Fuel characteristics of manure

Brændstofvurderinger på husdyrgødninger

The background of the project

In relation to experiments of combustion of manure the Danish EPA wishes to clarify the fuel and combustion properties of different types of manure.

The investigations in the project

Laboratory analyses have been carried out for the following eight types of manure:

Horse manure
Cattle manure fibres, from Kemira without polymer
Pig manure fibres, from Samson plant without polymer
Pig manure fibres, from Kemira plant with polymer
Mixed manure fibres after degasification, without polymer
Sitting hen manure
Broiler manure
Manure pellets

The tests are carried out according to the methods described in the technical specifications from CEN/TC 335 ”Solid biofuels” at FORCE Technology’s laboratory in Brøndby, department Chemical Analysis.

Evaluation of fuel quality is performed by Susanne Westborg, Specialist in the department Chemical Analysis and the evaluation of combustion technicalities is performed by Lars Peter Johansen, Project leader and Birgitte Holm Christensen, Head of Department, both from the department Biomass & Waste in Lyngby.

Application of manure as a fuel

For the examined samples of manure has been found that

Three of the samples contain sufficient energy for an utilisation of the energy by combustion. It is the sitting hen manure, the broiler manure and the manure pellets. The sample of cattle manure fibres contains that much water and so little energy that combustion without a support fuel is impossible. One of the pig manure samples and the mixed manure fibres are close to this borderline of a self-supporting combustion. Concerning the last-mentioned three samples, together with the second sample of pig manure and the sample of horse manure, are judged that the energy content is too low or on the borderline to secure a complete combustion.

The amount of energy consumed for the drying and pelletising of the manure pellets is not evaluated in this investigation.
The content of nitrogen in the manures is relatively high by which high concentrations of nitrogen oxide in the flue gas are possible during acombustion. If the nitrogen content in the manures is completely converted into nitrogen oxide in the flue gas, the highest concentrations would occur for the sitting hen manure, the broiler manure and the manure pellets. These are the same three manures which have the largest potential for energy utilisation by combustion. Whether higher concentrations compared to the concentrations observed concerning combustion of straw, actually will occur is difficult to judge. Conversions of the nitrogen in the fuel and in the combustion air to nitrogen oxide are both dependent on the specific combustion conditions as temperature, excess air and holding time.
Concerning formation of slag the horse manure and the cattle manure fibres properly will behave equivalent problematic as straw during a combustion – or worse as the content of ash in the manures are higher than in straw. The sitting hen manure has a very high content of ash but most probably the ash will not melt together. This is due to the high share of lime in the ash. The broiler manure presumably neither will give rise to severe slagging. Concerning the four last samples, there will be a potential risk of slagging in dependence of the actual plant conditions.

Vital fuel parameters

The content of water in five of the samples, the horse manure and the cattle manure, the pig manure and the mixed manure fibre samples is very high (above 60 weight percent). The net calorific value on as received basis concerning these manures is equally low, below 4 MJ/kg. On the other hand, the manure pellets stand out with a content of water at 11 weight percent and an equally higher net calorific value of 15 MJ/kg on as received basis. Concerning the sitting hen and the broiler manures the content of water is more moderate (20 – 40 weight percent) and the net calorific values of 7 – 10 MJ/kg on as received basis correspondingly moderate.

The content of ash forming components (”ash”) is high in all samples; 7 – 38 weight percent in the dry matter. The element composition of the ashes is very varying which influence the melting behaviour of the ash. For the ash from the sitting hen manure, having a very high calcium content and a limited content of silicone, very high ash melting temperatures are observed (above 1480 ^oC). On the other hand there are observed ash melting temperatures comparable to straw ash concerning the ashes from the horse manure and the cattle manure fibres. The content of silicone and potassium in these ashes is also comparable to the content of these elements in straw ash.

There is a high content of nitrogen in the manures, from 0,9 weight percent in the dry matter concerning the horse manure to 4 weight percent in the dry matter for the broiler manure. The content of nitrogen in straw typically is about 0,5 – 1 weight percent in the dry matter. Some biomasses however contain equally high amounts as e.g. grain and seeds with 2 – 4 weight percent in the dry matter. The level is also comparable to the nitrogen content in MDF and chipboards, typically with a nitrogen content of 3 – 5 weight percent. The content of sulfur in the manures, at 0,1 – 0,7 weight percent in the dry matter, is also higher than typically seen for straw. On the other hand the content of chlorine in the manures at 0,2 – 0,6 weight percent in the dry matter is equal to the typical chlorine content in straw.

Concerning the content of the nutrients potassium and phosphorus in the ash produced by combustion of the manures, there is a high content of phosphorus in all samples; at 8 – 30 weight percent in the ash as phosphoric pent oxide (P₂O₅). The content of potassium in the ashes varies from 4 – 5 weight percent as potassium oxide (K₂O) concerning ashes produced from the pig manure fibres, the mixed manure fibres, and the manure pellets to 20 – 30 weight percent in ashes produced from the horse manure, the cattle manure fibres, and the broiler manure.

Regarding the content of heavy metals in the manures, a very high content of zinc, at 100 – 150 mg/kg dry matter, and cupper, 20 – 300 mg/kg dry matter, compared to straw have been found. The content of nickel and arsenic is also higher compared to typical values for straw, respectively 2 – 10 mg/kg dry matter concerning nickel and 0.2 – 0.6 mg/kg dry matter concerning arsenic in the manures. On the other hand the contents of cadmium, chrome, mercury, lead and vanadium in the manures are approximately at the same level as the contents in straw. Concerning two of the samples the content of cadmium however is four times higher than in straw.