DEVELOPMENT OF AN EXCHANGE SCRUBBER: EXHAUST AIR CLEANING AND HEAT RECOVERY IN ONE PROCESSING STAGE

KROMMWEH, M. S.¹, BÜSCHER, W.¹

1 University of Bonn, Institute of Agricultural Engineering, Nußallee 5, 53115 Bonn, Germany

ABSTRACT: Exhaust air treatment systems (EATS) in animal husbandry are cost-intensive. To reduce overall costs of livestock buildings with EATS, a recuperative heat exchanger (HE) was integrated into a trickle-bed reactor in order to clean exhaust air and utilize thermal energy from exhaust air and scrubbing water for heating of incoming outside air. During winter and spring, initial tests under practical conditions on a pig farm were carried out using the

“exchange scrubber” in partial flow treatment. Relevant temperatures, air flow rates as well as power consumption were recorded.

While passing the HE, cool incoming supply air is heated up to the temperature level of the scrubbing water. In consequence, fossil fuel reserves are substituted, carbon dioxide emissions avoided, heating costs saved, and operating costs can be reduced. Supply air heating depends on temperature of outside air and scrubbing water. For outside temperatures below the scrubbing water temperature, the following applies: the lower the outside temperature, the higher the supply air heating and the heating output. Cooling effects could be observed at outside temperatures above the scrubbing water temperature.

The coefficient of performance was 7.8.

Keywords: Mitigation strategy, Energy saving, Swine, NH₃, Odour

INTRODUCTION: Intensive animal husbandry contributes to air pollution, by emitting particulate matter (PM), ammonia (NH₃), and odour. In practice, exhaust air treatment systems (EATS) are applied on mechanically ventilated livestock buildings for pigs and poultry to reduce these emissions, e. g. biofilters, trickle-bed reactors (air scrubber), acid scrubbers and multiple stage techniques (Grimm, 2008; Van der Heyden et al., 2015).

However, EATS are associated with high investment and operating costs (Grimm, 2008), which have to be borne by the plant operator. In order to reduce overall costs of a livestock building with EATS by saving heating costs (recovery of thermal energy), a new plant technology for reduction of emissions with simultaneous heat recovery from exhaust air and scrubbing water was developed as part of a research and development project. This plant

underflow suction. A part of the exhaust air was led into the exchange scrubber experimental plant. The internal floor area of the exchange scrubber was 2.3 m x 1.1 m, which results in a filter surface area of 2.53 m². The trickle-bed reactor technology used has already been certified by the German Agricultural Society (DLG) for livestock buildings with central above floor suction and is described in detail in the test report No. 6284 (DLG, 2015).

A recuperative heat exchanger was integrated into the trickle-bed reactor upside the packing material. Both were sprayed by scrubbing water. The heat exchanger has been made of polyvinyl chloride (PVC) with a height of 0.6 m and a specific heat exchange surface area of 136 m² m^-3. The heat exchanger surface was turned. It was a cross-flow heat exchanger, i.e.

exhaust air was led vertically through the heat exchanger, while outside air was led through horizontally. The heat transfer that ensued was convective over the PVC sheets exclusively.

During the examination time from November 26^th, 2015 to April 5^th, 2016, the following parameters were measured and recorded: temperatures of exhaust air (crude gas), outgoing air (clean gas), outside air, supply air, and scrubbing water (measuring interval: 5 min);

electricity consumption of electric consumers (e. g. exhaust air fan, supply air fan, recirculation pump) by means of electronic electricity meters; air flow rates of exhaust air and supply air by use of calibrated measuring fans (measuring interval: 5 min). The air flow rate of supply air and exhaust air was adjusted to 2,255 m³ h^-1 and 2,763 m³ h^-1, respectively.

In order to be able to compare the exchange scrubber with other technologies, the coefficient of performance (COP) was calculated according to VDI 3803-5 (2013), by relating thermal power delivered by heat recovery (Q„HR) to electrical power required (Pel):

JKL =^MN_Q^OP

RS (1)

2. RESULTS AND DISCUSSION: In general, the heating performance of the exchange scrubber was mainly influenced by the outside air temperature and subsidiary by scrubbing water temperature. The lower the outside air temperature, the higher the heating performance and vice versa (Figure 1). When outside air temperature exceeded scrubbing water temperature, incoming outside air was cooled down by passing the heat exchanger unit.

During the examination time, the exchange scrubber consumed 2,889 kWh electrical energy (Pel) and delivered 22,620 kWh thermal energy (Q„HR). This results in a coefficient of performance of 7.8 (Equation 1). The average monthly heating performance ranged between 6.5 kW and 8.5 kW (in terms of a constant supply air flow rate of 2,255 m³ h^-1). The highest heating performance was 21.0 kW at an outside temperature of -18°C (Figure 1).

Mitigation strategies

Figure 1: Heating performance of the exchange scrubber as a function of outside air temperature (y=-0.57x+9.17; R²=0.88) in January 2016.

3. CONCLUSION: The technology of the exchange scrubber contributes to a more environment-friendly and energy-efficient animal husbandry and is positively assessed under various aspects; saving energy costs and with it reduction of operating costs (economical aspect); fossil fuel reserves are spared (sustainability); avoiding of carbon dioxide emissions and removal of PM, NH3, and odour (ecological and environmental aspects); better air distribution on animal level (animal welfare). However, a detailed full cost accounting of the exchange scrubber has to be carried out. Because the previous investigations of the exchange scrubber are based on a partial flow treatment of the exhaust air with continuous air flow rates of exhaust and supply air, a long-term study of the plant is recommended for a well-founded economic evaluation of this technology. For that to happen, it is important that the entire ventilation of the livestock building as well as the cleaning of the exhaust air have to be carried out by the exchange scrubber. Further research is necessary to clarify how far the exchange scrubber can also be used to cool supply air in summer.

Acknowledgements. This project was carried out in close collaboration with Schönhammer Wärmetauscher und Lüftungstechnik GmbH (Mengkofen, Germany) and supported by the Federal Ministry of Food and Agriculture (BMEL) based on a decision of the Parliament of the Federal Republic of Germany via the Federal Office for Agriculture and Food (BLE) under the innovation support programme (funding code 28-1-408 75.011-11).

REFERENCES:

SOLID FLOORS WITH A SLOPE FOR RAPID URINE DRAINAGE: FIRST RESULTS FROM