Ceramic Tunnel Kilns in Ukraine

There are many old factories in Ukraine producing ceramic wall materials. They were built in the middle of the last century according to the standards of that time and in a hurry, i.e. in a short period of time, because it was necessary to quickly restore the infrastructure destroyed by the war. However, some of these factories are still operating today, although it must be admitted that several factories have been restored to European standards, but they are not enough.

Ukraine has clay raw materials suitable for the production of ceramic products [1]. On the territory of the Poltava region alone there are more than 200 studied clay deposits with a mineral composition and physical characteristics that meet the requirements for clay raw materials for ceramics [1].

Most ceramic factories were equipped with standard tunnel kilns with a capacity of about 30 million pieces/year or 100 thousand tons/year. The furnaces could run on both gas and coal. When using coal to remove ash, an ash accumulation chamber was organized under the firing zone. The productivity of factories was increased by increasing the number of furnaces. Currently, some kilns use coal dust as fuel, although European countries are already exploring the possibility of using hydrogen as a fuel in the production of ceramics [2]. Old factories are also far from organizing an environmentally neutral firing process for ceramic products, as is being studied in Europe [3]. There is also no research on cleaning flue gases from tunnel kilns to reduce harmful emissions into the atmosphere, although such research is already being carried out in European countries [4].

Most factories produce solid bricks of standard sizes 250 x 120 x 65 mm. The products do not meet the requirements of the standards, although their strength ranges from 10 to 15 MPa, which corresponds to local standards. Based on the above and taking into account that after the end of the war Ukraine will need to restore infrastructure and build residential buildings, we examined a typical tunnel kiln in order to save fuel and reduce emissions into the atmosphere.

Tunnel kiln inspection results.

Tunnel kiln for firing bricks and ceramic stones with a capacity of 30 million pieces normal format bricks using gas fuel.

General characteristics of the kiln

Total length – 120m.

Zone length:

- heating – 42m;

- firing – 30m;

- cooling – 48m.

Number of burners – 22 pcs.

Cycle duration – 48 hours.

Duration of work per year – 7920 hours.

There are no devices for recirculating combustion products and air curtains.

Loading the first trolley – 3000 pcs. bricks.

The mass of the trolley m_t = 2200 kg.

The mass of bricks on the trolley m_b = 11520 kg.

The initial moisture content of the raw material at the inlet is W = 5%.

The number of trolleys that are simultaneously in the cooling zone is 16 pcs.

The time spent by the first trolley in the cooling zone is 19.2 hours.

Temperatures by kiln zones:

- heating zone - 600ºC;

- firing zone - 1000ºC;

- cooling zone - 400ºC (average).

To determine the amount of heat loss through the kiln enclosing structures, heat transfer coefficients were determined for the walls and roof in different parts of the tunnel kiln.

The value of the heat transfer coefficient was determined by the formula:

К=1/[(1/α_in) + Σ(δ_i/λ_i) + (1/α_out)],   W/м² °C        (1)

where α_in is the heat transfer coefficient on the inner surface of the kiln enclosure. value α_in=17 W/m² °C;

δ_i – thickness of the layers of the wall structure, m;

λ_i – thermal conductivity coefficient of the wall material;

α_out – heat transfer coefficient on the outer surface of the fence. At wind speed v=5m/s, α_out=28W/m² °C.

The thermal conductivity coefficients of materials are assumed to be as follows:

red brick - λ = 0.81 W/m °C;

fireclay brick - λ = 1.33 W/m °C;

ordinary clay - λ = 1.0 W/m °C;

diatomaceous filling - λ = 0.15 W/m °C;

diatomaceous earth insulating brick - λ = 0.22 W/m °C;

boiler slag (poured onto the roof) - λ = 0.29 W/m °C.

Determination of the amount of heat flow that passes through the fences (heat loss to the environment) was calculated using the formula:

Q_cp = K * F * (t_in – t_out)   W,                    (2)

where F is the surface of the fence;

t_in – temperature of the medium in the furnace volume, ºC;

t_out – ambient temperature, taken according to the average temperature of the heating period, t_out = 0ºC.

Calculations of heat losses are shown in »Table.

As can be seen from the »Table, the heat loss of the kiln in the current situation is 898 kW, which is equal to almost half of the total thermal potential of the required fuel consumption. Taking this circumstance into account, we proposed to reconstruct the furnace. The basis for the reconstruction was the organization diagram of the gas-dynamic furnace circuit developed by the Keller company [6] and implemented at the Br. Laumans in Braacht.

The reconstruction project included the following activities:

Thermal insulation of the furnace with basalt wool;

Installation of a combustion products recirculation system in the heating zone;

Installation of an air curtain between the firing and cooling zones;

Installation of a cold air supply system to the cooling zone;

Installation of a system for localizing warm air and transporting it to dryers;

Selection of combustion products from the firing zone and heat exchanger to heat the air for the burners and supply heated air to the heating zone of the tunnel kiln.

We carried out thermal calculations to compare data before and after the implementation of reconstruction measures.

The heat balance of the furnace is determined by the equation:

Q_ng+ Q‘_tr+ Q_wb+ Q_air=Q_uh+ Q_wg+ Q‘‘_trout+ Q_hlb+ Q_hle,  кВт  (3)

where Q_ng is the heat that comes from the fuel – natural gas;

Q‘_tr is the physical heat that comes from the trolley, weighing 2200 kg;

Q_wb – physical heat that comes with raw unfired bricks weighing 11520 kg/vag;

Q_air – physical heat that comes with heated air;

Q_uh - useful heat that was spent on heating, firing bricks and heating the trolley, is determined on the condition that the temperature of the brick and trolley at the outlet of the kiln should not exceed +50ºC.

Determined by the formula:

Q_uh=(m_tr+ m_br) * с * t_out,                  (4)

where c is the average heat capacity of the trolley and bricks, c = 0.22 kJ/kg ºC;

Q_wg - heat consumption with waste gases that are formed in the burning zone when burning natural gas, is determined by the dependence:

Q_wg=[Р_gc(1+λ * V_t) * с_hcwg * (t_out-170)]K_ef,                     (5)

where P_gc is the consumption of natural gas in the kiln, m³/h;

λ – coefficient of excess air with which the kiln burners operate;

V_t – theoretical demand for air for combustion, V_t=9.8 nm³/nm³ of gas;

c_hcwg – volumetric heat capacity of waste gases, 1.3 kJ/nm³ ºC;

t_out –  the temperature of waste gases at the exit from the firing zone of the kiln, t_out = 1000 – 950 ºC;

170 ºC is the minimum possible temperature of the flue gases at the outlet of the kiln or kiln heat exchanger, which is necessary to avoid condensation of combustion products.

K_ef – coefficient that characterizes the efficiency of the heat recovery system of fuel combustion products;

Q‘‘_trout is a physical quantity that characterizes the amount of heat leaving with the trolley that comes out of the kiln (heat loss);

Q_hlb – heat loss with heated bricks that come out of the kiln;

Q_hle – heat loss to the environment through the enclosing structures of the kiln.

1. Distribution of heat balance items for the furnace in its current state

a – useful amount of heat for the implementation of the technological process - 0.55 MW (7.6%);

b – heat loss to the environment through enclosing structures  - 0.88 MW (12.4%);

c – heat loss with exhaust gases – 2.9 MW (40%);

d – heat loss with heated bricks – 2.9 MW (40%);

Overall: 7.2 MW (100%).

Kiln efficiency – 7.6 %;                                                               

Hourly gas consumption – 731 m³/h;

Gas consumption per year is 5.8 million m³.

2. Distribution of items in the heat balance of the furnace after the introduction of a cooling system for bricks and trolleys and the installation of thermal insulation of pipelines.

a – useful amount of heat for the implementation of the technological process - 0.55 MW (18.0%);

b – heat loss to the environment through the enclosing structures - 0.44 MW (14.6%);

c – heat loss with exhaust gases – 0.86 (28.4%);

d – heat loss with heated bricks 1.18 MW (39.0%);

Overall: 3.03 MW (100%)

Kiln efficiency – 18.0%;

Hourly gas consumption – 307 m³/h;

Gas consumption per year – 2.431 million. m³.

As can be seen from the above data, fuel consumption after applying reconstruction measures decreased by more than half, and the efficiency increased by more than two times.

Conclusions

1. In its current state, the tunnel kiln is characterized by significant fuel consumption for the production of bricks and low efficiency, which is explained by the following reasons:

low heat-protective characteristics of the furnace enclosing structures, which contributes to significant heat losses to the environment.

the absence of combustion product recirculation systems and air curtains in the furnace design to the existing state, which significantly reduces the efficiency of using the heat of fuel combustion products in the furnace, contributes to an increase in the stratification coefficient and leads to a deterioration in product quality;

complete absence of heat recovery from the firing zone, which constitutes approximately 40% of the heat balance of the furnace. This heat is lost irretrievably, which significantly worsens the thermal efficiency of the furnace.

2. It is proposed to increase the efficiency of the tunnel kiln through the introduction of a number of energy-saving measures outlined in the table.

3. The total annual efficiency of implementing measures is 25,400 thousand UAH, the payback period is 0.5 years.

4. Based on the results of the implementation of energy-saving measures, the efficiency of the furnace was increased from 7.6% to 18%.

5. The proposed measures did not allow achieving European levels of fuel consumption and furnace efficiency. In general, we did not expect that we would reach such a level, because to achieve such results when firing ceramics, the entire complex should be considered: starting from the design of the furnace, and the entire gas-dynamic system.

6. However, Ukraine will have to restore the infrastructure destroyed by the war and for this it will need a large amount of materials, including ceramics. These factories will continue to produce products even though the products do not meet standards.