The experimental materials used in this work, poplar, hemp, straw and cotton stalks, are suitable for briquetting. These materials belong to biomass so that their press characteristics are similar. The density-pressure function and stability-pressure function are also similar.

There is a correlation between the pressure and the briquette density. There is also a correlation between the pressure and the radial compressive strength of the briquette. Increasing the pressure will increase the briquette density digressively and its radial compressive strength progressively. Stable briquettes (radial compressive strength ≥ 0.25 N/mm² and dry matter density ≥ 0.8 g/cm²)can be produced with pressure ≥70 MPa depending on the material, particle size, moisture content and press diameter. The theoretical model developed by Clauß (2002) for computing the density as a function of pressure agrees well with the values measured in this work.

The dry matter density and the radial compressive strength generally increase with the increase in moisture content of the material. Increasing the moisture content to more than 10% will decrease the density values. The optimal moisture content for briquetting ranges between 8-15%. Cotton stalks in Egypt reach this range of moisture content after one-week of field storage in October (harvest month). Cotton stalks lose more than 80% of their initial moisture content in this period. This implies that these residues can be briquetted directly on the field one week after the harvest operation by a suitable type of briquetting machine without incurring any additional costs for drying. Generally, briquettes pressed at 10% moisture content were more stable compared with others pressed at 15%.

There is a limited effect of particle size on the briquette density for the particles sizes studied. However, small particles (x₅₀ < 3 mm) produce briquettes of a higher density than coarser particles (x₅₀ > 3 mm). The small particle sizes are able to fill the voids in the briquettes and this leads to an increase in briquette density compared with similar briquettes made of coarse particles.

The dry matter density and the radial compressive strength of the cylindrical briquettes have a close correlation. The density is considered as dry matter because the moisture does not have any influence on this connection. Increasing the pressure increases the briquette density and as a consequence the briquette durability also increases. Increasing the pressure up to 70 to 100 MPa for cotton and straw briquettes will not affect the density. The processed materials then reach the plastic stage and resist pressing. Above 100 MPa, increasing the pressure will increase the briquette density again.

The temperature has a major influence on the elimination of the Pink Bollworm. In order to kill these worms and their eggs or larvae, a temperature of 70 °C is required for at least 2 minutes. Fifteen minutes are required to kill the worms when the temperature is reduced to 60 °C. As Scholz and Fürll (1978) proved, pellets from fibrous materials can reach these temperatures in a cog-wheel pellet press (ΔT = 40 - 60 K). According to the measurements, briquettes from slow-running piston press machines do not reach this temperature (ΔT ≈ 10 K). In this case, the briquettes should be heated by heating and/or extending the press channel.

If briquettes of diameter 42.5 mm are warmed up after pressing, temperatures of 70 °C to 105 °C and heating times from 20 minutes to about 2 hours are required in order to reach 60 °C in the centre of the briquette. This is necessary to kill any worms and their eggs or larvae that may be found in the centre of the briquette. Temperatures above 105 °C should be avoided because the volatile components escape and a loss of dry matter will occur.

The experimental materials including cotton stalks are well suited for combustion. The ecologically and firing-relevant material content, the ash content and the heating value lie within the ranges of usual solid biofuels (volatile components: 66-75%; sulphur: 0.1-0.25; nitrogen: 0.6-0.9%; chlorine; 60-4400 mg/kg; ash 1.7-4.8%; lower heating value: 17.5 -19.4 MJ/kg). The high value of the chlorine (4400 mg/kg) and ash content (4.8%) of the cotton stalks lies within the range of non-woody biofuels.

Combustion experiments show that the combustion of briquettes is more regular than the combustion of loose materials. This is because of the high density of the briquettes. Therefore the thermal output of a combustion boiler with briquettes increases and the carbon monoxide (CO) value decreases by 30% compared with the value for the loose materials of poplar, hemp, and straw. The sulphur dioxide emissions (SO4) are also within the values limited by the German pollution control regulation TA-Luft.

The economic evaluation indicates that briquetting of cotton stalks is costly, but economically competitive compared with the use of kerosene as an energy source in households. Briquetting of cotton stalks costs 0.061 LE per kWh and thus only 5% more than the environmentally unfriendly process of burning cotton stalks on the field including the purchasable fuel. The use of unprocessed cotton stalks as a domestic fuel has an economic advantage, but it is ecologically undesired. The higher costs of briquetting cotton stalks (5%) will be more than returned to the farmer in the form of a clean environment. In addition, the extermination of the cotton pink bollworm through briquetting will increase the crop production by about 20%. This was the amount of losses because of this worm.

It can be stated that stable briquettes without binding material can be produced from cotton stalks and the other experimental materials. This depends on the material, sufficient pressure, and a suitable moisture content. This means the application of briquette technology can solve the phytosanitary, environmental and storage problems of cotton stalks. The dilemma of contradictory laws for the disposal of cotton stalks in Egypt would be overcome. The farmer’s household could cover its need for fossil fuels for cooking and baking by using a renewable energy source.

Further investigations should be directed towards developing a suitable adapted technology for briquetting cotton stalks in Egypt. Attention should also be paid to verifying the testing methods, as well as to the influence of the press on the mortality of cotton worms, their eggs and larvae.