DESCRIPTION Title of Invention: SOLID FUEL, AND METHOD AND APPARATUS FOR PRODUCING SAME Technical Field [0001] The present invention relates to a solid fuel obtained by heating biomass, and to a method and an apparatus for producing the same. Background Art [0002] As one of countermeasures for preventing global warming, biomass as a fuel derived from an organism is utilized as an energy source. For example, biomass is used as a partial alternative fuel of coal in coal fired-power generation equipment and the like. In order to use biomass in the coal fired-power generation equipment, it is necessary to pulverize the biomass so as to improve combustion efficiency. Since the coal fired-power generation equipment has a coal pulverizer for pulverizing coal, biomass is pulverized together with coal in the coal pulverizer, and the pulverized biomass and pulverized coal are co-combusted. [0003] Furthermore, since biomass generally has a high porosity, it has poor energy transporting property. Further, biomass has a low thermal energy density due to a high moisture content. Accordingly, the amount of heat generation is small when biomass is directly used as a fuel. Therefore, biomass is formed into pellets by being dried and pulverized, or is subjected a carbonization treatment when it is used. For example, Patent Literature 1 describes a method for combusting a biomass-based fuel used for co-combusting a char of biomass and coal. Furthermore, Patent Literature 2 discloses a content in which a pulverized product of palm shells is compressed, and the palm shells are used not as a fuel but as a microorganism base material for treating raw garbage. Citation List Patent Literatures [0004] Patent Literature 1: JP2005-114261A Patent Literature 2: JP2002-316128A Summary of Invention Technical Problem [0005] However, it is not easy to pulverize biomass in a coal pulverizer, and specifically in the case when biomass is shells left after obtaining kernel oil from palm fruit seeds (palm kernel shells), it has poorer pulverizing property than that of coal, and thus cannot be pulverized sufficiently. Furthermore, the char of biomass used in Patent Literature 1 is carbonized at a high temperature of 400 to 500°C, and much energy is lost during the carbonization, and the char is likely to be powderized during transport to cause environmental pollution. Patent Literature 2 describes that palm shells are compressed, but fails to describe that the compressed palm shells are used as a biomass fuel. The present invention aims at providing a solid fuel using palm kernel shells as biomass which is excellent in pulverizing property and high in calories and does not have dusting property, and a method and an apparatus for producing the solid fuel. Solution to Problem [0006] As a result of various studies by the inventors, it has been found that a solid fuel that solves the above-described problems is obtainable by a low temperature carbonization treatment of palm kernel shells. The present invention has been accomplished based on such finding. The present invention provides a solid fuel obtainable by heating shells left after obtaining kernel oil by oil-pressing palm fruit seeds. The solid fuel includes: 20 to 60 mass% of a fixed carbon; 30 to 66 mass% of a volatile content; 3 to 6 mass% of an ash content; and 6 mass% or less of a moisture content on air dried basis. The solid fuel has a higher heating value of 20 to 30 MJ/kg on air dried basis. The solid fuel preferably has 0.65