TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The gaseous acid is a halogen-based mineral acid (104) or a halogen-based organic acid; and has less than 10 wt.% of water or is dry. The gaseous acid is gaseous hydrochloric acid. The proton donor is a Lewis acid. The Lewis acid is lithium chloride, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, zinc chloride, aluminum chloride, and/or boron chloride.    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Component: The solid-gas separator is a cyclone, a filter, or a gravimetric system. The multiphase reactor is a fluidized bed reactor.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The method further comprises separating gaseous acid from the reaction mixture using a solid-gas separator, drying the separated gaseous acid, and feeding the dried gaseous acid into the multiphase reactor. The gaseous acid and the biomass are continuously fed into the multiphase reactor. The method also comprises adding a solvent to the reaction mixture, where the solvent solubilizes the substituted furan, and the addition produces a solution comprising the substituted furan; and separating the solution by a filter or a membrane system. The method further comprises distilling the solution to obtain the substituted furan, where the distilling produces a separated solvent. The separated solvent is combined with a second reaction mixture. The multiphase reactor has pressure of 0.001-350 atm and temperature of 50-500 degrees C. Preferred Component: The biomass comprises glycans, heteroglycans, lignin, inorganic salts, and/or cellular debris. The solvent is dichloromethane, ethyl acetate, hexane, cyclohexane, benzene, toluene, diethyl ether, tetrahydrofuran, acetone, dimethyl formamide, dimethyl sulfoxide, acetonitrile, methanol, ethanol, isopropanol, n-propanol, n-butanol, chloroform, dichloroethane, trichloroethane, furfural, furfuryl alcohol, and/or supercritical carbon dioxide. The reaction mixture further comprises levulinic acid (112), formic acid (114), and/or alkylfurfural. EXAMPLE - Lignocellulosic biomass originating from municipal wastewater was obtained and fed through a side-input into a 2000-L fluidized bed reactor at a rate of 50 kg/minute. Gaseous hydrochloric acid was fed through another side-input into the reactor at a rate of 4000 L/minute. The temperature inside the reactor was approximately 220 degrees C, and the pressure inside the reactor was approximately 15 atm. Fluidization occurred inside the reactor, allowing for thorough and uniform mixing of the sludge and gaseous acid. After two minute residence time, the crude reaction mixture of gas and solids reached the top of the reactor, and exited the reactor. As the gas and solids left the reactor, the crude reaction mixture was separated using a cyclone into gaseous hydrochloric acid and a solid/liquid mixture. A sample of the solid mixture was obtained and analyzed by liquid chromatography-mass spectrometry (LCMS). The solid mixture contained 5-(chloromethyl)furfural, 5-(hydroxymethyl)furfural, and furfural, and some amounts of levulinic acid and formic acid were observed in the reaction mixture. The gaseous hydrochloric acid was passed through a desiccant to remove water. The dried gaseous hydrochloric acid was fed back into the fluidized bed reactor. The solid/liquid mixture was mixed in line with dichloromethane to produce a mixture containing both solids, and an organic solution. This mixture was then filtered. Samples of the retentate solids and the permeate solution were obtained and analyzed by LCMS. The solution permeate contained 5-(chloromethyl)furfural, 5-(hydroxymethyl)furfural, and furfural. The solution was transferred to a distillation apparatus to isolate the reaction products. The fractions were collected and analyzed by LCMS, in which 5-(chloromethyl)furfural, 5-(hydroxymethyl)furfural, and furfural were obtained in high purity. The reaction also produced small quantities of levulinic acid and formic acid.