the fuel gas volume is required

The Wobbe index is 90 %

The adiabatic flame temperature is 1.08 times higher

The ignition rate is 7.4 times that of CH₄

The ignition range in which an explosive fuel gas/air mixture is formed is considerably larger (the explosive range is then similar to that of acetylene C₂H₂ with 2.3 – 78.0 vol%)

The ignition temperature is 40 K lower

The minimum combustion air required per m_N³ fuel gas is just 25 %, for the same amount of energy per MJ heating value, on the other hand, 83 %

The sum of the fuel gas and combustion air volume results for the CH₄-air mix in 0.294 m_N³/MJ and for the H₂-air mix in 0.314 m_N³/MJ, the volume of fuel gas-combustion air as a function of the amount of energy increases by 7 %

The wet exhaust gas volume per m_N³ fuel gas is just 27 %, for the same amount of energy per MJ heating value, on the other hand, 91 %

The water content per m_N³ fuel gas is just 50 %, per m_N³ wet exhaust gas, it amounts to 1.83 times the amount, for the same amount of energy, on the other hand, 1.67 times the amount of water is formed; as a result, the dew point of the exhaust gas increases from 59 °C by 14 K to 73 °C

Combustion takes place without CO₂ emissions (cf. Equation (2))

The density of the wet exhaust gas is reduced to 89 %, i.e. transport of the “thinner” exhaust gas required higher fan power for a theoretically equal amount of exhaust gas, however, this is compensated for with the lower exhaust gas volume of 91 %,

The specific heat capacity of the wet exhaust gas is unchanged, i.e. for the same exhaust gas quantity and temperature, almost the same quantity of heat is discharged

The emissions level of the wet exhaust gas increases as a function of the temperature: at 1 000 °C, it is 12 % lower, at 1 300 °C the two values are around the same, at 1 800 °C, it increases 1.36 fold; i.e. the heat transfer based on radiation changes, however, only by a small order of magnitude (cf. results from [7] under 3.3)

CH₄ cools down on pressure reduction (throttling) on account of the Joule Thomson effect (JT) and is therefore preheated, and consequently energy is needed for this. H₂, on the other hand, has a negative JT effect and heats up on the throttling, preheating of the gas is therefore not required [27]

Important for the firing process is the 3.33-fold fuel gas volume, which necessitates a high fuel gas supply rate and therefore different pipeline cross-sections and pressures. The standard volume made up of fuel gas and combustion air that exits at the non-premixed burner increases slightly. The higher combustion temperature leads to the increase in the exhaust gas volume during operation and has an adverse effect on the materials of existing burner nozzles. This necessitates a change in the burner design. In addition, with unchanged nozzle form, in comparison with CH₄- or natural gas combustion, more thermal NO_x is formed. The somewhat lower quantities of combustion air and exhaust gas in comparison with CH₄ can have a positive impact on the energy consumption of a kiln installation (»Table 9 in part 2 in ZI 2/2024). Depending on the quantities of air in the kiln, the increase in the water content in the exhaust gas can influence the product properties.

3.2 Properties of the gas mixes

The properties for different percentages of H₂ in the CH₄ calculated from the values in »Table 6 are shown in »1 and »2. In this context, it should be noted that real temperatures and ignition limits in gas mixes can deviate from the calculated values. The values apply for stoichiometric combustion with air (air factor λ = 1 without excess air) and were calculated from the respective contents of H₂ in CH₄.

For »3, selected properties of CH₄-H₂ mixes were correlated to the respective value of CH₄. They result in a relative change in the respective property of the gas mix in comparison with CH₄. »Table 7 shows by way of example some values from »3 for selected H₂ contents in the CH₄.

»3 documents the non-linear curve of the properties, which agrees with the literature values. For instance, in [27], for natural gas with a CH₄ content of 98 vol%, a 3.3-% reduction in the CO₂ emissions was calculated for a H₂ content of 10 vol% while 16.3-% reduction is calculated for a H₂ content of 20 vol%.

On the basis of the relative values, it is clear that for a practically possible addition of 50 vol% hydrogen to CH₄, the water mass in the exhaust gas increases by 15.2 % and the CO₂ mass decreases by only 23.1 %. The calculated values practically agree with the measurement results in [7] for natural gas with a CH₄ content of 98 %. The combustion air required is decreased by 4.2 % and the wet exhaust gas volume by 2.0 %. A reduction in the CO₂ emissions by more than 50 % is only achieved with a H₂ content above around 77 vol%.

How far the reduced combustion air and exhaust gas quantities and the increase water vapour emissions impact the radiation and therefore the heat transfer to the brick requires further testing.

3.3 Heat transfer and energy balance

The exhaust gas composition after combustion has a key influence on the heat transfer to the product. At the Gas-Wärme-Institut Essen (GWI), the influence of the fuel gas on heat transfer in a test flame tube was investigated. For this purpose, all input and output media temperatures and quantities were recorded and the heat flows for water and exhaust gas and the firing efficiency calculated [7]. The results showed that both the heat flows for water and exhaust gas and the efficiency were at the same level between 0 and 50 vol% H₂ content in the natural gas. Accordingly, up to a H₂ content of 50 vol%, no differences in the heat transfer compared to pure natural gas were determined.

In addition, the heat transfer for an H₂ content of 0 and 50 vol% in the natural gas was simulated with computational fluid dynamics (CFD). The results showed [7]:

Up to an H₂ content of 50 vol%, the exit temperature of the flame and the combustion temperature and the temperature distribution in the kiln chamber reached almost the same values as those for pure natural gas

the heat flows [J/h or W] and the heat flow densities [W/m²] as well as flame size as an indicator for the heat input increased slightly up to an H₂ content of 50 vol%

the exhaust gas composition and therefore also the content of the radiation-active molecules CO₂ and H₂O changed with the increasing H₂ content; up to a H₂ content of 50 vol% in the natural gas, a higher radiation heat flow density was calculated, however, no significant change in the heat transfer based on radiation was determined

up to an H₂ content of 50 vol%, the firing efficiency remained the same as that of the natural gas

It is not possible to draw any conclusions regarding how the heat transfer for the combustion of 100 % H₂ differs compared to pure natural gas since, as shown above, the exhaust gas composition only changes significantly from an H₂ content of around 80 % in the natural gas. For this reason, the energy consumption of a tunnel kiln was calculated for firing with 100 % natural gas and 100 % H₂ (cf. Section 4.1 in part 2 in ZI 2/2024).

3.4 Composition of the exhaust gas

On account of its different constituents, the composition of the exhaust gas influences the product properties. In addition to the combustion products water vapour and carbon dioxide CO₂ (»Table 6), depending on the burner design and the fuel gas, carbon monoxide (CO) and nitric oxides (NO_x) can also form. Extensive tests on this were also conducted at the GWI Essen [7]. So after the combustion of natural gas-H₂ mixes, the exhaust gas composition was measured and the water content in the exhaust gas calculated. The measured values for the H₂ content in the natural gas from 0 – 50 % were correlated to an O₂ content of 1 vol% and correspond to the values calculated in »Table 7. The volume of thermally formed NO_x as well as the water content increase with increasing H₂ content, the CO and CO₂ contents, on the other hand, decrease. The NO_x content cannot be anticipated as it depends essentially on the burner design and the flame temperature [7].

Further measurements were conducted on 100 % natural gas and natural gas with an H₂ content of 10 vol%. In a firing chamber, the composition of the atmosphere and the temperature during the combustion in the middle of the kiln chamber were measured by means of a mobile suction pyrometer. Here the concentration of CO and CO₂ decreased slightly at an H₂ content of 10 vol% in the entire firing chamber, but especially directly in the flame. The NO_x concentration increased slightly on account of the increased temperature in the firing chamber [7].

In addition, CFD simulations were conducted with an H₂ content up to 50 % in the natural gas and showed [7]:

At the burner exit, depending on the quantity of air supplied, CO was always formed (the more air was present, the lower was the CO content), for an H₂ content of 50 vol%, the CO content in the flame decreased slightly in comparison with natural gas

The CO₂ content in the exhaust gas decreased in comparison to natural gas, for an H₂ content of 10 % no decrease could be detected, at 50 vol% H₂, on the other hand, a considerable decrease could be detected

Up to a H₂ content of 50 vol%, the NO_x content in the exhaust gas was similar to that for natural gas and heavily dependent on the burner design; the tested low-NO_x burner also showed a low NO_x content in the exhaust gas when H₂ was used

The change in the gas radiation of the exhaust gas as result of the combustion of H₂ in comparison to the natural gas was shown in the increase in the intensity of the radiation bands of the water mainly at 1.5 µm, 1.9 – 2.0 µm and 2.5 – 3.5 µm and the decrease in the CO₂ intensities between 2.8 and 3.0 µm as well as between 4.0 and 4.5 µm.

(Read part 2 in ZI 2/2024)