Agricultural Energy Programs Reauthorization Act of 2018 This bill amends the Farm Security Rural Investment Act of 2002 to reauthorize through FY2023 several Department of Agriculture (USDA) energy programs, including: the Biobased Markets Program; the Biorefinery, Renewable Chemical, and Biobased Product Manufacturing Assistance Program; the Bioenergy Program for Advanced Biofuels; the Rural Energy For America Program (REAP); the Biomass Research and Development Initiative; the Feedstock Flexibility Program; and the Community Wood Energy Program. The bill modifies and reauthorizes the Biomass Crop Assistance Program to: rename it the Biofuels and Biobased Product Feedstock and Wildland Fire Protection Program; require USDA to provide assistance under the program for hazardous woody fuel reduction projects; and expand the definition of eligible materials to include algae and animal waste byproducts, including fat, oil, grease, and manure. The bill provides mandatory funding through FY2023 for several of the reauthorized programs. The bill modifies the Biobased Markets Program to: (1) include producers of renewable chemicals in the voluntary labeling program, and (2) require USDA and the Department of Commerce to jointly develop North American Industry Classification system codes for renewable chemical manufacturers and biobased products manufacturers. The bill modifies REAP to prohibit more than 30% of the funds from being used each year for: (1) any one form of renewable energy, or (2) technologies to improve the efficiency of energy usage.